ISO 23705

Space systems — Identifying, evaluating and avoiding collisions between orbiting objects

Consistent with Guideline A.3.2b and B.4 of the UN COPUOS LTS Guidelines [1] to “address the safety and reliability of outer space activities under the entity’s control, during all phases of a mission life cycle;” and to “perform conjunction assessment during all orbital phases of controlled flight,” this proposed standard describes the workflow and technical requirements for perceiving, evaluating, and avoiding collisions among orbiting objects, data requirements for these tasks, identifies techniques that can be used to estimate the probability and/or consequence of collision, and provides guidance and requirements for executing collision avoidance maneuvers. [1] “Guidelines for the Long-term Sustainability of Outer Space Activities”, United Nations General Assembly document A/AC.105/C.1/L.366, 17 July 2018.

Systèmes spatiaux — Identification, évaluation et évitement des collisions entre objets en orbite

General Information

• Status: Not Published
• ICS: 49.140 - Space systems and operations
• Technical Committee: ISO/TC 20/SC 14 - Space systems and operations
• Drafting Committee: ISO/TC 20/SC 14 - Space systems and operations
• Current Stage: 6000 - International Standard under publication
• Start Date: 05-May-2026
• Completion Date: 09-May-2026

Overview

ISO 23705: Space systems - Identifying, evaluating and avoiding collisions between orbiting objects is an international standard developed by the International Organization for Standardization (ISO) under ISO/TC 20/SC 14. This standard provides comprehensive guidelines and technical requirements for perceiving, evaluating, and preventing collisions among orbiting objects in space. Covering the full mission lifecycle, ISO 23705 aligns with the United Nations COPUOS LTS Guidelines, advocating for safer and more reliable outer space activities through rigorous conjunction assessment and risk mitigation procedures.

Key Topics

• Conjunction Assessment (CA): Detailed workflow and responsibilities for identifying potential close approaches (conjunctions) between orbiting objects, and methodologies for evaluating the risks associated.
• Data Requirements: Specification of the essential data-including positional information, ephemerides, orbital elements, and covariance matrices-necessary for accurate collision risk assessment, referencing ISO 26900 for standardized data sharing.
• Collision Probability Estimation: Overview of accepted techniques for quantifying the likelihood of collision, including considerations of measurement errors and uncertainties in orbit determination.
• Collision Avoidance Maneuvers (CAM): Requirements and best practices for planning and executing collision avoidance maneuvers, including collaboration between operators and clear thresholds for action.
• Stakeholder Coordination: Clarifies the roles and responsibilities of Space Situational Awareness (SSA) systems, SSA service providers, and spacecraft operators for risk assessment, data exchange, and notification procedures.
• Reporting and Notification: Guidance for issuing timely alerts about potential collisions, including recommended content for notifications and coordination strategies.
• Spacecraft Operator Responsibilities: Criteria for operator decision-making, data sharing expectations, and management of maneuverability categories, as well as documentation and operational concepts to support ongoing safe operations.

Applications

ISO 23705 serves as a critical resource for:

• Spacecraft Operators: Enabling responsible management of orbital assets, implementing risk mitigation strategies, and ensuring compliance with international best practices.
• SSA Service Providers: Assisting with cataloging, tracking, and risk analysis services for satellites, debris, and other orbiting objects, facilitating collaboration and standardized communication with operators.
• National and International Agencies: Supporting regulatory compliance and the long-term sustainability of space activities through the application of consistent collision avoidance protocols.
• Space Traffic Coordination Entities: Enhancing global coordination, data exchange, and incident response between commercial, governmental, and international stakeholders.

ISO 23705 is especially practical for organizations managing large constellations, high-value assets (such as crewed space stations), or operations in densely populated orbital regions (LEO, MEO, GEO).

Related Standards

Organizations looking to implement or reference ISO 23705 for space traffic management and collision avoidance should also be aware of related ISO standards that provide supporting terminology and protocols:

• ISO 26900: Space data and information transfer systems - Orbit data messages. Defines standards for sharing orbit data, fundamental for effective conjunction assessment.
• ISO 19389: Space data and information transfer systems - Conjunction data message. Outlines messaging formats for notification of conjunctions.
• ISO 9490: Focuses on space traffic coordination, ensuring harmonized terminology and roles across agencies.
• ISO 21740: Addresses launch collision avoidance (LCOLA), extending risk management to the launch phase.
• ISO 14950, ISO 17666, ISO 19971: Provide additional operational concepts and guidance on mission assurance and collision avoidance for specific mission phases.

Adhering to ISO 23705 and its related standards ensures a harmonized approach to collision risk management in space, fostering transparency, sustainability, and the collective safety of all space actors.

Standards Content (Sample)

DRAFT
International
Standard
ISO/DIS 23705
ISO/TC 20/SC 14
Space systems — Identifying,
Secretariat: ANSI
evaluating, and avoiding collisions
Voting begins on:
between orbiting objects
2025-03-05
ICS: 49.140
Voting terminates on:
2025-05-28
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
This document is circulated as received from the committee secretariat.
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 23705:2025(en)
DRAFT
ISO/DIS 23705:2025(en)
International
Standard
ISO/DIS 23705
ISO/TC 20/SC 14
Space systems — Identifying,
Secretariat: ANSI
evaluating, and avoiding collisions
Voting begins on:
between orbiting objects
ICS: 49.140
Voting terminates on:
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
This document is circulated as received from the committee secretariat. BE CONSIDERED IN THE LIGHT OF THEIR
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
or ISO’s member body in the country of the requester.
NATIONAL REGULATIONS.
ISO copyright office
RECIPIENTS OF THIS DRAFT ARE INVITED
CP 401 • Ch. de Blandonnet 8
TO SUBMIT, WITH THEIR COMMENTS,
CH-1214 Vernier, Geneva
NOTIFICATION OF ANY RELEVANT PATENT
Phone: [phone]
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ISO/DIS 23705:2025(en)
ii
ISO/DIS 23705:2025(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
1.1 Breakdown of space safety constituents across ISO standards .1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and abbreviated terms. 3
4.1 Symbols .3
4.2 Abbreviated terms .3
5 CA Requirements for SSA systems and organizations . 4
5.1 General Requirements .4
5.1.1 Documentation describing conjunction assessment methodology .4
5.1.2 Conjunction assessment operational concept .4
5.2 SSA systems and organizations responsibilities .4
5.2.1 Data exchange with SSA systems and organizations.4
5.2.2 Interface with S/C operators .4
5.2.3 Establishment of service conditions between S/C operators and SSA
organisations .4
5.2.4 Update of the space object catalog for debris and non-cooperating operators .4
5.2.5 Update of the space object catalog for cooperating operators .4
5.2.6 Quality Assessment of the data used .5
5.2.7 Coordination with third parties .5
5.3 Risk assessment methodology .5
5.3.1 Screening threshold.5
5.3.2 CDM computation .5
5.3.3 Adopted spacecraft or debris equivalent cross-sectional area .5
5.3.4 Adopted spacecraft or debris mass .5
5.3.5 Spacecraft activity status .5
5.3.6 Notification and reporting of upcoming conjunctions .6
6 CA Requirements for Spacecraft Operators . 6
6.1 General Requirements .6
6.1.1 Documentation describing collision avoidance methodology .6
6.1.2 Collision avoidance operational concept. .6
6.2 Spacecraft operator collision risk mitigation and notification responsibilities .8
6.2.1 General .8
6.2.2 Spacecraft collision avoidance metrics and thresholds .8
6.2.3 Data exchange with relevant stakeholders . .8
6.2.4 Spacecraft attitude reorientation to minimize collision risk .9
6.2.5 Collision avoidance manoeuvre coordination .9
6.2.6 Spacecraft manoeuvrability categories .10
6.2.7 Communication of operator interpretation of manoeuvre rules, planned
avoidance manoeuvres, and spacecraft status.10
6.2.8 Collision avoidance manoeuvre Go/No-Go thresholds .10
6.2.9 Post-CAM collision probability targets .10
6.2.10 Obtaining sufficiently accurate STC . .11
6.2.11 Avoiding introduction of new collisions .11
6.2.12 Timeline of collision avoidance manoeuvres .11
Annex A (informative) Collision probability estimation methods .12
Annex B (informative) Covariance realism and relationship between sigma span and
probability density percentage .20
Annex C (informative) Perceiving close approaches .23

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ISO/DIS 23705:2025(en)
Annex D (informative) Notification of potential collisions for warning and further action .26
Annex E (informative) Relationship between combined object size, combined positional error,
and maximum probability .30
Annex F (informative) Probability contour visualization .32
Bibliography .42

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ISO/DIS 23705:2025(en)
Foreword
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The procedures used to develop this document and those intended for its further maintenance are described
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This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 14, Space systems and operations.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

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ISO/DIS 23705:2025(en)
Introduction
This document specifies the workflow for perceiving, evaluating, and avoiding collisions among orbiting
objects, data requirements for these tasks, techniques that can be used to estimate the probability of
collision and guidance for executing avoidance manoeuvres.
The process begins with obtaining orbital data from observations provided by spacecraft operators or
sensor systems developed for this purpose. Spacecraft orbits can be compared with each other to discern
physically feasible approaches that could result in collisions. The trajectories so revealed can then be
examined more closely to estimate the probability of collision. Where the possibility of a collision has been
identified within the criteria established by each spacecraft operator, the spectrum of feasible maneuvers is
examined.
The avoidance process begins with obtaining the best possible positional information on all potentially
[1]
conjuncting objects obtained from a combination of spacecraft operator and/or STC systems. The content
[2]
of which is specified in ISO 26900. The data can be provided by collaborating spacecraft operators and
from observers who are capable of viewing spacecraft. It is also important to know the nature of each object
if possible. This information includes size, mass, geometry, and the operational state (e.g., active or inactive).
Collision probability estimates consider the inevitable imprecision associated with orbit determination and
other hypotheses and measurements. This information then allows decision makers to take an effective
course of action as shown in the top-level workflow contained in Figure 1.
Figure 1 — Top-level collision avoidance workflow
There are several different approaches to conjunction assessment. All have merits and deficiencies.
Many focus on how closely spacecraft approach each other. This is often very uncertain since spacecraft
orbits generally change more rapidly under the influence of non-conservative forces than observations of
spacecrafts in orbit can be acquired and employed to improve orbit estimates. Spacecraft operators require
the fullness of orbit data to judge the credibility and quality of conjunction perception. This information
includes the moment of time of the last elaboration of orbit (the epoch) and the standard time scale employed,
positional ephemerides or orbit state vector values or elements at that moment in time, the coordinate system
description that presents the orbital data, the forces model description that was used for orbital predictions,
and information about the estimated positional errors associated with those predictions. Essential elements
[2]
of information for this purpose are specified in ISO 26900 .
There are also diverse approaches to estimating the probability that a close approach might really result
in a collision. This is a statistical process very similar to weather forecasting. Meteorologists no longer
make definitive predictions. They provide the probability of precipitation, not whether it will rain. All

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ISO/DIS 23705:2025(en)
conjunction assessment approaches are in some way founded in probabilities. Probability of collision is also
a highly desirable element of data. It can be accompanied by metadata that allows operators to interpret the
information within their own operational procedures.
How near two spacecraft might be to each other and the probability they might collide if they were that
close are only two discriminants of potentially catastrophic events. Since the objective is that the spacecraft
survives despite many potential close approaches, cumulative probability of survival is also important
information. Responding precipitously to the close approach nearest at hand might only delay the demise of
the spacecraft or even contribute to a subsequent more serious event. The evolution of close approaches is
thus also important.
Finally, the state of each of the conjunction partners, their ability to maneuver or otherwise avoid contact,
and the outcomes of past events that are similar guide courses of action.

vii
DRAFT International Standard ISO/DIS 23705:2025(en)
Space systems — Identifying, evaluating, and avoiding
collisions between orbiting objects
1 Scope
This document provides the workflow and technical requirements for perceiving, evaluating, and avoiding
collisions among orbiting objects, data requirements for these tasks, identifies techniques that can be used
to estimate the probability and/or consequence of collision, and provides guidance and requirements for
executing collision avoidance maneuvers.
1.1 Breakdown of space safety constituents across ISO standards
The space flight safety-relevant topics of Space Traffic Coordination, on-orbit collision avoidance, and launch
collision avoidance are closely related. To minimize duplication and maximize document consistency, the
various content that serve as the basis for these three disciplines has been divided up as shown in Figure 2.
Figure 2 — Division of space safety operations content spanning several ISO standards.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO/TR 11233, Space systems — Orbit determination and estimation — Process for describing techniques
ISO 26900, Space data and information transfer systems — Orbit data messages
ANSI/AIAA S-131-2010, Best Practices in Astronautics: Propagation
AIAA G-043-1992, Guide to Developing Operational Concepts

ISO/DIS 23705:2025(en)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
collision
act of colliding; an instance of one object striking another
3.2
collision probability
A quantification of the likelihood of two objects impacting each other during a specific encounter
3.3
conjunction
apparent meeting or passing of two or more objects in space
3.4
covariance
measure of how much variables change together
Note 1 to entry: For multiple dependent variables, a square, symmetric, positive definite matrix of dimensionality
N × N, where N is the number of variables.
3.5
encounter plane
plane normal to the relative velocity at the time of closest approach.
3.6
ephemeris
A time history of positional and velocity (and optionally acceleration and covariance) state information.
3.7
false alarm
statistical error, when a statistical test fails to reject a false null hypothesis.
3.8
interface control document
specification that describes the characteristics that must be controlled at the boundaries between systems,
subsystems, and other elements.
3.9
mission operations concept
description, in operator and user terms, of the operational attributes of a mission’s flight and ground
elements
3.10
orbital elements
parameters that describe the evolution of the trajectory and which can be used to estimate the trajectory in
the future.
3.11
scaled Pc
Scaled collision probability, where the collision probability is estimated using covariance matrices that have
been numerically adjusted by the square of a scaling factor.

ISO/DIS 23705:2025(en)
4 Symbols and abbreviated terms
4.1 Symbols
(TBD)
4.2 Abbreviated terms
ANSI American National Standards Institute
AIAA American Institute of Aeronautics and Astronautics
CA Conjunction Assessment
CAM Collision avoidance manoeuvre
CCSDS Consultative Committee for Space Data Systems
CDM Conjunction Data Message
COLA COLlision Avoidance
ESA European Space Agency
FDS Flight Dynamics Staff
GEO Geostationary earth orbit
HEO High earth orbit
JCA Just-in-time Collision Avoidance
LCOLA "Launch COLlision Avoidance" or "Launch COLA"
LEO Low earth orbit
LTS Long-Term Sustainability
MEO Medium earth orbit
O/O Owner/Operator
NASA National Aeronautics and Space Administration
PDF Probability Distribution Function
RCS Radar Cross Section
SANA Space Assigned Numbers Authority
S/C Spacecraft
SSA Space situational awareness
SST Space Surveillance and Tracking
STM Space traffic management
STC Space traffic coordination

ISO/DIS 23705:2025(en)
TCA Time of Closest Approach
UTC Coordinated Universal Time
5 CA Requirements for SSA systems and organizations
5.1 General Requirements
5.1.1 Documentation describing conjunction assessment methodology
SSA systems and organizations shall create and maintain documentation describing their own catalogue
update strategies (including detection of new objects), update of external orbits (ephemerides from
spacecraft operators, catalog maintained by other SSA entities), assessment of the quality of the data used,
conjunctions detection, risks assessment and mitigation strategy provision.
NOTE These data requirements enable a well-understood workflow and interactions among the potentially
several organizations that can interact to mitigate the potential consequences of conjunctions and collisions.
5.1.2 Conjunction assessment operational concept
SSA systems and organizations shall develop and maintain an operational concept that describes the roles,
relationships, tasks, processes, and information flows amongst stakeholders and the way systems and
processes will be used to provide conjunction assessment to spacecraft operators.
5.2 SSA systems and organizations responsibilities
5.2.1 Data exchange with SSA systems and organizations
SSA systems and organizations shall have the capability to exchange orbits and measurements with other
SSA systems and organizations, and with SST data providers.
NOTE The use of machine-to-machine interfaces as well as CCSDS standards is recommended when applicable.
5.2.2 Interface with S/C operators
Organizations conducting conjunction assessments shall provide an interface to operators to receive input
data (such as ephemerides) and generate CDMs.
5.2.3 Establishment of service conditions between S/C operators and SSA organisations
Exact service conditions shall be achieved between the satellite operator and the SSA organization(s) before
the start of the service and clarify the liability of the SSA systems and organizations; and unless specified
otherwise, the owners and operators are responsible for deciding mitigating courses of action.
NOTE Such conditions may already be established for all participants in a common definition of services, adopted
data exchange formats, and definition of liability. In cases where these are not established by default or if not well-
suited for a particular spacecraft operator, an interface document may be required to delineate such terms as agreed
between SSA organizations and spacecraft operators (cf 6.2.3.3).
5.2.4 Update of the space object catalog for debris and non-cooperating operators
Organizations conducting conjunction assessments shall obtain or maintain the most current, complete,
timely, and accurate SSA catalog data on debris and non-cooperating actively maneuvering spacecraft, by
building their own catalog and/or by acquiring data from an external entity.
5.2.5 Update of the space object catalog for cooperating operators
SSA system or organization shall keep up to date their database of orbits shared by spacecraft operators.

ISO/DIS 23705:2025(en)
5.2.6 Quality Assessment of the data used
Organizations conducting conjunction assessments shall check the consistency of external data before use,
by performing comparisons with statistical analysis, with measurements or with other orbits sources.
NOTE Organizations conducting conjunction assessments should notify S/C operators if the ephemerides or
covariances provided does not pass the quality checks.
5.2.7 Coordination with third parties
In case of a conjunction between two or more active S/C, organizations shall have the capability to describe
their risk assessment process, methodology, and analysis results to any parties unfamiliar with them.
5.3 Risk assessment methodology
5.3.1 Screening threshold
Organizations conducting conjunction assessments shall use the risk assessment methodology, screening
metrics, and associated thresholds as specified by their decision authority.
NOTE 1 Suggested minimum metrics and thresholds are provided in informative C.
NOTE 2 See [3] for representative metrics and thresholds used by commercial spacecraft operators.
5.3.2 CDM computation
Organizations conducting conjunction assessments shall consider all information available to compute CDM
and provide an easy mean to enable the user to differentiate the data source (O/O ephemerides, own catalog,
external catalogs) used for the primary and secondary objects.
NOTE Organizations conducting conjunction assessments should be able to identify which orbit source is the
most adequate for a given conjunction at a given time.
5.3.3 Adopted spacecraft or debris equivalent cross-sectional area
Organizations conducting conjunction assessments shall obtain or define the estimated “Equivalent Cross-
Sectional Area” for the spacecraft and/or debris object involved in the conjunction based on the following
priority order: data provided by Spacecraft operator or debris originator, computed from geometry available
on open-source information, default values derived from RCS or optical magnitude information, or a default
value when no information is available.
5.3.4 Adopted spacecraft or debris mass
Organizations conducting conjunction assessments shall obtain or define the estimated mass of the
spacecraft and/or debris object involved in the conjunction based on the following priority order: data
provided by spacecraft operator or debris originator, open-source information available, default values
derived from RCS or optical magnitude information, or a default value when no information is available.
5.3.5 Spacecraft activity status
Organizations conducting conjunction assessments shall obtain the operational status of any spacecraft
involved in a conjunction.
ISO/DIS 23705:2025(en)
5.3.6 Notification and reporting of upcoming conjunctions
Organizations conducting conjunction assessments shall alert operators of conjunctions that exceed
predefined metric thresholds as specified by their respective decision authority.
NOTE 1 Examples of conjunction metrics and thresholds are provided in informative Annex D. Such thresholds
typically support the identification of three separate phases of the analysis: (1) initial screening; (2) warning
associated with the adopted reporting criteria; and (3) exceedance of the Go/No-Go avoidance maneuver threshold.
NOTE 2 Organizations conducting conjunction assessments should be able to provide mitigation actions
recommendation for a few standard cases (such as a chemical maneuver performed 0,5 orbits before TCA) and help
the Spacecraft operator to define its mitigation strategy.
6 CA Requirements for Spacecraft Operators
6.1 General Requirements
6.1.1 Documentation describing collision avoidance methodology
Spacecraft operators shall create and maintain documentation describing their collision avoidance detection,
evaluation, and mitigation strategies.
NOTE These data requirements enable a well-understood workflow and interactions among the potentially
several organizations that can interact to mitigate the potential consequences of conjunctions and collisions.
6.1.2 Collision avoidance operational concept.
Spacecraft operators shall develop and maintain an operational concept that describes the roles,
relationships, tasks, processes, and information flows amongst stakeholders and the way systems and
processes will be used to perform collision avoidance actions.
NOTE 1 Since conjunction and collision assessment involves multiple stakeholders, providers, and action recipients,
a commonly understood, normative operational concept is essential. Figure 3 and Figure 4 provide a representative
operational concept depicting each of the elements in the collision avoidance workflow.
NOTE 2 There are several guides for developing and maintaining operational concepts (e.g. see ISO 14950,
ISO 17666 and ISO 19971).
ISO/DIS 23705:2025(en)
Figure 3 — Representative collision avoidance operational concept
Figure 4 provides details for how to “Select and execute course of action” box from Figure 2.

ISO/DIS 23705:2025(en)
Figure 4 — Diagram detailing components of selecting and executing a course of action.
6.2 Spacecraft operator collision risk mitigation and notification responsibilities
6.2.1 General
The previous discussion leads to operational collision risk mitigation requirements for avoiding collisions
and documentary requirements for providing relevant spacecraft, conjunction, and collision risk
information.
6.2.2 Spacecraft collision avoidance metrics and thresholds
Spacecraft operators shall mitigate collision risk which exceeds their respective decision authority’s
specified collision avoidance metrics and associated thresholds.
NOTE Collision risk mitigation measures should take priority over the mission of the spacecraft.
6.2.3 Data exchange with relevant stakeholders
6.2.3.1 Sharing of spacecraft physical characteristics
Spacecraft operators shall provide, to the extent such sharing is allowable, sufficient spacecraft size,
dimensions, equivalent cross-sectional area, and/or geometry and attitude/orientation to other relevant
spacecraft operators and stakeholders using ISO 26900 to permit the recipient’s independent assessment of
collision risk.
ISO/DIS 23705:2025(en)
6.2.3.2 Sharing of spacecraft manoeuvrability characteristics and planned manoeuvres
Spacecraft operators shall provide, to the extent such sharing is allowable, spacecraft manoeuvrability
characteristics as well as any planned manoeuvres to conjunction partners and any relevant stakeholders
using ISO 26900.
6.2.3.3 Sharing of ephemerides
As directed by their approving agent, spacecraft operators shall, either directly or by a proxy SSA
organization, generate and regularly share spacecraft ephemerides, inclusive of planned manoeuvres (for
manoeuvrable spacecraft) and, whenever possible, associated covariance time histories for their S/C.
NOTE When associated covariance information is not available directly from the SSA service provider, suitable
equivalents should be obtained to enable operators to evaluate collision probabilities and assess collision risk. An
[4]
example of such an approach to estimate covariance time histories is provided in .
6.2.3.4 Ephemeris screening duration and time step
Shared spacecraft ephemerides shall employ an appropriate duration and time-step as specified by their
approving agent. Sample screening durations and time steps are provided in Annex C.
6.2.3.5 Service agreement with SSA systems and organizations
As directed by their approving agent and prior to providing the service, S/C operators shall establish an
agreement via an interface document with an SSA system or organization that levies requirements for the
provision of SSA and space safety services containing the following mandatory information:
— Operational Points of Contacts
— S/C to be monitored
— S/C attributes: Hard Body Radius (i.e. radius of the sphere englobing the S/C) or an upper bound value,
and the operability and current category of a spacecraft’s manoeuvring system. Other S/C attributes
may also be provided such as mass, surface, drag coefficient, SRP coefficient
— Description of the format of ephemerides and maneuver plan (if provided)
— Severity threshold (e.g. ALERT, WARNING)
— Collision risk mitigation preferred strategies
— Post mitigation action threshold
6.2.4 Spacecraft attitude reorientation to minimize collision risk
Spacecraft operators shall consider reorienting their spacecraft to minimize collision risk where possible.
NOTE 1 Collision risk is a direct function of the effective cross-sectional area of the spacecraft parallel to the
conjunction encounter plane. Attitude can be changed with minimal energy expenditure to present the smallest cross-
section to the relative velocity vector at the time of closest approach.
NOTE 2 Large spacecraft likely have large solar panels. Most of the solar panel cross section can have low spatial
density, which is therefore less likely to fragment but more likely to remain in orbit.
6.2.5 Collision avoidance manoeuvre coordination
Collision avoidance manoeuvres shall be coordinated with the other spacecraft operator(s).
NOTE This can be accomplished either in real time or via bilaterally or multilaterally negotiated coordination
protocol or agreement and implemented as applicable.

ISO/DIS 23705:2025(en)
6.2.6 Spacecraft manoeuvrability categories
Spacecraft shall be categorized into the following five manoeuvrability categories:
i. Non-manoeuvrable; total inability to effect flight safety-relevant orbital changes.
ii. Minimally Manoeuvrable Robotic: Only able to perturb one’s orbit to a very small degree, e.g., using low
duty cycle low-thrust manoeuvres or differential drag perturbations with accelerations below 0,01 m/
s . This includes on-orbit servicers and active debris removal vehicles with their client space object
currently in custody and being towed.
iii. Manoeuvrable Robotic: Able to easily alter using available propellant and within one orbital revolution
the spacecraft course to mitigate the threat of collision.
iv. Automated on-board collision avoidance (COLA) manoeuvrable capability (robotic spacecraft). Operators
of systems with automated on-board COLA capabilities shall publish, where permitted by national laws,
regulations, and policies, information for peer review by affected operators and STC system decision
authorities regarding how the automation system works and coordinate with other operators (to
include establishment of bilateral agreements) to ensure that their avoidance manoeuvre decision/
plan is shared with affected operators and/or STC platforms at least 10 revolutions/hours before the
avoidance manoeuvre takes place, and verify that conducted avoidance manoeuvres effectively reduce
collision risk as intended.
v. Crewed (presumed manoeuvrable): A crewed spacecraft, able to alter their orbit to avoid collision.
6.2.7 Communication of operator interpretation of manoeuvre rules, planned avoidance
manoeuvres, and spacecraft status
Where directed by their approving agent, spacecraft operators shall communicate their interpretation
of manoeuvre rules and their avoidance manoeuvre plans with all known spacecraft operators of active
spacecraft involved in the conjunction for predicted close approaches exceeding alert thresholds (see
Annex C), even if the other spacecraft is/are un-manoeuvrable or minimally manoeuvrable.
6.2.8 Collision avoidance manoeuvre Go/No-Go thresholds
Collision avoidance manoeuvres (CAMs) (or equivalently effective mitigation strategies achieved by
cancelling or adjusting existing planned stationkeeping manoeuvres, etc.) shall be conducted for predicted
encounters having an estimated probability of collision or miss distance that exceeds the minimum
thresholds specified by their approving agent, unless more stringent bilaterally agreed spacecraft operator
thresholds necessitate the use of more conservative keep out or probability thresholds.
NOTE 1 Example avoidance maneuver thresholds are provided in informative Annex D.
NOTE 2 A mitigation action may not be possible in some cases such as reception of degraded SSA product,
impossibility to perform a mitigation action without raising the risk level of other conjunctions or incompatibility
between the required mitigation action and the S/C constraints.
NOTE 3 Go/No-Go thresholds for crewed space stations may be much more stringent than for robotic spacecraft
and/or debris.
6.2.9 Post-CAM collision probability targets
CAMs shall be designed to reduce collision risk to the degree specified by their approving agent.
NOTE 1 It is suggested that at a minimum, the collision probability target of conducted collision avoidance
[14]
manoeuvres should be a factor of at least 10 below the applicable collision probability threshold .

ISO/DIS 23705:2025(en)
6.2.10 Obtaining sufficiently accurate STC
When planning for a collision avoidance manoeuvre, operators shall obtain STC information that is of sufficient
[14]
accuracy to support the collision probability and/or miss distance metrics and thresholds used, such that
when predicted forward to an upcoming close approach, effective avoidance manoeuvres can be made.
6.2.11 Avoiding introduction of new collisions
When planning for a collision avoidance manoeuvre, spacecraft operators or responsible SSA system
organization shall use avoidance manoeuvre plan optimization techniques (i.e., to minimize fuel usage,
mission downtime, or mission risk) or conduct secondary CA screening to ensure that selected avoidance
manoeuvres do not introduce serious collision risk with the secondary object at other proximate times or
with other secondary objects.
6.2.12 Timeline of collision avoidance manoeuvres
6.2.12.1 Avoidance maneuver execution times
Spacecraft operators shall consider typical collision avoidance manoeuvre execution times as directed by
their conjunction assessment approving agent.
NOTE 1 Typical maneuver execution times are shown in Annex C.
NOTE 2 The typical sequence of events when conducting collision avoidance manoeuvres is as follows:
1. The relevant SSA and STC systems detect and notify the operator(s) of the impending collision threat.
2. Spacecraft Flight Dynamics Staff study the conjunction and plan a suitable avoidance manoeuvre (specifying
manoeuvre time, direction, magnitude).
3. FDS makes a recommendation to management, and/or jointly decides whether to conduct the avoidance
manoeuvre.
4. FDS obtains management approval.
5. The avoidance manoeuvre is conducted.
6. The passage of time allows the effect of the avoidance manoeuvre to effectively mitigate collision risk. Manoeuvres
typically must be conducted at least half an orbital revolution prior to TCA for maximum effectiveness.
7. The spacecraft remains in the collision avoidance phase (or “avoidance orbit”).
NOTE 2 Regarding avoidance manoeuvre strategies, some operators act as early as possible to mitigate the risk
of close approach well before this risk results in a high probability of collision, while other operators wait as long as
[14]
possible to refine the available data and get a better estimate of the geometry of the encounter and its risk level .
NOTE 3 The time required to plan, implement, and conduct avoidance manoeuvres, and the time required for those
manoeuvres to be effective, varies widely depending upon the operator’s selected avoidance strategy, orbital regime,
quality of SSA information, remaining spacecraft manoeuvring fuel and/or consumables, and type of propulsion
system utilized.
6.2.12.2 Incorporation of SSA accuracy and error growth when designing avoidance manoeuvres
When evaluating whether to conduct an avoidance manoeuvre, spacecraft operators shall incorporate and
account for the estimated accuracy and error growth of the conjunction assessment SSA positional products
at the estimated time of closest approach.

ISO/DIS 23705:2025(en)
Annex A
(informative)
Collision probability estimation methods
A.1 General
This annex discusses some current methods to compute collision probability for short- and long-term
encounters between space-borne objects. Many current formulations are based on the positional Gaussian
distribution and use covariances obtained from orbit determination methods. In broad general terms, all
the methods attempt to determine the value of the three-dimensional probability equation based on the
volume of probability density as the objects pass by each other.
A.2 Collision probability assessment methods
A.2.1 Selection of a suitable collision probability formulation
The analyst must first perform an assessment of the conjunction(s) in question to determine whether the
linearized collision probability formulation can be used or whether a non-linear formulation is required.
A.2.2 The linear collision probability formulation
One of the simplest analytical methods to assess collision probability, the linear conjunction assumes that
the time-varying r
...

FINAL DRAFT
International
Standard
ISO/FDIS 23705
ISO/TC 20/SC 14
Space systems — Identifying,
Secretariat: ANSI
evaluating and avoiding collisions
Voting begins on:
between orbiting objects
2026-03-09
Voting terminates on:
2026-05-04
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Reference number
ISO/FDIS 23705:2026(en) © ISO 2026

FINAL DRAFT
ISO/FDIS 23705:2026(en)
International
Standard
ISO/FDIS 23705
ISO/TC 20/SC 14
Space systems — Identifying,
Secretariat: ANSI
evaluating and avoiding collisions
Voting begins on:
between orbiting objects
Voting terminates on:
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
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INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
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TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
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ISO/FDIS 23705:2026(en) © ISO 2026

ii
ISO/FDIS 23705:2026(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 3
4.1 Abbreviated terms .3
5 CA Requirements for SSA systems and SSA service providers . 4
5.1 General requirements .4
5.1.1 Documentation describing conjunction assessment methodology .4
5.1.2 Conjunction assessment operational concept .4
5.2 SSA systems and SSA service providers responsibilities .4
5.2.1 Data exchange with SSA systems and SSA service providers .4
5.2.2 Interface with spacecraft operators .4
5.2.3 Establishment of service conditions between spacecraft operators and SSA
service providers .4
5.2.4 Update of the space object catalogue for debris and non-cooperating operators .5
5.2.5 Update of the space object catalogue for cooperating operators.5
5.2.6 Quality assessment of the data used .5
5.2.7 Coordination with third parties .5
5.3 Risk assessment methodology .5
5.3.1 Screening threshold.5
5.3.2 CDM computation .5
5.3.3 Adopted spacecraft or debris equivalent cross-sectional area .5
5.3.4 Adopted spacecraft or debris mass .5
5.3.5 Spacecraft activity status .6
5.3.6 Notification and reporting of upcoming conjunctions .6
6 CA requirements for spacecraft operators . 6
6.1 General requirements .6
6.1.1 Documentation describing collision avoidance methodology .6
6.1.2 Collision avoidance operational concept. .6
6.2 Spacecraft operator collision risk mitigation and notification responsibilities .7
6.2.1 Spacecraft collision avoidance metrics and thresholds .7
6.2.2 Data exchange with relevant stakeholders . .8
6.2.3 Spacecraft attitude reorientation to minimize collision risk .9
6.2.4 Collision avoidance manoeuvre coordination .9
6.2.5 Spacecraft manoeuvrability categories .9
6.2.6 Communication of operator interpretation of manoeuvre rules, planned
avoidance manoeuvres, and spacecraft status.9
6.2.7 Collision avoidance manoeuvre go/no-go thresholds .10
6.2.8 Post-CAM collision probability targets .10
6.2.9 Obtaining sufficiently accurate STC . .10
6.2.10 Avoiding introduction of new collisions .10
6.2.11 Timeline of collision avoidance manoeuvres .10
Annex A (informative) Collision probability estimation methods .12
Annex B (informative) Covariance realism and relationship between sigma span and
probability density percentage .15
Annex C (informative) Perceiving close approaches .18
Annex D (informative) Notification of potential collisions for warning and further action .21

iii
ISO/FDIS 23705:2026(en)
Annex E (informative) Relationship between combined object size, combined positional error,
and maximum probability .25
Annex F (informative) Probability contour visualization .28
Bibliography .36

iv
ISO/FDIS 23705:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
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related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 14, Space systems and operations.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
ISO/FDIS 23705:2026(en)
Introduction
The process for perceiving, evaluating and avoiding collisions among orbiting objects begins in stage 1 as
shown in the top-level workflow contained in Figure 1. In this stage, the best possible positional information
[1]
on all potentially conjuncting objects is obtained from a combination of either spacecraft operators or STC
[2]
systems or both. A standardized way to share such information is specified in ISO 26900 . Spacecraft
positional information can be provided by collaborating spacecraft operators and from observers who are
capable of viewing spacecraft. This data can then be compared with each other to discern physically feasible
approaches that could result in collisions. The trajectories so revealed can then be examined more closely to
estimate the probability of collision. It is also important to know the nature of each object if possible. This
information includes the size, mass, geometry, and the operational state (e.g. active or inactive). Collision
probability estimates consider the inevitable imprecision associated with orbit determination and other
hypotheses and measurements. This information then allows analysts in stage 2 to screen for threatening
close approaches. Where the possibility of a collision has been identified which violates the thresholds
established by a spacecraft operator, the spectrum of feasible maneuvers is examined in stage 3 to take an
effective course of action.
Figure 1 — Top-level collision avoidance workflow
There are several different approaches to conjunction assessment. All have merits and deficiencies. Many
focus on how closely spacecraft approach each other. This is often very uncertain since changes in spacecraft
orbits under the influence of non-conservative forces, as a rule:
a) evolve more rapidly than observations of spacecrafts in orbit can be acquired; and
b) cannot be modelled accurately enough to obtain credible results of propagation even when the initial
orbit determination is accurate.
Spacecraft operators require the fullness of orbit data to judge the credibility and quality of conjunction
perception.
This information includes the moment in time of the last elaboration of orbit (the epoch) and the standard
time scale employed, positional ephemerides or orbit state vector values or elements at that moment in
time, the coordinate system description that presents the orbital data, the forces model description that is
used for orbital predictions, and information about the estimated positional errors associated with those
[2]
predictions. Essential elements of information for this purpose are specified in ISO 26900 .

vi
ISO/FDIS 23705:2026(en)
There are also diverse approaches to estimating the probability that a close approach can result in a
collision. This is a statistical process very similar to weather forecasting. Meteorologists no longer make
definitive predictions. They provide the probability of precipitation, not whether it will rain. All conjunction
assessment approaches are in some way founded in probabilities. Probability of collision is also a highly
desirable element of data. It can be accompanied by metadata that allows operators to interpret the
information within their own operational procedures.
How near two spacecraft can be to each other and the probability they can collide if they were that close are
only two discriminants of potentially catastrophic events. Since the objective is that the spacecraft survives
despite many potential close approaches, cumulative probability of survival is also important information.
Responding precipitously to the close approach nearest at hand can only delay the demise of the spacecraft
or even contribute to a subsequent more serious event. The evolution of close approaches is thus also
important.
Finally, the state of each of the conjunction partners, their ability to manoeuvre or otherwise avoid contact,
and the outcomes of past events that are similar guide courses of action.
The space flight safety-relevant topics of space traffic coordination, on-orbit collision avoidance, and launch
collision avoidance are closely related. To minimize duplication and maximize document consistency, the
various content that serve as the basis for these three disciplines is divided up as shown in Table 1.
[1]
Table 1 — Division of space safety operations content between ISO 9490, ISO 23705 and
[3]
ISO 21740
ISO 9490: STC ISO 23705: Avoid collision ISO 21740: LCOLA
— STC terminology — Conjunction assessment and collision — LCOLA terminology:
avoidance terminology safety LCOLA and mission
— STC system, QC, IS, reliability
assurance LCOLA
— Risk mitigation strategies
— STC roles/responsibilities for
— LCOLA launch window
launch providers, S/C operators, — Suggested collision miss distance and
screening process
and SSA providers probability/risk metrics and associated
minimum thresholds — LCOLA products
— STC and SSA services
— Mathematical techniques: — Launch range
— Orbit determination, required
coordination
accuracy, timeliness — Conjunction assessment
— LCOLA data exchange
— Manoeuvrability RotR/ — Collision probability
(LDM)
— Manoeuvre recommendations — Linear, non-linear,
— LCOLA mathematical
techniques
— Data exchange and launch, — Asymmetric, max prob
manoeuvre, anomaly, and
— CAM planning, optimize
fragmentations notifications
— CA nomograms
— STC mathematical techniques
vii
FINAL DRAFT International Standard ISO/FDIS 23705:2026(en)
Space systems — Identifying, evaluating and avoiding
collisions between orbiting objects
1 Scope
This document provides the workflow and technical requirements for perceiving, evaluating and avoiding
collisions among orbiting objects, data requirements for these tasks, identifies techniques that can be used
to estimate either the probability or the consequence of collisions, or both, and provides requirements and
guidance for executing collision avoidance manoeuvres.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
3.1
collision
instance of one object striking another
3.2
collision probability
quantification of the likelihood of two space objects impacting each other during a conjunction (3.3) event
3.3
conjunction
event where the positional separation between two objects is at a local minimum and that minimum is either
a) closer than a specified minimum distance threshold;
b) the estimated probability of collision (3.1) at this local minimum exceeds a specified collision probability
(3.2) threshold; or
c) a combination of miss distance, probability and/or other metrics (e.g. “risk”)
3.4
covariance
measure of how much variables change together
Note 1 to entry: For multiple dependent variables, covariance is a square, symmetric, positive definite matrix of
dimensionality N × N, where N is the number of variables.

ISO/FDIS 23705:2026(en)
3.5
encounter plane
plane normal to the relative velocity at the time of closest approach
3.6
ephemeris
time history of positional and velocity (and optionally acceleration and covariance (3.4)) state information
3.7
false alarm
statistical error, when a statistical test fails to reject a false null hypothesis
3.8
interface control document
specification that describes the characteristics that must be controlled at the boundaries between systems,
subsystems and other elements
[SOURCE: ISO 15388:2022, 3.1.22, modified — The abbreviated term "ICD" has been removed.]
3.9
orbital elements
parameters that describe the evolution of the trajectory and which can be used to estimate the trajectory in
the future
3.10
scaled P
c
scaled collision probability
collision probability (3.2) that is estimated using covariance (3.4) matrices that have been numerically
adjusted by the square of a scaling factor
3.11
space situational awareness
SSA
knowledge and characterization of the space environment to facilitate decisions that support safe, stable,
and sustainable space activities
Note 1 to entry: SSA includes all artificial space objects (spacecraft, rocket bodies, mission-related objects and
fragments), natural objects, asteroids (including near-Earth objects or NEOs), comets and meteoroids, effects from
[35]
space weather, including solar activity and its radiation. Assessed risks include potential risks to humans and
property in space, on the ground and in the air space due to accidental or intentional re-entries, on-orbit explosions
and release events, on-orbit collisions (3.1), radio frequency interference, and occurrences that can disrupt missions
and services.
Note 2 to entry: This definition is adopted from Reference [4].
3.12
SSA service provider
space situational awareness service provider
entity that operates a space situational awareness (SSA) (3.11) system to maintain an operational picture of
the space environment and potential risks
Note 1 to entry: SSA services can be provided by a combination of one or more commercial, governmental, non-
governmental, or international entities, as well as by a mandated or delegated entity assigned by applicable national
regulation.
3.13
SSA system
space situational awareness system
set of information gathering and exchange protocols enabling the characterization of the space environment
and dissemination of its knowledge

ISO/FDIS 23705:2026(en)
3.14
space surveillance and tracking
SST
detection, observation, monitoring, cataloguing and prediction of the movement of space objects and the
identification and alerting of derived risks
Note 1 to entry: Space surveillance and tracking is generally accomplished through the operation and calibration of
ground-based or space-based tracking sensors using radar, optical, or passive RF technology.
3.15
space traffic coordination
STC
cooperative planning, harmonization, data and information sharing, and synchronization of space activities
to avoid collision (3.1) and radio frequency interference during spacecraft and launch vehicle operations in
space
Note 1 to entry: This definition is adopted from Reference [4].
4 Symbols and abbreviated terms
4.1 Abbreviated terms
ANSI American National Standards Institute
AIAA American Institute of Aeronautics and Astronautics
CA conjunction assessment
CAM collision avoidance manoeuvre
CCSDS Consultative Committee for Space Data Systems
CDM conjunction data message
COLA collision avoidance
ESA European Space Agency
FDS flight dynamics staff
GEO geostationary Earth orbit
LCOLA "launch collision avoidance" or "launch COLA"
LEO low Earth orbit
MEO medium Earth orbit
O/O owner/operator
NASA National Aeronautics and Space Administration
PDF probability distribution function
RCS radar cross section
SANA Space Assigned Numbers Authority
S/C spacecraft
ISO/FDIS 23705:2026(en)
SSA space situational awareness
SST space surveillance and tracking
STC space traffic coordination
TCA time of closest approach
UTC Coordinated Universal Time
5 CA Requirements for SSA systems and SSA service providers
5.1 General requirements
5.1.1 Documentation describing conjunction assessment methodology
SSA systems and SSA service providers shall create and maintain documentation describing their own
catalogue update strategies (including detection of new objects), update of external orbits (ephemerides
from spacecraft operators, catalogue maintained by other SSA entities), assessment of the quality of the data
used, conjunctions detection, risks assessment and mitigation strategy provision.
NOTE These data requirements enable a well-understood workflow and interactions among the potentially
multiple organizations involved in mitigating the potential consequences of conjunctions and collisions.
5.1.2 Conjunction assessment operational concept
SSA systems and SSA service providers shall develop and maintain an operational concept that describes the
roles, relationships, tasks, processes, and information flows amongst stakeholders and the way systems and
processes will be used to provide conjunction assessment to spacecraft operators.
5.2 SSA systems and SSA service providers responsibilities
5.2.1 Data exchange with SSA systems and SSA service providers
SSA systems and SSA service providers shall have the capability to exchange orbits and measurements with
other SSA systems and SSA service providers, and with SST service providers.
Where applicable, machine-to-machine interfaces adhering to CCSDS standards such as ISO 26900 orbit
data message, ISO 19389 conjunction data message, and CCSDS attitude data message, along with their
accompanying SANA registry normative content, should be used.
5.2.2 Interface with spacecraft operators
SSA service providers shall provide an interface to operators to receive input data (such as ephemerides)
and generate CDMs.
5.2.3 Establishment of service conditions between spacecraft operators and SSA service providers
Exact service conditions shall be established between the satellite operator and the SSA service provider(s)
before the start of the service. The liability of the SSA systems and SSA service providers shall be clarified.
Unless specified otherwise, the owners and operators shall be responsible for deciding mitigating courses of
action.
NOTE Such conditions can already be established for all participants in a common definition of services document,
adopted data exchange formats, and definition of liability. In cases where these are not established by default or if not
well-suited for a particular spacecraft operator, an interface control document can be established to delineate such
terms as agreed between SSA service providers and spacecraft operators (see 6.2.2).

ISO/FDIS 23705:2026(en)
5.2.4 Update of the space object catalogue for debris and non-cooperating operators
SSA service providers shall obtain or maintain the most current, complete, timely, and accurate SSA
catalogue data on debris and non-cooperating actively manoeuvring spacecraft, either by building their own
catalogue or by acquiring data from an external entity, or both.
5.2.5 Update of the space object catalogue for cooperating operators
The SSA system or SSA service provider shall keep up to date their database of orbits shared by spacecraft
operators.
5.2.6 Quality assessment of the data used
SSA service providers shall check the consistency and accuracy of external data before use, by performing
comparisons with statistical analysis, with measurements or with other orbits sources.
SSA service providers should notify spacecraft operators if the ephemerides or covariances provided does
not pass the quality checks.
5.2.7 Coordination with third parties
In case of a conjunction between two or more active S/C, organizations shall have the capability to describe
their risk assessment process, methodology and analysis results to any parties unfamiliar with them.
5.3 Risk assessment methodology
5.3.1 Screening threshold
SSA service providers shall use the risk assessment methodology, screening metrics and associated
thresholds as specified by their decision authority.
Minimum metrics and thresholds should be selected in accordance with Annex C and Annex D.
NOTE See Reference [6] and Clause D.2 for representative metrics and thresholds used by commercial spacecraft
operators.
5.3.2 CDM computation
SSA service providers should be able to identify which orbit source is the most adequate for a given
conjunction at a given time. SSA service providers shall clearly indicate the data source (O/O ephemerides,
own catalogue, external catalogues) used for the primary and secondary objects in the generated CDMs.
5.3.3 Adopted spacecraft or debris equivalent cross-sectional area
SSA service providers shall use sources for estimated “equivalent cross-sectional area” for spacecraft and
debris objects deemed most reliable.
NOTE 1 Generally, the following prioritization order can be used: data provided by the spacecraft operator or
debris originator, values computed from geometry available on open-source information, default values derived from
RCS or optical magnitude information, or a default value when no other information is available.
NOTE 2 Default values can be derived from statistical median “typical” values of other objects of known cross-
sections within a mission type, orbit class or category, ballistic coefficient estimates, or year of manufacture.
5.3.4 Adopted spacecraft or debris mass
SSA service providers shall obtain or define the estimated mass of either the spacecraft or the debris object,
or both, involved in the conjunction based on the following priority order: data provided by the spacecraft
operator or debris originator, open-source information available, default values derived from RCS or optical
magnitude information, or a default value when no information is available.

ISO/FDIS 23705:2026(en)
5.3.5 Spacecraft activity status
SSA service providers shall obtain the operational status of any spacecraft involved in a conjunction.
5.3.6 Notification and reporting of upcoming conjunctions
SSA service providers shall alert operators of conjunctions that exceed predefined metric thresholds as
specified by their respective decision authority.
NOTE 1 Examples of conjunction metrics and thresholds are provided in Annex D. Such thresholds typically support
the identification of three separate phases of the analysis: a) initial screening; b) warning associated with the adopted
reporting criteria; and c) exceedance of the go/no-go avoidance manoeuvre threshold.
NOTE 2 The notification can contain recommended mitigation actions for a few standard cases (such as a chemical
manoeuvre performed 0,5 orbits before TCA) to help the spacecraft operator select and define their mitigation
strategy.
6 CA requirements for spacecraft operators
6.1 General requirements
6.1.1 Documentation describing collision avoidance methodology
Spacecraft operators shall create and maintain documentation describing their collision avoidance detection,
evaluation and mitigation strategies.
NOTE These data requirements enable a well-understood workflow and interactions among the potentially
multiple organizations involved in mitigating the potential consequences of conjunctions and collisions.
6.1.2 Collision avoidance operational concept.
Spacecraft operators shall develop and maintain an operational concept that describes the roles,
relationships, tasks, processes and information flows amongst stakeholders and the way systems and
processes will be used to perform collision avoidance actions.
NOTE 1 Since conjunction and collision assessment involves multiple stakeholders, providers and action recipients,
a commonly understood, normative operational concept is essential. Figure 3 and Figure 4 provide a representative
operational concept depicting each of the elements in the collision avoidance workflow.
[7] [8] [9]
NOTE 2 See, for example, ISO 14950, ISO 17666 and ISO 19971 for guidance on developing and maintaining
operational concepts.
ISO/FDIS 23705:2026(en)
Figure 3 — Representative collision avoidance operational concept
Figure 4 provides details for the “Select and execute course of action” box from Figure 3, including
[10]
consequence assessment .
Figure 4 — Diagram detailing components of selecting and executing a course of action
6.2 Spacecraft operator collision risk mitigation and notification responsibilities
6.2.1 Spacecraft collision avoidance metrics and thresholds
Spacecraft operators shall mitigate collision risk which exceeds their respective decision authority’s
specified collision avoidance metrics and associated thresholds.

ISO/FDIS 23705:2026(en)
Collision risk mitigation measures should take priority over the mission of the spacecraft.
NOTE 1 See Reference [6], 5.3.1 and Clause D.2 for representative metrics and thresholds used by commercial
spacecraft operators.
NOTE 2 See Annex E and Annex F for information relating to, and visualizations of, the relationship between
combined object size, combined positional error, and maximum probability.
6.2.2 Data exchange with relevant stakeholders
6.2.2.1 Sharing of spacecraft physical characteristics
Spacecraft operators should provide sufficient spacecraft size, dimensions, equivalent cross-sectional area,
and geometry and attitude/orientation to other relevant spacecraft operators, SSA service providers, and
stakeholders in accordance with ISO 26900 and its underlying SANA registry normative content to permit
the recipient’s independent assessment of collision risk.
6.2.2.2 Sharing of spacecraft manoeuvrability characteristics and planned manoeuvres
Spacecraft operators should provide spacecraft manoeuvrability characteristics as well as any planned
manoeuvres to conjunction partners and any relevant stakeholders in accordance with ISO 26900 and its
underlying SANA registry normative content.
6.2.2.3 Sharing of ephemerides
As directed by their approving agent, spacecraft operators shall, either directly or by a proxy SSA service
provider, generate and regularly share spacecraft ephemerides, inclusive of planned manoeuvres (for
manoeuvrable spacecraft) and associated realistic covariance time histories for their S/C (see Annex B for
discussion of covariance realism).
When associated covariance information is not available directly from the SSA service provider, suitable
equivalents should be obtained to enable operators to evaluate collision probabilities and assess collision
risk. An example of such an approach to estimate covariance time histories is provided in Reference [101].
6.2.2.4 Ephemeris screening duration and time step
Shared spacecraft ephemerides shall employ an appropriate duration and time-step as specified by their
approving agent. Sample screening durations and time steps are provided in Annex C.
6.2.2.5 Service agreement with SSA systems and SSA service providers
As directed by their approving agent and prior to providing the service, spacecraft operators shall establish
an agreement with an SSA system or SSA service provider that levies requirements for the provision of SSA
and space safety services, which shall contain the following information:
— operational points of contacts;
— S/C to be monitored;
— S/C attributes: hard body radius (i.e. radius of the sphere englobing the S/C) or an upper bound value,
and the operability and current category of a spacecraft’s manoeuvring system; other S/C attributes may
also be provided such as mass, surface, drag coefficient, and solar radiation pressure coefficient;
— description of the format of ephemerides and manoeuvre plan (if provided);
— severity threshold (e.g. ALERT, WARNING);
— preferred strategies for collision risk mitigation;
— post-mitigation action threshold.

ISO/FDIS 23705:2026(en)
NOTE In cases where these are not established by default or if not well-suited for a particular spacecraft operator,
an interface control document can be established to delineate such terms as agreed between SSA service providers
and spacecraft operators.
6.2.3 Spacecraft attitude reorientation to minimize collision risk
Spacecraft operators should reorient their spacecraft to minimize collision risk where possible.
NOTE 1 Collision risk is a direct function of the effective cross-sectional area of the spacecraft parallel to the
conjunction encounter plane. The attitude can be changed with minimal energy expenditure to present the smallest
cross-section to the relative velocity vector at the time of closest approach.
NOTE 2 Large spacecraft likely have large solar panels. Most of the solar panel cross section can have low spatial
density, which is therefore less likely to fragment but more likely to remain in orbit.
NOTE 3 Reorientation can be especially useful in cases where it enables the operator to meet the risk reduction
target specified in 6.2.8.
6.2.4 Collision avoidance manoeuvre coordination
Collision avoidance manoeuvres shall be coordinated with all known spacecraft operators of active
[1]
spacecraft that may conjunct with an operator’s spacecraft, as required in ISO 9490 .
NOTE This can be accomplished either in real time or via bilaterally or multilaterally negotiated coordination
protocol or agreement and implemented as applicable.
6.2.5 Spacecraft manoeuvrability categories
Spacecraft shall be categorized into the following six manoeuvrability categories:
a) non-manoeuvrable: total inability to make flight safety-relevant orbital changes;
b) minimally manoeuvrable robotic: only able to perturb one’s orbit to a very small degree (i.e. mitigation
measure cannot be completed within one orbital revolution);
c) manoeuvrable robotic: able to easily alter the spacecraft course within one orbital revolution using
available propellant to mitigate the risk of collision;
d) automated on-ground collision avoidance (COLA) manoeuvrable capability (robotic spacecraft)
performing manoeuvres based on on-ground automation (i.e. computation of the manoeuvre on-
ground);
e) automated on-board collision avoidance (COLA) manoeuvrable capability (robotic spacecraft), where
the automated in-space decision can only be inferred based upon ground simulation or understanding;
f) inhabitable (presumed manoeuvrable): an inhabitable space station that can alter its path to avoid
collision.
NOTE These categories can change over time for a spacecraft, e.g. if its manoeuvring system or attitude control
system degrade or fail.
Spacecraft that rely solely on differential-drag control shall be categorised as minimally manoeuvrable
robotic.
6.2.6 Communication of operator interpretation of manoeuvre rules, planned avoidance
manoeuvres, and spacecraft status
Spacecraft operators should communicate, at least once, their interpretation of manoeuvre rules and their
avoidance manoeuvre plans with other spacecraft operators involved in the conjunction for close approaches
exceeding the operator’s collision risk threshold and with known relevant STC entities for all predicted close
approaches, even if the other spacecraft is/are un-manoeuvrable or minimally manoeuvrable.

ISO/FDIS 23705:2026(en)
NOTE In the case of several collocated spacecraft constellations, this requirement can be met by the initial sharing
of each other’s mitigation action plans for any future conjunctions.
6.2.7 Collision avoidance manoeuvre go/no-go thresholds
Collision avoidance manoeuvres (CAMs) (or equivalently effective mitigation strategies achieved by
cancelling or adjusting existing planned station-keeping manoeuvres, etc.) shall be conducted for predicted
encounters having an estimated probability of collision or miss distance exceeding the minimum thresholds
specified by their approving agent, unless more stringent bilaterally agreed spacecraft operator thresholds
necessitate the use of more conservative keep out or probability thresholds.
NOTE 1 Example avoidance manoeuvre thresholds are provided in Annex D.
NOTE 2 A mitigation action can be impossible in some cases such as reception of degraded SSA product,
impossibility to perform a mitigation action without raising the risk level of other conjunctions or incompatibility
between the required mitigation action and the S/C constraints.
NOTE 3 Go/no-go thresholds for crewed space stations can be much more stringent than for robotic spacecraft or
debris.
6.2.8 Post-CAM collision probability targets
CAMs shall be designed to reduce collision risk to the degree specified by their approving agent, unless the
resulting manoeuvre would cause significant degradation to S/C operations or exceed the S/C capability.
In that case, the spacecraft operator should implement a manoeuvre that decreases the risk to the greatest
extent possible.
At a minimum, the collision probability target of conducted collision avoidance manoeuvres should be a
[11]
factor of at least 10 below the applicable collision probability threshold .
6.2.9 Obtaining sufficiently accurate STC
When planning for a collision avoidance manoeuvre, operators shall obtain STC information that is of
[11]
sufficient accuracy to support the collision probability and miss distance metrics and thresholds used,
such that when predicted forward to an upcoming close approach, effective avoidance manoeuvres can be
made.
Analysts shall use a validated, accepted collision probability assessment method to generate STC products.
NOTE Information on a variety of suitable collision probability assessment methods is provided in Annex A.
6.2.10 Avoiding introduction of new collisions
When planning for a collision avoidance manoeuvre, spacecraft operators and responsible SSA service
providers shall use avoidance manoeuvre plan optimization techniques (i.e. to minimize fuel usage,
mission downtime, or mission risk) or conduct secondary CA screening to ensure that selected avoidance
manoeuvres do not introduce serious collision risk with the secondary object or with other space objects
within the minimum timeframe specified by the approving agent.
6.2.11 Timeline of collision avoidance manoeuvres
6.2.11.1 Avoidance manoeuvre execution times
Spacecraft operators shall consider typical collision avoidance manoeuvre execution times as directed by
their conjunction assessment approving agent.
NOTE 1 Typical manoeuvre execution times are shown in Annex C.
NOTE 2 The typical sequence of events when conducting collision avoidance manoeuvres is as follows:
a) The relevant SSA and STC systems detect and notify the operator(s) of the impending collision threat.

ISO/FDIS 23705:2026(en)
b) Spacecraft fli
...

ISO/DIS FDIS 23705:2025(en)
ISO/TC 20/SC 14/WG 3
Secretariat: ANSI
Date: 2025-08-212026-02-20
Space systems — Identifying, evaluating and avoiding collisions
between orbiting objects
FDIS stage
COMSPOC – Internal
ISO/FDIS 23705:2026(en)
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication
may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying,
or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO
at the address below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
EmailE-mail: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
ISO/FDIS 23705:2026(en)
Contents
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 3
4.1 Abbreviated terms . 3
5 CA Requirements for SSA systems and SSA service providers . 4
5.1 General requirements . 4
5.2 SSA systems and SSA service providers responsibilities . 5
5.3 Risk assessment methodology . 6
6 CA requirements for spacecraft operators . 7
6.1 General requirements . 7
6.2 Spacecraft operator collision risk mitigation and notification responsibilities . 10
Annex A (informative) Collision probability estimation methods . 15
Annex B (informative) Covariance realism and relationship between sigma span and
probability density percentage . 19
Annex C (informative) Perceiving close approaches . 22
Annex D (informative) Notification of potential collisions for warning and further action . 26
Annex E (informative) Relationship between combined object size, combined positional error,
and maximum probability . 31
Annex F (informative) Probability contour visualization . 36
Bibliography . 10

iii
ISO/FDIS 23705:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO documentsdocument should be noted. This document was drafted in accordance with the editorial rules
of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 14, Space systems and operations.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
ISO/FDIS 23705:2026(en)
Introduction
The process for perceiving, evaluating and avoiding collisions among orbiting objects begins in stage ①1 as
shown in the top-level workflow contained in Figure 1Figure 1. In this stage, the best possible positional
information on all potentially conjuncting objects is obtained from a combination of either spacecraft
[1]i
operators or STC systems or both. A standardized way to share such information is specified in
[2] [ii]
ISO 26900 . . Spacecraft positional information can be provided by collaborating spacecraft operators and
from observers who are capable of viewing spacecraft. This data can then be compared with each other to
discern physically feasible approaches that could result in collisions. The trajectories so revealed can then be
examined more closely to estimate the probability of collision. It is also important to know the nature of each
object if possible. This information includes the size, mass, geometry, and the operational state (e.g. active or
inactive). Collision probability estimates consider the inevitable imprecision associated with orbit
determination and other hypotheses and measurements. This information then allows analysts in stage ②2
to screen for threatening close approaches. Where the possibility of a collision has been identified which
violates the thresholds established by a spacecraft operator, the spectrum of feasible maneuvers is examined
in stage ③3 to take an effective course of action.

v
ISO/FDIS 23705:2026(en)
Figure 1— Top-level collision avoidance workflow
There are several different approaches to conjunction assessment. All have merits and deficiencies. Many
focus on how closely spacecraft approach each other. This is often very uncertain since changes in spacecraft
orbits under the influence of non-conservative forces, as a rule, (a) :
a) evolve more rapidly than observations of spacecrafts in orbit can be acquired; and
b) (b) cannot be modelled accurately enough to obtain credible results of propagation even when the initial
orbit determination is accurate.
Spacecraft operators require the fullness of orbit data to judge the credibility and quality of conjunction
perception.
This information includes the moment in time of the last elaboration of orbit (the epoch) and the standard
time scale employed, positional ephemerides or orbit state vector values or elements at that moment in time,
the coordinate system description that presents the orbital data, the forces model description that is used for
orbital predictions, and information about the estimated positional errors associated with those predictions.
[2][2]
Essential elements of information for this purpose are specified in ISO 26900 .
There are also diverse approaches to estimating the probability that a close approach can result in a collision.
This is a statistical process very similar to weather forecasting. Meteorologists no longer make definitive
predictions. They provide the probability of precipitation, not whether it will rain. All conjunction assessment
approaches are in some way founded in probabilities. Probability of collision is also a highly desirable element
of data. It can be accompanied by metadata that allows operators to interpret the information within their
own operational procedures.
How near two spacecraft can be to each other and the probability they can collide if they were that close are
only two discriminants of potentially catastrophic events. Since the objective is that the spacecraft survives
despite many potential close approaches, cumulative probability of survival is also important information.
Responding precipitously to the close approach nearest at hand can only delay the demise of the spacecraft or
even contribute to a subsequent more serious event. The evolution of close approaches is thus also important.
vi
ISO/FDIS 23705:2026(en)
Finally, the state of each of the conjunction partners, their ability to manoeuvre or otherwise avoid contact,
and the outcomes of past events that are similar guide courses of action.

1.1 Breakdown of space safety constituents across ISO standards
The space flight safety-relevant topics of space traffic coordination, on-orbit collision avoidance, and launch
collision avoidance are closely related. To minimize duplication and maximize document consistency, the
various content that serve as the basis for these three disciplines is divided up as shown in Table 1.
Field Code Changed
[] [1]
Table 1 — Division of space safety operations content between ISO 9490 , , ISO 23705, and ISO
[3][iii]
21740 .
ISO 9490: STC ISO 23705: Avoid collision ISO 21740: LCOLA
— STC terminology — Conjunction assessment and collision — LCOLA terminology:
avoidance terminology safety LCOLA and mission
assurance LCOLA
— STC system, QC, IS, reliability
— Risk mitigation strategies
— STC roles/responsibilities for — LCOLA launch window
launch providers, S/C operators, — Suggested collision miss distance and screening process
and SSA providers probability/risk metrics and associated
minimum thresholds — LCOLA products
— STC and SSA services
— Mathematical techniques: — Launch range
— Orbit determination, required coordination
accuracy, timeliness — Conjunction assessment
— LCOLA data exchange
— Manoeuvrability RotR/ — Collision probability (LDM)
— Manoeuvre recommendations — Linear, non-linear, — LCOLA mathematical
techniques
— Data exchange and launch, — Asymmetric, max prob
manoeuvre, anomaly, and
fragmentations notifications — CAM planning, optimize
— STC mathematical techniques — CA nomograms
vii
DRAFT International Standard ISO/DIS 23705:2025(en)

Space systems — Identifying, evaluating and avoiding collisions
between orbiting objects
21 1 Scope
This document provides the workflow and technical requirements for perceiving, evaluating and
avoiding collisions among orbiting objects, data requirements for these tasks, identifies techniques that
can be used to estimate either the probability or the consequence of collisions, or both, and provides
requirements and guidance for executing collision avoidance manoeuvres.
32 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 26900, Space data and information transfer systems — Orbit data messages
ISO 19389, Space data and information transfer systems – Conjunction data messageThere are no
normative references in this document.

43 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/
4.13.1 3.1
collision
instance of one object striking another
4.23.2 3.2
collision probability
quantification of the likelihood of two space objects impacting each other during a conjunction (3.3) event.
4.33.3 3.3
conjunction
event where the positional separation between two objects is at a local minimum and that minimum is
either (a) closer than a specified minimum distance threshold, (b) the estimated probability of collision
at this local minimum exceeds a specified collision probability threshold, or (c) a combination of miss
distance, probability, and/or other metrics (e.g., “risk”).
a) closer than a specified minimum distance threshold;
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ISO/FDIS 23705:2026(en)
the estimated probability of collision (3.1) at this local minimum exceeds a specified collision
probability (3.23.4
b) ) threshold; or
c) a combination of miss distance, probability and/or other metrics (e.g. “risk”)
3.4
covariance
measure of how much variables change together
Note 1 to entry: For multiple dependent variables, covariance is a square, symmetric, positive definite matrix of
dimensionality N × N, where N is the number of variables.
4.43.5 3.5
encounter plane
plane normal to the relative velocity at the time of closest approach
4.53.6 3.6
ephemeris
time history of positional and velocity (and optionally acceleration and covariance (3.4))) state
information
4.63.7 3.7
false alarm
statistical error, when a statistical test fails to reject a false null hypothesis
4.73.8 3.8
interface control document
specification that describes the characteristics that must be controlled at the boundaries between
systems, subsystems and other elements [ISO 15388:2022, 3.1.22].
3.9[SOURCE: ISO 15388:2022, 3.1.22, modified — The abbreviated term "ICD" has been removed.]
4.83.9
orbital elements
parameters that describe the evolution of the trajectory and which can be used to estimate the trajectory
in the future
4.93.10 3.10
scaled Pc
scaled collision probability
collision probability (3.2(3.2)) that is estimated using covariance (3.4) matrices that have been
numerically adjusted by the square of a scaling factor
3.11
Space Situational Awareness
3.11 Knowledge
space situational awareness
SSA
knowledge and characterization of the space environment to facilitate decisions that support safe, stable,
[4]
and sustainable space activities .
NOTE IncludesNote 1 to entry: SSA includes all artificial space objects (spacecraft, rocket bodies, mission-
related objects and fragments), natural objects, asteroids (including Nearnear-Earth Objectsobjects or NEOs), comets
[35]
and meteoroids, effects from space weather, including solar activity and its radiation [3 ]. . Assessed risks include
potential risks to humans and property in space, on the ground and in the air space due to accidental or intentional
ISO/DIS 23705:2025(en)
re-entries, on-orbit explosions and release events, on-orbit collisions (3.1,), radio frequency interference, and
occurrences that couldcan disrupt missions and services.
Note 2 to entry: This definition is adopted from Reference [4]3.12
Space Situational Awareness Service Provider
.
3.12
SSA service provider
space situational awareness service provider
entity that operates a space situational awareness (SSA) (3.11an SSA) system to maintain an operational
picture of the space environment and potential risks.
NOTE Note 1 to entry: SSA services can be provided by a combination of one or more commercial,
governmental, non-governmental, or international entities, as well as by a mandated or delegated entity assigned
by applicable national regulation.
3.13
3.13 Space Situational Awareness
SSA system
space situational awareness system
set of information gathering and exchange protocols enabling the characterization of the space
environment and dissemination of its knowledge.
3.14
3.14 Space Surveillance
space surveillance and Trackingtracking
SST
detection, observation, monitoring, cataloguing and prediction of the movement of space objects and the
identification and alerting of derived risks
NOTE Note 1 to entry: Space Surveillancesurveillance and Trackingtracking is generally accomplished through
the operation and calibration of ground-based or space-based tracking sensors using radar, optical, or passive RF
technology.
3.15
Space Traffic Coordination
3.15
space traffic coordination
STC
cooperative planning, harmonization, data and information sharing, and synchronization of space
activities to avoid collision (3.1) and radio frequency interference during spacecraft and launch vehicle
operations in space.
NOTE DefinitionNote 1 to entry: This definition is adopted from Reference [4]“SPACE TRAFFIC MANAGEMENT
TERMINOLOGY,” IAF STM Task Force [].
54 Symbols and abbreviated terms
5.14.1 Abbreviated terms
ANSI American National Standards Institute
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AIAA American Institute of Aeronautics and Astronautics
CA conjunction assessment
CAM collision avoidance manoeuvre
CCSDS Consultative Committee for Space Data Systems
CDM conjunction data message
COLA collision avoidance
ESA European Space Agency
FDS flight dynamics staff
GEO geostationary Earth orbit
LCOLA "launch collision avoidance" or "launch COLA"
LEO low Earth orbit
MEO medium Earth orbit
O/O owner/operator
NASA National Aeronautics and Space Administration
PDF probability distribution function
RCS radar cross section
SANA Space Assigned Numbers Authority
S/C spacecraft
SSA space situational awareness
SST space surveillance and tracking
STC space traffic coordination
TCA time of closest approach
UTC Coordinated Universal Time
65 CA Requirements for SSA systems and SSA service providers
6.15.1 General requirements
6.1.15.1.1 Documentation describing conjunction assessment methodology
SSA systems and SSA service providers shall create and maintain documentation describing their own
catalogue update strategies (including detection of new objects), update of external orbits (ephemerides
from spacecraft operators, catalogue maintained by other SSA entities), assessment of the quality of the
data used, conjunctions detection, risks assessment and mitigation strategy provision.
NOTE These data requirements enable a well-understood workflow and interactions among the potentially
multiple organizations involved in mitigating the potential consequences of conjunctions and collisions.
6.1.25.1.2 Conjunction assessment operational concept
SSA systems and SSA service providers shall develop and maintain an operational concept that describes
the roles, relationships, tasks, processes, and information flows amongst stakeholders and the way
systems and processes will be used to provide conjunction assessment to spacecraft operators.
ISO/DIS 23705:2025(en)
6.25.2 SSA systems and SSA service providers responsibilities
6.2.15.2.1 Data exchange with SSA systems and SSA service providers
SSA systems and SSA service providers shall have the capability to exchange orbits and measurements
with other SSA systems and SSA service providers, and with SST service providers.
Where applicable, machine-to-machine interfaces adhering to CCSDS standards such as ISO 26900 Orbit
Data Messageorbit data message, ISO 19389 Conjunction Data Messageconjunction data message, and
CCSDS Attitude Data Messageattitude data message, along with their accompanying SANA registry
normative content, should be used.
6.2.25.2.2 Interface with spacecraft operators
SSA service providers shall provide an interface to operators to receive input data (such as ephemerides)
and generate CDMs.
6.2.35.2.3 Establishment of service conditions between spacecraft operators and SSA service
providers
Exact service conditions shall be established between the satellite operator and the SSA service
provider(s) before the start of the service. The liability of the SSA systems and SSA service providers shall
be clarified.
Unless specified otherwise, the owners and operators shall be responsible for deciding mitigating courses
of action.
NOTE Such conditions can already be established for all participants in a common definition of services
document, adopted data exchange formats, and definition of liability. In cases where these are not established by
default or if not well-suited for a particular spacecraft operator, an interface control document can be established
to delineate such terms as agreed between SSA service providers and spacecraft operators (see 6.2.2).
Field Code Changed
6.2.45.2.4 Update of the space object catalogue for debris and non-cooperating operators
SSA service providers shall obtain or maintain the most current, complete, timely, and accurate SSA
catalogue data on debris and non-cooperating actively manoeuvring spacecraft, either by building their
own catalogue or by acquiring data from an external entity, or both.
6.2.55.2.5 Update of the space object catalogue for cooperating operators
The SSA system or SSA service provider shall keep up to date their database of orbits shared by spacecraft
operators.
6.2.65.2.6 Quality assessment of the data used
SSA service providers shall check the consistency and accuracy of external data before use, by performing
comparisons with statistical analysis, with measurements or with other orbits sources.
SSA service providers should notify spacecraft operators if the ephemerides or covariances provided
does not pass the quality checks.
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ISO/FDIS 23705:2026(en)
6.2.75.2.7 Coordination with third parties
In case of a conjunction between two or more active S/C, organizations shall have the capability to
describe their risk assessment process, methodology and analysis results to any parties unfamiliar with
them.
6.35.3 Risk assessment methodology
6.3.15.3.1 Screening threshold
SSA service providers shall use the risk assessment methodology, screening metrics and associated
thresholds as specified by their decision authority.
Minimum metrics and thresholds should be selected in accordance with Annex CAnnex C and
Annex DAnnex D.
NOTE See Reference [6] [ ] and Clause D.2section for representative metrics and thresholds used by
commercial spacecraft operators.
6.3.25.3.2 CDM computation
SSA service providers should be able to identify which orbit source is the most adequate for a given
conjunction at a given time. SSA service providers shall clearly indicate the data source (O/O
ephemerides, own catalogue, external catalogues) used for the primary and secondary objects in the
generated CDMs.
6.3.35.3.3 Adopted spacecraft or debris equivalent cross-sectional area
SSA service providers shall use sources for estimated “equivalent cross-sectional area” for spacecraft and
debris objects deemed most reliable.
NOTE 1 : Generally, the following prioritization order can be used: data provided by the spacecraft operator or
debris originator, values computed from geometry available on open-source information, default values derived
from RCS or optical magnitude information, or a default value when no other information is available.
NOTE 2 : Default values can be derived from statistical median “typical” values of other objects of known cross-
sections within a mission type, orbit class or category, ballistic coefficient estimates, or year of manufacture.
6.3.45.3.4 Adopted spacecraft or debris mass
SSA service providers shall obtain or define the estimated mass of either the spacecraft or the debris
object, or both, involved in the conjunction based on the following priority order: data provided by the
spacecraft operator or debris originator, open-source information available, default values derived from
RCS or optical magnitude information, or a default value when no information is available.
6.3.55.3.5 Spacecraft activity status
SSA service providers shall obtain the operational status of any spacecraft involved in a conjunction.
6.3.65.3.6 Notification and reporting of upcoming conjunctions
SSA service providers shall alert operators of conjunctions that exceed predefined metric thresholds as
specified by their respective decision authority.
NOTE 1: Examples of conjunction metrics and thresholds are provided in Annex DAnnex D. Such thresholds
typically support the identification of three separate phases of the analysis: a) initial screening; b) warning
associated with the adopted reporting criteria; and c) exceedance of the go/no-go avoidance manoeuvre threshold.
ISO/DIS 23705:2025(en)
NOTE 2 : The notification can contain recommended mitigation actions for a few standard cases (such as a
chemical manoeuvre performed 0,5 orbits before TCA) to help the spacecraft operator select and define their
mitigation strategy.
76 CA requirements for spacecraft operators
7.16.1 General requirements
7.1.16.1.1 Documentation describing collision avoidance methodology
Spacecraft operators shall create and maintain documentation describing their collision avoidance
detection, evaluation and mitigation strategies.
NOTE These data requirements enable a well-understood workflow and interactions among the potentially
multiple organizations involved in mitigating the potential consequences of conjunctions and collisions.
7.1.26.1.2 Collision avoidance operational concept.
Spacecraft operators shall develop and maintain an operational concept that describes the roles,
relationships, tasks, processes and information flows amongst stakeholders and the way systems and
processes will be used to perform collision avoidance actions.
NOTE 1 Since conjunction and collision assessment involves multiple stakeholders, providers and action
recipients, a commonly understood, normative operational concept is essential. Figure 3Figure 3 and
Figure 4Figure 4 provide a representative operational concept depicting each of the elements in the collision
avoidance workflow.
[7] [7] [8][8] [9][9]
NOTE 2 See, for example, ISO 14950 , , ISO 17666 and ISO 19971 for guidance on developing and
maintaining operational concepts.
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ISO/FDIS 23705:2026(en)
Figure 3— Representative collision avoidance operational concept
ISO/DIS 23705:2025(en)
Figure 4Figure 4 provides details for the “Select and execute course of action” box from Figure 3Figure 3,,
[10] [10]
including consequence assessment .

Figure 4— Diagram detailing components of selecting and executing a course of action
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ISO/FDIS 23705:2026(en)
7.26.2 Spacecraft operator collision risk mitigation and notification responsibilities
7.2.16.2.1 Spacecraft collision avoidance metrics and thresholds
Spacecraft operators shall mitigate collision risk which exceeds their respective decision authority’s
specified collision avoidance metrics and associated thresholds.
Collision risk mitigation measures should take priority over the mission of the spacecraft.
NOTE 1 : See Reference [6] [], 5.3.1 and Clause D.2sections and for representative metrics and thresholds used
by commercial spacecraft operators.
NOTE 2 : See Annex EAnnex E and Annex FAnnex F for information relating to, and visualizations of, the
relationship between combined object size, combined positional error, and maximum probability.
7.2.26.2.2 Data exchange with relevant stakeholders
7.2.2.16.2.2.1 Sharing of spacecraft physical characteristics
Spacecraft operators should provide sufficient spacecraft size, dimensions, equivalent cross-sectional
area, and geometry and attitude/orientation to other relevant spacecraft operators, SSA service
providers, and stakeholders in accordance with ISO 26900 and its underlying SANA registry normative
content to permit the recipient’s independent assessment of collision risk.
7.2.2.26.2.2.2 Sharing of spacecraft manoeuvrability characteristics and planned manoeuvres
Spacecraft operators should provide spacecraft manoeuvrability characteristics as well as any planned
manoeuvres to conjunction partners and any relevant stakeholders in accordance with ISO 26900 and its
underlying SANA registry normative content.
7.2.2.36.2.2.3 Sharing of ephemerides
As directed by their approving agent, spacecraft operators shall, either directly or by a proxy SSA service
provider, generate and regularly share spacecraft ephemerides, inclusive of planned manoeuvres (for
manoeuvrable spacecraft) and associated realistic covariance time histories for their S/C (see Annex B
Field Code Changed
for discussion of covariance realism).
When associated covariance information is not available directly from the SSA service provider, suitable
equivalents should be obtained to enable operators to evaluate collision probabilities and assess collision
risk. An example of such an approach to estimate covariance time histories is provided in Reference [101]
[].
7.2.2.46.2.2.4 Ephemeris screening duration and time step
Shared spacecraft ephemerides shall employ an appropriate duration and time-step as specified by their
approving agent. Sample screening durations and time steps are provided in Annex CAnnex C.
7.2.2.56.2.2.5 Service agreement with SSA systems and SSA service providers
As directed by their approving agent and prior to providing the service, spacecraft operators shall
establish an agreement with an SSA system or SSA service provider that levies requirements for the
provision of SSA and space safety services, which shall contain the following information:
— — operational points of contacts;
— — S/C to be monitored;
ISO/DIS 23705:2025(en)
— — S/C attributes: hard body radius (i.e. radius of the sphere englobing the S/C) or an upper bound
value, and the operability and current category of a spacecraft’s manoeuvring system; other S/C
attributes may also be provided such as mass, surface, drag coefficient, and solar radiation pressure
coefficient;
— — description of the format of ephemerides and manoeuvre plan (if provided);
— — severity threshold (e.g. ALERT, WARNING);
— — preferred strategies for collision risk mitigation;
— — post-mitigation action threshold.
NOTE : In cases where these are not established by default or if not well-suited for a particular spacecraft
operator, an interface control document can be established to delineate such terms as agreed between SSA service
providers and spacecraft operators.
7.2.36.2.3 Spacecraft attitude reorientation to minimize collision risk
Spacecraft operators should reorient their spacecraft to minimize collision risk where possible.
NOTE 1 Collision risk is a direct function of the effective cross-sectional area of the spacecraft parallel to the
conjunction encounter plane. The attitude can be changed with minimal energy expenditure to present the smallest
cross-section to the relative velocity vector at the time of closest approach.
NOTE 2 Large spacecraft likely have large solar panels. Most of the solar panel cross section can have low spatial
density, which is therefore less likely to fragment but more likely to remain in orbit.
NOTE 3 Reorientation can be especially useful in cases where it enables the operator to meet the risk reduction
target specified in 6.2.8.
Field Code Changed
7.2.46.2.4 Collision avoidance manoeuvre coordination
Collision avoidance manoeuvres shall be coordinated with all known spacecraft operators of active
[1][]
spacecraft that may conjunct with an operator’s spacecraft, as required in ISO 9490 .
NOTE This can be accomplished either in real time or via bilaterally or multilaterally negotiated coordination
protocol or agreement and implemented as applicable.
7.2.56.2.5 Spacecraft manoeuvrability categories
Spacecraft shall be categorized into the following six manoeuvrability categories:
a) Nonnon-manoeuvrable;: total inability to make flight safety-relevant orbital changes.;
b) Minimally Manoeuvrable Robotic: Onlyminimally manoeuvrable robotic: only able to perturb one’s
orbit to a very small degree (i.e.,. mitigation measure cannot be completed within one orbital
revolution).);
c) Manoeuvrable Robotic: Ablemanoeuvrable robotic: able to easily alter the spacecraft course within
one orbital revolution using available propellant to mitigate the risk of collision.;
d) Automatedautomated on-ground collision avoidance (COLA) manoeuvrable capability (robotic
spacecraft) performing manoeuvres based on on-ground automation (i.e. computation of the
manoeuvre on-ground).);
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ISO/FDIS 23705:2026(en)
e) Automatedautomated on-board collision avoidance (COLA) manoeuvrable capability (robotic
spacecraft), where the automated in-space decision can only be inferred based upon ground
simulation or understanding. ;
f) Inhabitableinhabitable (presumed manoeuvrable): Anan inhabitable space station that can alter its
path to avoid collision.
NOTE 1: These categories maycan change over time for a spacecraft, e.g., should. if its manoeuvring system or
attitude control system degrade or fail.
NOTE 2: Spacecraft that rely solely on differential-drag control shall be categorised as Minimally
Manoeuvrable Roboticminimally manoeuvrable robotic.
7.2.66.2.6 Communication of operator interpretation of manoeuvre rules, planned avoidance
manoeuvres, and spacecraft status
Spacecraft operators should communicate, at least once, their interpretation of manoeuvre rules and
their avoidance manoeuvre plans with other spacecraft operators involved in the conjunction for close
approaches exceeding the operator’s collision risk threshold and with known relevant STC entities for all
predicted close approaches, even if the other spacecraft is/are un-manoeuvrable or minimally
manoeuvrable.
Note: NOTE In the case of several collocated spacecraft constellations, this requirement can be met by the initial
sharing of each other’s mitigation action plans for any future conjunctions.
7.2.76.2.7 Collision avoidance manoeuvre go/no-go thresholds
Collision avoidance manoeuvres (CAMs) (or equivalently effective mitigation strategies achieved by
cancelling or adjusting existing planned station-keeping manoeuvres, etc.) shall be conducted for
predicted encounters having an estimated probability of collision or miss distance exceeding the
minimum thresholds specified by their approving agent, unless more stringent bilaterally agreed
spacecraft operator thresholds necessitate the use of more conservative keep out or probability
thresholds.
NOTE 1 Example avoidance manoeuvre thresholds are provided in Annex DAnnex D.
NOTE 2 A mitigation action can be impossible in some cases such as reception of degraded SSA product,
impossibility to perform a mitigation action without raising the risk level of other conjunctions or incompatibility
between the required mitigation action and the S/C constraints.
NOTE 3 Go/no-go thresholds for crewed space stations can be much more stringent than for robotic spacecraft
or debris.
7.2.86.2.8 Post-CAM collision probability targets
CAMs shall be designed to reduce collision risk to the degree specified by their approving agent, unless
the resulting manoeuvre would cause significant degradation to S/C operations or exceed the S/C
capability. In that case, the spacecraft operator should implement a manoeuvre that decreases the risk to
the greatest extent possible.
At a minimum, the collision probability target of conducted collision avoidance manoeuvres should be a
[11][11]
factor of at least 10 below the applicable collision probability threshold .
7.2.96.2.9 Obtaining sufficiently accurate STC
When planning for a collision avoidance manoeuvre, operators shall obtain STC information that is of
[11][]
sufficient accuracy to support the collision probability and miss distance metrics and thresholds used,
ISO/DIS 23705:2025(en)
such that when predicted forward to an upcoming close approach, effective avoidance manoeuvres can
be made.
Analysts shall use ana validated, accepted collision probability assessment method as listed in Annex Ato
generate STC products.
NOTE Information on a variety of suitable collision probability assessment methods is provided in Annex A.
7.2.106.2.10 Avoiding introduction of new collisions
When planning for a collision avoidance manoeuvre, spacecraft operators and responsible SSA service
providers shall use avoidance manoeuvre plan optimization techniques (i.e. to minimize fuel usage,
mission downtime, or mission risk) or conduct secondary CA screening to ensure that selected avoidance
manoeuvres do not introduce serious collision risk with the secondary object or with other space objects
within the minimum timeframe specified by the approving agent.
7.2.116.2.11 Timeline of collision avoidance manoeuvres
7.2.11.16.2.11.1 Avoidance manoeuvre execution times
Spacecraft operators shall consider typical collision avoidance manoeuvre execution times as directed by
their conjunction assessment approving agent.
NOTE 1 Typical manoeuvre execution times are shown in Annex CAnnex C.
NOTE 2 The typical sequence of events when conducting collision avoidance manoeuvres is as follows:
a) a) The relevant SSA and STC systems detect and notify the operator(s) of the impending collision
threat.
b) b) Spacecraft flight dynamics staff study the conjunction and plan a suitable avoidance manoeuvre
(specifying manoeuvre time, direction, magnitude).
c) c) FDS either make a recommendation to management or jointly decide with the management, or
both, whether to conduct the avoidance manoeuvre.
d) d) FDS obtain management approval.
e) e) The avoidance manoeuvre is conducted.
f) f) The passage of time allows the effect of the avoidance manoeuvre to effectively mitigate collision
risk. Manoeuvres typically must be conducted at least half an orbital revolution prior to TCA for maximum
effectiveness.
g) g) The spacecraft remains in the collision avoidance phase (or “avoidance orbit”).
NOTE 3 Regarding avoidance manoeuvre strategies, some operators act as early as possible to mitigate the risk
of close approach well before this risk results in a high probability of collision, while other operators wait as long
as possible to refine the available data and get a better estimate of the geometry of the encounter and its risk
[11][]
level .
NOTE 4 The time required to plan, implement and conduct avoidance manoeuvres, and the time required for
those manoeuvres to be effective, varies widely depending upon the operator’s selected avoidance strategy, orbital
regime, quality of SSA information, remaining spacecraft manoeuvring fuel or consumables (or both), and type of
propulsion system utilized.
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7.2.11.26.2.11.2 Incorporation of SSA accuracy and error growth when designing
avoidance manoeuvres
When evaluating whether to conduct an avoidance manoeuvre, spacecraft operators shall incorporate
and account for the estimated accuracy and error growth of the conjunction assessment SSA positional
products at the estimated time of closest approach.
ISO/DIS 23705:2025(en)
Annex A
(informative)
Collision probability estimation methods
A.1 General
This annex discusses some current methods to compute collision probability for short- and long-term
encounters between space-borne objects. Many current formulations are based on the positional
Gaussian distribution and use covariances obtained from orbit determination methods. In broad general
terms, all the methods attempt to determine the value of the three-dimensional probability equation
based on the volume of probability density as the objects pass by each other.
A.2 Collision probability assessment methods
A.2.1 Selection of a suitable collision probability formulation
The analyst first performs an assessment of the conjunction(s) in question to determine whether the
linearized collision probability formulation can be used or whether a non-linear formulation is required.
A.2.2 The linear collision probability formulation
One of the simplest analytical methods to assess collision probability, the linear conjunction assumes that
the time-varying relative motion (x, y, z) is considered linear for the encounter by assuming that the effect
of relative acceleration is dwarfed by that of the velocity. The positional errors are assumed to be zero-
mean, Gaussian, uncorrelated, and constant for the encounter. The relative velocity at the time of closest
approach (TCA) is deemed sufficiently large to ensure a brief encounter time and static covariance with
principal axes standard deviations of σ , σ , and σ .
x y z
Another typical assumption assumptions of the linear collision probability formulation is that the
physical hard body shape of the two conjuncting objects is modelled as spheres, thus eliminating the need
for attitude information.
In this formulation, the cumulative collision probability P is found by integrating the three-dimensional,
Gaussian, relative position density over the volume V (collision tube) that is swept out by the combined
hard body of the two space objects over a s
...