ISO/DTR 25775.2

• Abstract:

  Explaining the future increase in constellation programmes, the increase in collision risk, the necessity of complying with the requirements, and the technical basis of the requirements.

• 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/WG 3 - Operations and support systems
• Current Stage: 6000 - International Standard under publication

Overview

ISO/DTR 25775.2 addresses the increasing complexity and risks associated with the deployment of large spacecraft constellations in Earth orbit. With the rapid expansion of constellation programmes, this ISO technical report highlights the growing threat of orbital congestion, increased collision risk, and the necessity for clear and robust compliance with standardized requirements. The document serves as a justification for the requirements outlined in ISO/TS 6434:2024, offering valuable context and technical rationale for decision-makers, operators, and manufacturers in the space sector.

As the space environment becomes more crowded with both active satellites and space debris, adhering to internationally recognized requirements ensures the sustainable and safe operation of large satellite constellations. This guidance is vital for mission planners, operators, and international stakeholders in managing risks and supporting the long-term use of outer space.

Key Topics

• Growth of Spacecraft Constellations: Details the significant increase in spacecraft launches since 2013, driven primarily by the proliferation of large constellations, and forecasts continued growth to 2031.
• Orbital Congestion and Collision Risk: Explores how densely populated altitude bands, especially between 450 km and 650 km, lead to elevated collision probabilities and challenges in operational planning.
• Accumulation of Orbital Debris: Explains that the total number of catalogued objects in orbit, including operational spacecraft and debris, will continue to rise, intensifying the need for effective debris mitigation.
• Technical Rationale for Requirements: Outlines the reasoning behind compliance with ISO/TS 6434:2024, including measures like radial separation, coordinated orbital selection, and collision avoidance to minimize risk.
• Operational Coordination: Emphasizes the importance of cooperation and data sharing among constellation operators to facilitate traffic management and safe disposal strategies.
• Long-term Sustainability: Supports international guidelines aiming to balance constellation growth with the preservation of the near-Earth environment for future generations.

Applications

ISO/DTR 25775.2 is applicable to a wide range of entities in the space domain:

• Constellation Programme Owners & Operators: Provides critical guidance for mission design, orbit selection, and operational coordination to minimize collision risk and comply with international best practices.
• Manufacturers & Integrators: Informs spacecraft design decisions by highlighting requirements for safe integration into congested orbits and plans for post-mission disposal.
• National Space Agencies: Assists in policy development, licensing, and international compliance for large constellation projects.
• Space Traffic Management Experts: Supplies a foundation for risk analysis, conjunction assessment, and the development of new traffic coordination protocols.
• Insurance & Regulatory Bodies: Offers authoritative context for risk assessment related to satellite collisions and orbital debris.

By adhering to the recommendations of ISO/DTR 25775.2, stakeholders can improve operational safety, protect valuable orbital resources, and demonstrate proactive responsibility in space stewardship.

Related Standards

• ISO/TS 6434:2024: Space systems - Requirements for spacecraft large constellations. This is the primary reference guiding the detailed requirements justified in ISO/DTR 25775.2.
• ISO/TC 20/SC 14: The ISO technical committee and subcommittee responsible for standardization in space systems and operations.
• CCSDS (Consultative Committee for Space Data Systems): Provides complementary guidelines for space data systems, relevant to constellation coordination and telemetry sharing.
• UN Guidelines for the Long-term Sustainability of Outer Space Activities: Offers a broader framework for sustainable space operations referenced by this technical report.

Practical Value

Implementing the guidance from ISO/DTR 25775.2 helps space sector organizations:

• Reduce the probability of in-orbit collisions
• Ensure compliance with international expectations for space sustainability
• Plan missions and develop spacecraft compatible with an increasingly complex orbital environment
• Support global coordination for the safe and reliable use of space

With the international landscape rapidly evolving, this ISO report enables organizations to keep pace with emerging best practices and regulatory trends, securing their place in the future of space operations.

FINAL DRAFT
Technical
Report
ISO/DTR 25775
ISO/TC 20/SC 14
Space systems — Justification of
Secretariat: ANSI
requirements for a spacecraft large
Voting begins on:
constellation
2026-04-24
Voting terminates on:
2026-06-19
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.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
ISO/DTR 25775:2026(en) © ISO 2026

FINAL DRAFT
ISO/DTR 25775:2026(en)
Technical
Report
ISO/DTR 25775
ISO/TC 20/SC 14
Space systems — Justification of
Secretariat: ANSI
requirements for a spacecraft large
Voting begins on:
constellation
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
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ISO/DTR 25775:2026(en) © ISO 2026

ii
ISO/DTR 25775:2026(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 1
4.1 Symbols .1
4.2 Abbreviated terms .1
5 Orbital environment occupied with active spacecraft and catalogued debris . 3
5.1 Launch history .3
5.2 Accumulation of manmade objects trackable from the ground .4
5.3 Launch history and plan of low Earth orbit (LEO) large constellations .5
5.4 High spatial density caused by large constellations which is hardly prospected even a
few years before .7
5.5 The threat of collision relating to large constellations.9
5.6 Trend of generation of orbital objects and their decay . 13
6 Purpose, rationale and feasibility of requirements . 14
6.1 Mission design.14
6.2 Spacecraft design .19
Bibliography .31

iii
ISO/DTR 25775:2026(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
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/DTR 25775:2026(en)
Introduction
More than a dozen large constellation programs are planned to be launched in the next several years.
While large constellations can provide societal benefits to humanity, they can also put pressure on orbital
and electro-magnetic environments, introducing challenges to mission design, hardware design, launch,
operations and disposal actions and the long-term sustainability of space activities.
Corresponding to the above situation, to ensure the long-term sustainability of space operations,
ISO/TS 6434 was published in January 2024, providing a set of standard practices based on the voluntary
practices applied in the existing world major large constellation programs. It also reflects the guidelines
developed by academic organizations.
This document explains the necessity to conform to the requirements of ISO/TS 6434, and the technical
basis of the requirements.
v
FINAL DRAFT Technical Report ISO/DTR 25775:2026(en)
Space systems — Justification of requirements for a
spacecraft large constellation
1 Scope
This document provides the justification for the requirements of ISO/TS 6434. It also explains the growth
of future large constellation programs, the increasing collision risk, and the technical rationale for the
requirements.
This document is intended to support owners of large constellations and manufacturers or operators acting
under their responsibility.
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
large constellation of spacecraft
large constellation
system of a hundred or more spacecraft working together
Note 1 to entry: While providing a specific quantity of spacecraft, the size, mass, complexity and function of the
spacecraft also have a bearing. There are several cases that are excluded from the scope of a large constellation even if
it is composed of more than 100 vehicles.
[SOURCE: ISO/TS 6434:2024, 3.1, modified — Last sentence added to Note 1 to entry.]
4 Symbols and abbreviated terms
4.1 Symbols
For the purposes of this document, the following symbols apply.
Ec expected number of casualties
4.2 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.

ISO/DTR 25775:2026(en)
API advanced publication of information
AT acceptance test
CAM collision avoidance manoeuvre
CARA conjunction assessment and risk analysis
CCSDS Consultative Committee for Space Data System
CSpOC Combined Space Operations Center, United States Space Forces – Space.
C/O check-out
DoA dead on arrival
DoD US Department of Defense
EoM end of mission
FSOA French Space Operations Act
IADC Inter-Agency Space Debris Coordination Committee
ITU international telecommunication union
LST local sun time
LTS long-term sustainability of outer space activities
NASA National Aeronautics and Space Administration
NIST National Institute of Standards and Technology, US Department of Commerce
NMI NASA Management Instruction
PMD post mission disposal
PSD probability of successful disposal
QT qualification test
RB radiocommunication bureau (in ITU)
RF radio frequency
SAA space act agreement
SSA space situational awareness
TT&C telemetry, tracking and command
UN United Nations
V&V verification and validation

ISO/DTR 25775:2026(en)
5 Orbital environment occupied with active spacecraft and catalogued debris
5.1 Launch history
Figure 1 shows the number of spacecraft launched into Earth's orbit by the end of 2019. Figure 2 shows the
number of spacecraft launched by the end of 2025.
From those two figures, it is observed that the number of nano- and pico-spacecraft began to increase form
2013, and the large number (larger than several thousands) of spacecraft has begun to be launched due to
several large constellation programs from 2020.
NOTE The method of classification of spacecraft mass is provided by the Seradata database. There is no intention
to claim this method to apply widely.

Key
Y launch year
L
N number of spacecraft launched in a year
M large spacecraft (>2 500 kg)
L
M medium spacecraft (from 1 000 kg to 2 500 kg)
Med
M small spacecraft (from 500 kg to 1 000 kg)
S
M mini spacecraft (from 100 kg to 500 kg)
min
Figure 1 — Number of spacecraft launched into Earth's orbit by the end of 2019
1)
[SOURCE: Reproduced with permission from the Seradata database, 2026/1/5]

1) Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

ISO/DTR 25775:2026(en)
Key
Y launch year
L
N number of spacecraft launched in a year
M large spacecraft (>2 500 kg)
L
M medium spacecraft (from 1 000 kg to 2 500 kg)
Med
M small spacecraft (from 500 kg to 1 000 kg)
S
M mini spacecraft (from 100 kg to 500 kg)
min
Figure 2 — Number of spacecraft to be launched by the end of 2025
2)
[SOURCE: Reproduced with permission from the Seradata database, 2026/1/5]
5.2 Accumulation of manmade objects trackable from the ground
From Figure 3, the total number of catalogued objects will reach more than 32 000 at the end of 2025, and
the number of spacecraft will surpass the number of catalogued fragments.
2) Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

ISO/DTR 25775:2026(en)
Key
Y launch year
l
N number of catalogued objects accumulated in the earth orbit each year
a
O number of fragments and released objects accumulated in the earth orbit each year
D
O number of spacecraft accumulated in the earth orbit each year
S
O number of launch vehicle orbital stages including assist modules accumulated in the earth orbit each year
L
NOTE Data source: CSpOC, 2026/01/05.
Figure 3 — Number of accumulated catalogued objects excluding decayed objects each year
5.3 Launch history and plan of low Earth orbit (LEO) large constellations
Figure 4 shows the launch schedule up to 2031. The tendency of the large constellation programs does not
end in 2025, and even larger constellation programs are expected to follow until 2031, and perhaps longer.
Figure 5 shows the number of catalogued spacecraft that are in LEO at 2025/07/04 and others that will be
launched by 2031/12/31 according to their operation altitude.
The most congested altitude range is from 450 km to 650 km.
Anther critical situation will occur in the range from 600 km to 1 100 km, where an extremely large
constellation has large eccentricity covering such altitude range. In Figure 5, the number of spacecraft
of large eccentricity orbit is expressed with the number that is allocated to each altitude band according
to their time occupation rate in each band. Historically, the altitude range is used for Earth observation
missions, and some constellation programs are being planned. These large constellations with large orbital
eccentricities pose a greater risk due to frequent collisions with other constellations, large collision angles
and high collision velocities resulting in large collision energies, and the difficulty of early predictions of
collisions. Furthermore, frequent collision avoidance operations hinder stable mission operations.
Also, even for spacecraft operated in circular orbit, if they are disposed in high eccentricity disposal orbit
with setting their perigee to very low altitude, the disposal orbit will cause high frequent conjunctions

ISO/DTR 25775:2026(en)
with many operation corridors of other large constellation groups. Therefore, it is expected that a disposal
strategy will be set appropriately, as described in 6.2.1.4.
Key
Y launch year
L
N number of spacecraft of large constellations
G G60 Qianfan (Thousand Sails)
Q
K Kuiper (Amazon)
P
S Starlink
L
G GuoWang (China SatNet)
W
O OneWeb
W
O Other large constellations
t
NOTE Figure 4 shows the number of spacecraft of large constellations launched by the 22nd of August 2025 and
planned to be launched until the end of 2031 as of 2025/08/22.
Figure 4 — Number of spacecraft of large constellations planned to be launched by the end of 2031
3)
[SOURCE: Reproduced with permission from the Seradata database, 2026/8/22]

3) Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

ISO/DTR 25775:2026(en)
Key
N is number of spacecraft
A is altitude of large constellations
L
G is G60 Qianfan (Thousand Sails)
Q
K is Kuiper (Amazon)
P
S is Starlink
L
G is GuoWang (China SatNet)
W
O is OneWeb
W
O is Other large constellations
t
NOTE The number of GuoWang program spacecraft (total 6495 spacecraft), which has large eccentricity (from
perigee 600 m to apogee 1 100 km), are allocated to each altitude band according to the rate of time occupation per
altitude band calculated by the orbit analysis.
Figure 5 — Distribution of spacecraft along their altitude as a snapshot on 2025-07-04, and that of
the constellation spacecraft to be launched by the end of 2031
4)
[SOURCE: Reproduced with permission from CSpOC and the Seradata database, 2025/07/04]
5.4 High spatial density caused by large constellations which is hardly prospected even a
few years before
Figure 6 shows the quasi-spatial density along a 10 km altitude band for the end of 2016, July 2025, and end
of 2031.
The proper spatial density value is calculated considering the allocated time along the altitude band where
each object interferes, considering eccentricity. In the quasi-spatial density in Figure 6, objects in highly
eccentric orbits with a mean altitude of more than 2 000 km are ignored, so the quasi-spatial density is
smaller than the actual value. But since the majority of large constellation spacecraft are circular orbits, they
4) Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

ISO/DTR 25775:2026(en)
are unlikely to cause any major problems. As shown in Figures 6, the range between 450 km and 650 km
altitude is very congested.
The European Space Agency has declared they will revise MASTER annually from 2025, but the large
constellation programs will not be announced, so that it is difficult to analyse the collision probability during
the operation phase in the planning or design phase.
If the new MASTER does not take into account the existence of active spacecraft because of lower impact
probability, due to their collision avoidance capability, it will not be possible to prospect conjunction
probability, which will need to estimate propellants for avoidance operations.
International academic committees and the United Nations Committee on the Peaceful Uses of Outer Space
have prospected these situations and started discussions regarding the long-term sustainability of space
activities beyond the debris issue in the early 2000s. The main theme is collision avoidance with worldwide
coordination for the surveillance of active orbital assets, detection of the conjunction, data sharing of
ephemeris of active assets.
Key
A is average orbital altitude (higher value of altitude band), km
S is quasi-spatial density of catalogued objects whose average altitude exist 10 km altitude band, 1/km
D
D is the quasi-spatial density of catalogued objects that exist in 10 km altitude band in 2016, 1/km
D is the quasi-spatial density in 2025, 1/km
D is the quasi-spatial density in 2031, 1/km
NOTE 1 Quasi-spatial density (QSD) is the number of catalogued objects whose average altitude exists 10 km
altitude band (N) per spatial volume of the shell for altitude 10 km band (V) (QSD = N/V).
NOTE 2 The number of spacecraft of large eccentricity orbit is expressed with the number that is allocated to each
altitude band according to their time occupation rate in each band.

ISO/DTR 25775:2026(en)
NOTE 3 In Figure 6, in the case of 2031, the number of orbital stages and fragments potentially generated after
2025 July are ignored.
Figure 6 — Quasi-spatial density calculated from average altitude of catalogued objects for every
10 km altitude band for calendar year of 2016, 2025 and 2031
5)
[SOURCE: Reproduced with permission from CSpOC/SSR and the Seradata database, 2025/07/23]
5.5 The threat of collision relating to large constellations
In general, there are no threats of collision between multiple spacecraft orbiting at the same altitude in a
unified orbital plane. Spacecraft in different orbital planes will not collide if they are in separate altitude.
In the case of a constellation, the orbital planes are distributed and arranged to maximize the coverage
of the Earth's surface and ensure uniformity of communications or observations. Currently, in the orbital
turning areas at the northernmost and southernmost ends of the orbit, where spacecraft fly in an east-
west direction and form densely clouded belts, (referred to as Northern and Southern densely clouded belts
in this document). In polar orbits, North and South polar congested regions are formed (see Figure 7). To
reduce the probability of collision, it is possible to separate the altitude (or give eccentricity).
Even in the mid-latitude bands, other than the Northern and Southern densely clouded belts, all spacecraft
in the constellation have a threat of collision at all intersections of the mid-latitude region, so it is believed
that collision management within the intra-constellation is performed by strictly managing the passing
timing, and each spacecraft controls its orbit at every moment.
Furthermore, there is the issue of congested regions in the sun-synchronous orbits (SSO). In SSO, the orbital
plane rotates in synchronous with the Earth's orbital period, so the angle of incidence of the sun on the
orbital plane remains constant throughout the year. For this reason, it is the most orthodox special orbit
frequently used by observation spacecraft. Local Sun Times (LSTs) of 6:00, 10:30, 13:30, and 18:00 are
frequently used. As a result, the orbital planes corresponding to these popular LSTs are congested with SSO
spacecraft.
The intersection between the congested region in the altitude direction of large constellations and the
congested orbital planes of SSO is a danger zone.
As one of available solutions, ISO/TS 6434:2024, 5.1.1.2 introduces the concept of radial separation,
defined in ISO/TS 6434:2024, 3.3. Since there is no way to resolve these intersections, the importance of
international space traffic rules to ensure safe traffic and rules to separate operation altitude is recognized.
In addition to the above potential threats, there is the problem of interference with other constellations
with high eccentricities and interference with spacecraft disposed into high eccentric orbits. Objects on
highly eccentric orbits tend to reach higher velocities near perigee, causing greater fragmentation in the
event of a collision, and the conjunction warnings tend to be delayed. There is also the issue of the invasion
of spacecraft and launch vehicles that will be added to the constellation. Currently large constellations are
equipped with autonomous collision avoidance systems (automatic avoidance AI) to avoid objects including
uncooperative objects (dead spacecraft, disposed spacecraft, and spacecraft without avoidance functions).
However, since the only means of avoiding collision with uncooperative objects is on the active spacecraft
side, an excessive share of the responsibility lies with the operators of active constellation spacecraft.
5) Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

ISO/DTR 25775:2026(en)
Figure 7 — North/South polar congested regions and densely clouded belts
Figure 8 shows the inclination of large constellations along the periods of 87 minutes to 110 minutes (roughly
at altitudes of 130 km to 1200 km), and STARLINK and Kuiper have almost the same period of 95 minutes,
with the difference in orbital inclination within 1 degree. The figure also shows the proximity of STARLINK
to Jilin 1 and Hawkeye 360.
Furthermore, Figure 9 shows an enlarged polar dense area, and in the narrow range from 97,2 to 97,7
degrees in north latitude, multiple constellations with almost the same period and altitude are densely
packed together, raising concerns about collisions. It also appears to be preventing ordinary polar-orbiting
spacecraft from entering at an altitude of around 500 km.

ISO/DTR 25775:2026(en)
Key
P is orbital period (min)
I is inclination (degree)
Ge is Geesat
Hw is Hawkeye 360
J is Jilin 1
K is Kuiper (Amazon)
P
L is Lemur
M
O is OneWeb
W
P is Planet Constellations
C
S is Starlink
L
O is Other Chinees large constellations
t
NOTE Data source: US CSpOC/Element Set (ELSET) Data /Low Earth Orbit (LEO) @20250704.
Figure 8 — Interference among large constellations, in the northern congested region, presented by
orbital period and inclination, below altitudes of 1 200 km (period: 110 minutes)

ISO/DTR 25775:2026(en)
Key
P is orbital period (min)
I is inclination (degree)
Hw is Hawkeye 360
J is Jilin 1
L is Lemur
M
P is Planet Constellations
C
S is Starlink
L
NOTE Data source: US CSpOC/Element Set (ELSET) Data /Low Earth Orbit (LEO) @20250704.
Figure 9 — Large constellations around north pole congested region expressed with orbital period
and inclination
In the future, the following measures are likely to be discussed:
a) Promotion of traffic management that employs different orbital altitudes for each orbital plane.
— Information sharing among constellation operators who are operating polar orbits.
— Allocation of orbital altitudes for each orbital plane.
— Sharing information of specific events such as disposal or new launch and injection into constellations.
— Coordination among operators and STC coordinators to avoid collisions.
b) Control of the timing of polar passes.
— Coordination among operators for the timing of their polar passes to avoid collisions.

ISO/DTR 25775:2026(en)
5.6 Trend of generation of orbital objects and their decay
Figure 10 shows the trend in the number of generated orbital objects (including new launch vehicle orbital
stages, fragments and released objects) and number of re-entries. The recent increase in the number of re-
entries since 2022 is temporary due to the Crona Mass Ejections, but it is possible that the number of re-
entries will increase in the future according to the disposal of the large constellation spacecraft. The safety
against the threat of re-entering objects is another issue relating to large constellations.
Key
Y calendar year
L
N number of catalogued objects each year
C
N number of decayed objects each year
D
C catalogued number of fragments and released objects each year
D
C catalogued number of spacecraft each year
S
C catalogued number of launch vehicle orbital stages and assist modules each year
L
D decreased number of launch vehicle orbital stages and assist modules each year
L
D decreased number of spacecraft each year
S
NOTE Data source: CSpOC, 2025/07/01.
Figure 10 — Number of objects catalogued or decayed each year

ISO/DTR 25775:2026(en)
6 Purpose, rationale and feasibility of requirements
6.1 Mission design
6.1.1 Orbit selection of large constellation and maintenance to minimize collision risk
6.1.1.1 Selection of orbit of large constellation considering collision risk with other space objects
ISO/TS 6434:2024, 5.1.1.1 requires the selection of mission orbit in order to minimize the probability of
collision with other space objects. The more hazardous collision with the neighbouring large constellations
is mentioned in ISO/TS 6434:2024, 5.1.1.2, particularly. Another hazardous intra-constellation collision is
mentioned in ISO/TS 6434:2024, 5.1.2.
For general spacecraft (such as Earth observation spacecraft), the orbital characteristics are almost
fixed according to the mission requirements, and the degree of freedom in orbit selection is low. Large
constellation program owners are therefore expected to respect their existence.
6.1.1.2 Selection of orbit of large constellation considering collision risk with neighbouring large
constellations
6.1.1.2.1 General
ISO/TS 6434:2024, 5.1.1.2 requires that constellations be designed and operated to avoid interaction
with neighbouring large constellations to assure safe operations under both nominal and anomalous
vehicle operating conditions, and notes that maintaining an appropriate radial distance from other large
constellations is an effective way to minimize collision risks.
Here, radial separation, though not called out as an explicit requirement, is a very effective way to minimize
collision risk between neighbouring large constellations.
Figure 11 illustrates the rate of close encounter one should expect for a new spacecraft placed in a circular
orbit at the specified altitude and inclination. The entire public catalogue is used as the background
population. The encounter rate is a strong function of inclination and altitude. As such, peak encounter rates
can be observed for the new spacecraft in a polar orbit (i.e., 90 degrees inclination), even though the existing
resident space object catalogue has orbits in lower inclination orbits, since the higher inclination orbit can
conjunct with all objects in lower inclination orbits which share its altitude.

ISO/DTR 25775:2026(en)
Key
A is altitude [km]
i is inclination [degree]
Ea is annual encounter rate with catalogued objects (number of encounters approaching shorter than 1 km)
NOTE The figure is based on the catalogued objects registered on 25 March 2025.
Figure 11 — Spacecraft encounter rates vs inclination and altitude
Certainly, other operational solutions can also be effective, including:
— assessments of cumulative collision probability to demonstrate that risk is controlled;
— ensuring the close and reliable cooperation between neighbouring operators of the individual
constellations;
— careful interleaving of two or more coexisting constellations to limit collision risk.
However, considering the congested region mentioned in 5.5, it can be difficult to ensure the safety of the
operation with keeping multiple large constellations in same radius of orbit only by the operational efforts,
without radial separation.
As explained in 5.5, the risk of inter-constellation collisions is of particular concern in the following three
areas:
a) Collision in the polar congested region among polar orbital constellations with different orbital planes
at the same altitude.
b) Collision in the Northern and Southern densely clouded belts among constellations with adjacent orbital
planes, at the same altitude and inclination.
c) Collisions in the mid-latitude congested area among intersections where constellations in adjacent
orbital planes at the same altitude. Also, the intersection of the congested region in the altitude
direction of large constellations and the congested orbital planes of SSO is an unavoidable danger zone
and requires special management.

ISO/DTR 25775:2026(en)
These collision avoidance measures are coordinated by the relevant constellation operators and STC service
providers. However, collision avoidance manoeuvres can result in mission interruptions (or additional
work to avoid service interruptions), losses such as propellant consumption, and potential new collisions
at the collision avoidance point in congested areas. Preventing collisions in dangerous areas is the most
desirable approach. While possible solutions to reduce the probability of collision include mean radial
separation, radial separation at eccentricity-based epochs, orbital inclination adjustments, and orbital plane
adjustments, radial separation is the most basic approach.
Countries or companies planning to deploy large constellations in the future can argue that using radial
separation as the sole criterion would introduce a first-come, first-served approach, potentially inconsistent
with the free access to space guaranteed by the Outer Space Treaty. However, the key issue here is preventing
large-scale fragmentation that can trigger Kessler Syndrome. The monopolization of vast swaths of space by
large constellations for the benefit of a few companies in a few countries poses a significant threat to world
space users, who feel that free access to space is already being restricted by large constellation operators.
The increased threat posed by additional constellations can push the Kessler Syndrome beyond its critical
point. ISO/TS 6434:2024 exists to help prevent this.
As explained in 5.5, to reduce the probability of collision with other constellations and general spacecraft, the
optimal selection should be made, considering all intersections in the polar congested regions, the Northern
and Southern densely clouded belts and the mid-latitude congested area. including radial separation, orbital
inclination, eccentricity, orbital plane, and coordination among operators of relating constellations.
Among these, radial separation is probably the most basic measure. Considering all possible factors, such as
loss of function due to unexpected deterioration in quality, errors in workmanship during manufacturing,
space weather, or other unforeseen circumstances, such as the failure of operators to make appropriate
adjustments or take appropriate action in the case of large constellations that do not inherently have
collision avoidance capabilities, radial separation is one of the safest and most reliable methods.
6.1.1.2.2 Abnormal situation
As long as we cannot deny that there are potential errors of the automatic collision avoidance system,
defects in the SSA system, or workmanship errors of the coordination system, collisions can occur. Collisions
can occur by human error regardless of manual operations or automated operations for collision avoidance.
Operating a large constellation becomes more reliable through automation, but is imperfect due to potential
uncooperative objects.
Concerning the addition of new constellations, the passage of many disposed spacecraft, and securing
transportation of additional spacecraft and carrier vehicles, greater burden will lie with operators in the
future. The separation of altitude must be ensured in order to reduce the risk of collision.
6.1.1.2.3 Situations in other frameworks
Requiring a large radial separation distance limits the number of constellations in congested altitude
zones. Some constellation program owners will possibly not welcome the introduction of such restrictions.
However, if necessary for safe space utilization, they will consider it.
The radial separation is mentioned favourably in both the IADC statement and The French Space Operations
Act (FSOA) Technical Regulation.
The IADC statement recommends radial separation as follows:
a) It is recommended that sufficient radial separation be considered between all parts of the large
constellation to minimize collision risks between large constellation members.
b) Radial separation at the intersection between the orbital planes of the large constellations has been
shown to have a positive impact on the environment if it is large enough to avoid interplane mergers.
c) To minimize the collision risk, it is recommended to consider sufficient radial separation with respect
to other large constellations, frequently used orbits, and orbits with a high concentration of other space
debris.
ISO/DTR 25775:2026(en)
[13]
The IADC Debris Mitigation Guidelines, issued in January 2025, state that sufficient separation distances
are recommended and provide simpler guidelines on separation distances, as follows:
1) spacing between all components of a large constellation to minimize potential collision risks between
members of the large constellation;
2) intersections of orbital planes to avoid close contact between the orbital planes of the large constellation;
3) with respect to other large constellations and spacecraft in highly utilized orbits and other densely
populated orbits to minimize potential collision risks.
FSOA Technical Regulation also requires considering the risk of interference between adjacent large
constellations by either guaranteeing adequate radial separation between these large constellations, or if
not possible, by demonstrating robustness regarding the risk of collision between both large constellation
spacecraft.
While it has been acknowledged that ensuring a radial separation is an effective measure to minimize
collision risks between large constellations, some more measures can be considered to mitigate the risks
associated with adjacent large constellations. For instance, considerations of cumulative probability
of collision can also demonstrate the risk is controlled, or a coordination with the operator of the other
constellation can be sufficient in some specific cases.
Another aspect is that eccentricities are generated unintentionally in real operations.
The overlap among several constellation programs can be observed at altitudes of 350 km to 570 km among
Aleph, Kepler, Kuiper and Flock. Most Flocks have only differential drag manoeuvrability. We can also see
that their mean altitudes are significantly different. Even if each constellation was planned at a separate
altitude, coarse altitude control can cause interference due to variations in mean altitude and eccentricity.
Taking these factors into consideration, planning for sufficient orbital separation from the outset is
important.
6.1.2 Intra-constellation collision avoidance
6.1.2.1 Allocation of orbit of member spacecraft to minimize collision risk
ISO/TS 6434:2024, 5.1.2.1, as mentioned in 5.5, outlines that there are concerns about collisions within the
constellation, besides the cross points in the mid-latitude congested area, in more critical area, the North
and South polar congested region and in the Northern and Southern densely clouded belts.
These can be avoided by allocating orbital planes, separating altitude, or adding eccentricity. Intersections
in the mid-latitudes are also an area to be aware of. An automatic collision avoidance system is being
implemented for the operation of the constellation.
When commissioning the launch of additional spacecraft to a constellation:
a) conditions must be met to avoid immediate entry into congested areas in the North and South polar
congested region or Northern and Southern densely clouded belts after launch;
b) the orbital plane must be accurately specified (appropriate orbital plane placement, specification of an
orbital plane that can be penetrated);
c) orbital conditions must be shared between the constellation and the launch vehicle;
d) when a launch vehicle ascending beyond the corridors of existing constellations and at higher altitudes,
it is practically difficult for launch vehicle operators to obtain high-precision orbital information in real
time, so the optimal transit timing must be coordinated with the large constellations.

ISO/DTR 25775:2026(en)
6.1.2.2 Allocation of orbit considering the contingencies
[1]
ISO/TS 6434:2024 ,5.1.2.2 requires that constellations shall be configured such that constituent failures
do not significantly elevate intra-constellation collision risk (e.g. by separating the orbit planes and radial
profiles to avoid intersection points).
To expect short orbital lifetime for spacecraft and fragments, lower operation altitude may be selected.
6.1.2.3 Orbit control of spacecraft during operation
According to ISO/TS 6434:2024, 5.1.2.3, when adding member spacecraft to a large constellation or deciding
on their operation and disposal orbits, they must be selected by considering the collision risk and the orbital
life after the end of operation, depending on the mission objectives and constraints.
Potentially dangerous moments are not only orbit change manoeuvres, but also the addition of many
spacecraft to an existing spacecraft fleet. Orbit change manoeuvres and ingesting additional spacecraft into
a constellation pose additional risks that should be mitigated by taking into account collision avoidance,
disposal operations, and orbital lifetime.
6.1.3 Space debris mitigation
ISO/TS 6434:2024, 5.1.3 and 5.2.3 require satisfying the space debris mitigation requirements in
ISO 24113:2023 as a set of standardized requirements, but add stricter specific requirements for collision
avoidance, disposal, and passivation, etc.
The specific (and additional) requirements for large constellations are given in the following standards:
a) A detailed set of collision avoidance procedures is given in ISO/TS 6434:2024, 5.2.2, and 5.4.2.
b) Post-mission orbital lifetime are required to be controlled to be within the same duration as designed to
be operational, up to a maximum of five years as specified in ISO/TS 6434:2024, 5.5.1 and 5.2.1.4 a).
c) Keeping collision avoidance operation is required even after disposal manoeuvre until re-entry or as
late as technically feasible as specified in ISO/TS 6434:2024, 5.5.4. Passivation is conducted at the end of
the collision avoidance operation, if needed, as specified in ISO/TS 6434:2024, 5.5.5.
Starlink spacecraft are propulsively deorbited within weeks of their end-of-mission life. For this
operation, enough propellant is reserved to deorbit from their operational altitude. Once the spacecraft
reach an appropriate altitude, they coordinate with the 18th Space Control Squadron. Once coordinated,
they initiate a high drag mode, causing the
...

ISO/DTR 25775:2025(E)
ISO /TC 20/SC 14/WG 3
Secretariat: ANSI
Date: 2026-01-2104-09
Space systems — — Justification of requirements for a spacecraft
large constellation [Technical Report]

Warning for WDs and CDs
This document is not an ISO TR. It is distributed for review and comment. It is subject to change without notice and
may not be referred to as an International Standard.
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.

ISO/DTR 25775:2025(E)
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.orgwww.iso.org
Published in Switzerland
© ISO 25775 – All rights reserved iii

ISO/DTR 25775:2025(E)
Contents
Foreword . v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 1
4.1 Symbols . 1
4.2 Abbreviated terms . 2
5 Orbital environment occupied with active spacecraft and catalogued debris . 3
5.1 Launch history . 3
5.2 Accumulation of manmade objects trackable from the ground . 6
5.3 Launch history and plan of low Earth orbit (LEO) large constellations . 7
5.4 High spatial density caused by large constellations which is hardly prospected even
a few years before . 11
5.5 The threat of collision relating to large constellations . 13
5.6 Trend of generation of orbital objects and their decay . 19
6 Purpose, rationale and feasibility of requirements . 20
6.1 Mission design . 20
6.2 Spacecraft design . 27
Bibliography . 44

iv © ISO 25775 – All rights reserved

ISO/DTR 25775:2025(E)
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).
Field Code Changed
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.
Field Code Changed
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.
© ISO 25775 – All rights reserved v

ISO/DTR 25775:2025(E)
Introduction
More than a dozen large constellation programs are planned to be launched in the next several years.
While large constellations can provide societal benefits to humanity, they can also put pressure on orbital
and electro--magnetic environments, introducing challenges to mission design, hardware design, launch,
operations and disposal actions and the long-term sustainability of space activities.

Corresponding to the above situation, to ensure the long-term sustainability of space operations, ISO/TS
[1]
6434:2024 was published in January 2024, providing a set of standard practices based on the voluntary
practices applied in the existing world major large constellation programs. It also reflects the guidelines
developed by academic organizations.

This document aims to explainexplains the necessity to comply withconform to the requirements of
[1]
ISO/TS 6434:2024 ,, and the technical basis of the requirements.

vi © ISO 25775 – All rights reserved

ISO/DTR 25775:2025(E:(en)
© ISO 25775 2026 – All rights reserved
vii
ISO/DTR 25775:(en)
Space systems — — Justification of requirements for a spacecraft large
constellation
1 Scope
[1]
This document provides the justification for the requirements of ISO/TS 6434:2024 . . It also explains the
growth of future large constellation programs, the increasing collision risk, and the technical rationale for the
requirements.
This document is intended to support owners of large constellations and manufacturers or operators acting
under their responsibility.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
[1]
ISO/TS 6434:2024 , 3.1 defines a "large constellation." That is, a For the purposes of this document, the
following terms and definitions apply.
large constellation of 100 or more spacecraft currently active and working together.
[1]
As the note in ISO/TS 6434:2024 , 3.1 clearly states that "providing a specific number of spacecraft, the size,
mass, complexity and functionality of the spacecraft are also relevant," there are several cases that are
excluded from the scope of a large constellation even if it is composed of more than 100 vehicles.
Other than above mentioned terms, no terms and definitions are listed in this document.
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
large constellation of spacecraft
large constellation
system of a hundred or more spacecraft working together
Note 1 to entry: While providing a specific quantity of spacecraft, the size, mass, complexity and function of the spacecraft
also have a bearing. There are several cases that are excluded from the scope of a large constellation even if it is composed
of more than 100 vehicles.
[SOURCE: ISO/TS 6434:2024, 3.1, modified — Last sentence added to Note 1 to entry.]
4 Symbols and Abbreviatedabbreviated terms
4.1 Symbols
Ec expected number of casualties.
For the purposes of this document, the following symbols apply.
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Ec expected number of casualties
4.2 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.
API advanced publication of information
AT acceptance test
CAM collision avoidance manoeuvre
CARA conjunction assessment and risk analysis
CCSDS Consultative Committee for Space Data System
CSpOC Combined Space Operations Center, United States Space Forces – Space.
C/O check-out
DoA dead on arrival
DoD US Department of Defense
EoM end of mission
FSOA French Space Operations Act
IADC Inter-Agency Space Debris Coordination Committee
ITU international telecommunication union
LST local sun time
LTS long-term sustainability of outer space activities
NASA National Aeronautics and Space Administration
NIST National Institute of Standards and Technology, US Department of Commerce
NMI NASA Management Instruction
PMD post mission disposal
PSD probability of successful disposal
QT qualification test
RB radiocommunication bureau (in ITU)
RF radio frequency
SAA space act agreement
SSA space situational awareness
TT&C telemetry, tracking and command
UN United Nations
V&V verification and validation
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
5 Orbital environment occupied with active spacecraft and catalogedcatalogued
debris
5.1 Launch history
Figure 1Figure 1 shows the number of spacecraft launched into EarthEarth's orbit by the end of 2019.
Figure 2Figure 2 shows the number of spacecraft launched by the end of 2025.
From those two figures, it is observed that the number of nano- and pico-spacecraft began to increase form
2013, and the large number (larger than several thousands) of spacecraft has begun to be launched due to
several large constellation programs from 2020.
(Note: NOTE The method of classification of spacecraft mass is provided by the Seradata database. There is no
intention to claim this method to apply widely.).

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
Y launch year
L
N number of spacecraft launched in a year
ML large spacecraft (>2 500 kg)
MMed medium spacecraft (from 1 000 kg to 2 500 kg)
MS small spacecraft (from 500 kg to 1 000 kg)
M mini spacecraft (from 100 kg to 500 kg)
min
Figure 1— Number of spacecraft launched into the EartheEarth's orbit by the end of 2019 [
1)
[SOURCE: Reproduced with permission from the Seradata database @, 2026/1/5]

1)
Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
YL launch year
N number of spacecraft that willlaunched in a year
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
ML large spacecraft (>2 500 kg)
MMed medium spacecraft (from 1 000 kg to 2 500 kg)
MS small spacecraft (from 500 kg to 1 000 kg)
Mmin mini spacecraft (from 100 kg to 500 kg)
Figure 2— Number of spacecraft to be launched by the end of 2025.
2)
[[SOURCE: Reproduced with permission from the Seradata database @, 2026/1/5]

5.2 Accumulation of manmade objects trackable from the ground
From Figure 3Figure 3,, the total number of catalogued objects will reach more than 32 000 at the end of 2025,
and the number of spacecraft will surpass the number of catalogued fragments.

2)
Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
Yl launch year
Na number of catalogued objects accumulated in the earth orbit each year
OD number of fragments and released objects accumulated in the earth orbit each year
OS number of spacecraft accumulated in the earth orbit each year
O number of launch vehicle orbital stages including assist modules accumulated in the earth orbit each year
L
NOTE Data source: CSpOC, 2026/01/05.
Figure 3— Number of accumulated catalogedcatalogued objects excluding decayed objects each year
[Ref: CSPOC @2026-01-05]
5.3 Launch history and plan of low Earth orbit (LEO Large) large constellations
Figure 4Figure-4 shows the launch schedule up to 2031. The tendency of the large constellation programs
isdoes not end atin 2025, and even larger constellation programs isare expected to follow until 2031, and
perhaps longer.
Figure 5Figure 5 shows the number of catalogedcatalogued spacecraft that are in the LEO at 2025/07/04 and
others that will be launched by 2031/12/31 according to their operation altitude.
The most congested altitude range is from 450 –km to 650 km.
Anther critical situation will occur in the range from 600 km to 1100 1 100 km, where an extremely large
constellation that has large eccentricity covering such altitude range. (In Figure 5Figure – 5,, the number of
spacecraft of large eccentricity orbit is expressed with the number that is allocated to each altitude band
according to their time occupation rate in each band.). Historically, the altitude range is used for Earth
observation missions, alsoand some constellation programs are being planned. These large constellations with
large orbital eccentricities pose a greater risk due to frequent collisions with other constellations, large
collision angles and high collision velocities resulting in large collision energies, and the difficulty of early
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
predictions of collisions. Furthermore, frequent collision avoidance operations hinder stable mission
operations.
Also, even for the spacecraft operated in circular orbit, if they are disposed in high eccentricity disposal orbit
with setting their perigee to very low altitude, the disposal orbit will cause high frequent conjunctions with
many operation corridors of other large constellation groups. Therefore, it is expected that a disposal strategy
will be set appropriately, as described in 6.2.1.4Section 6.2.1.4.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
Y launch year
L
N number of spacecraft of large constellations
GQ G60 Qianfan (Thousand Sails)
KP Kuiper (Amazon)
SL Starlink
G GuoWang (China SatNet)
W
O OneWeb
W
Ot Other large constellations
NOTE shows the number of spacecraft of large constellations launched by the 22nd of August 2025 and planned to be
launched until the end of 2031 as far as cleared by MOU/LOI/Announcement as of 2025-/08-/22 [Ref.
Figure 4— Number of spacecraft of large constellations planned to be launched by the end of 2031
3)
[SOURCE: Reproduced with permission from the Seradata database @2025-08-, 2026/8/22]

3)
Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
N is number of spacecraft
AL is altitude of large constellations
G is G60 Qianfan (Thousand Sails)
Q
K is Kuiper (Amazon)
P
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
SL is Starlink
GW is GuoWang (China SatNet)
OW is OneWeb
Ot is Other large constellations
NOTE The number of GuoWang program spacecraft (total 6495 spacecraft), which has large eccentricity (from
perigee 600 m to apogee 1 100 km), are allocated to each altitude band according to the rate of time occupation per
altitude band calculated by the orbit analysis.
Figure 5— Distribution of spacecraft along their altitude as a snapshot on 2025-07-04, and that of the
constellation spacecraft to be launched by the end of 2031.
4)
[CSPOC @20250704,[SOURCE: Reproduced with permission from CSpOC and the Seradata database
@20250704, 2025/07/04]
5.4 High spatial density caused by large constellations which is hardly prospected even a
few years before
Figure 6Figure 6 shows the quasi-spatial density along a 10 km altitude band for the end of 2016, July 2025,
and end of 2031 .
The proper spatial density value is calculated considering the allocated time along the altitude band where
each object interferes, considering eccentricity. In the quasi-spatial density in Figure 6Figure 6,, objects in
highly eccentric orbits with a mean altitude of more than 2000 2 000 km are ignored, so the quasi-spatial
density is smaller than the actual value. But since the majority of large constellation spacecraft are circular
orbits, they are unlikely to cause any major problems. As shown in Figures 6Figures 6,, the range between
450 km and 650 km altitude is very congested.

ESAThe European Space Agency has declared tothey will revise MASTER annually sincefrom 2025, but the
large constellation programs will not be announced timely, so that it is almost impossibledifficult to
analyzeanalyse the collision probability during the operation phase in the planning or design phase.
Also, ifIf the new MASTER will ignoredoes not take into account the existence of the active spacecraft because
of lesslower impact probability, due to their collision avoidance capability, peopleit will not be ablepossible to
prospect conjunction probability, which will need to estimate propellants for avoidance operations.

The worldInternational academic committees and UNCOPUOSthe United Nations Committee on the Peaceful
Uses of Outer Space have prospected these situations and started discussion fordiscussions regarding the
long-term sustainability of space activities beyond the debris issue in the early 2000s. The main theme tends
to be theis collision avoidance with worldwide coordination for the surveillance of active orbital assets,
detection of the conjunction, data sharing of ephemeris of active assets.

4)
Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
A  is average orbital altitude (higher value of altitude band）[), km]
SD  is quasi-spatial density of catalogued objects whose average altitude exist 10 km altitude band [, 1/km ]
D1  is the quasi-spatial density of catalogued objects that exist in 10 km altitude band in 2016 [, 1/km ]
D2  is the quasi-spatial density in 2025 [, 1/km ]
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
D3  is the quasi-spatial density in 2031 [, 1/km ]

NOTE 1  Quasi-spatial density (QSD) is the number of catalogedcatalogued objects whose average altitude exists
10 km altitude band (N)") per "spatial volume of the shell for altitude 10 km band (V)".  ) (QSD = N/V).
NOTE 2  The number of spacecraft of large eccentricity orbit is expressed with the number that is allocated to each
altitude band according to their time occupation rate in each band.
NOTE 3 In Figure 6, in the case of 2031, the number of orbital stages and fragments potentially generated after 2025
July are ignored.
Figure 6— Quasi-spatial density calculated from average altitude of catalogued objects for every
10 km altitude band for calendar year of 2016, 2025 and 2031
[Sources: Existing objects[SOURCE: Reproduced with permission from CSpOC/SSR and prospected data fromthe
5)
Seradata database @, 2025/07/23] (In the case of 2031, number of orbital stages and fragments, potentially
generated after 2025 July are ignored.),

5.5 The threat of collision relating to large constellations
In general, there are no threats of collision between multiple spacecraft orbiting at the same altitude in a
unified orbital plane. Spacecraft in different orbital planes will not collide if they are in separate altitude.

In the case of a constellation, the orbital planes are distributed and arranged to maximize the coverage of the
Earth's surface and ensure uniformity of communications or observations. Currently, in the orbital turning
areas at the northernmost and southernmost ends of the orbit, where spacecraft fly in an east-west direction
and form densely clouded belts, (referred to as “Northern/ and Southern Densely Clouded Belts” heredensely
clouded belts in this document). In polar orbits, a “North /and South Polar Congested Regions”polar congested
regions are formed (see Figure 7.). To reduce the probability of collision, it is also consideredpossible to
separate the altitude (or give eccentricity). (See Figure-7)

Even in the mid-latitude bands, other than the “Northern/ and Southern Densely Clouded Belts”, there is so-
called
densely clouded belts, all spacecraft in the constellation have a threat of collision at all intersections of the
mid-latitude region, so it is believed that collision management within the intra-constellation is performed by
strictly managing the passing timing, and each spacecraft controls its orbit at every moment.

Furthermore, there is the issue of congested regions in the sun-synchronous orbits (SSO). In SSO, the orbital
plane rotates in synchronous with the Earth's orbital period, so the angle of incidence of the sun on the orbital
plane remains constant throughout the year. For this reason, it is the "most orthodox special orbit" frequently
used by observation spacecraft. Especially, Local Sun Times (LSTs) of 6:00, 10:30, 13:30, and 18:00 are

5)
Seradata is the trade name or trademark of a product supplied by Slingshot Aerospace. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of the product named.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
frequently used. As a result, the orbital planes corresponding to these popular LSTs are congested with SSO
spacecraft.
The intersection between “the congested region in the altitude direction of large constellations” and “the
congested orbital planes of SSO” is ana danger zone.

[1]
As one of available solutions, ISO/TS 6434:2023 ,2024, 5.1.1.2 shows a NOTE to introduceintroduces the
[1]
concept of radial separation, defined in ISO/TS 6434:2023 ,2024, 3.3. Since there is no way to resolve these
intersections, it is recognized the importance to setof international space traffic rules to ensure safe traffic and
establish the rules to separate operation altitude or othersis recognized. In addition to the above potential
threats, there is the problem of interference with other constellations with high eccentricities and interference
with spacecraft disposed into high eccentric orbits. Objects on highly eccentric orbits tend to reach higher
velocities near perigee, causing greater fragmentation in the event of a collision, and the conjunction warnings
tend to be delayed. There is also the issue of the invasion of spacecraft and launch vehicles that will be added
to the constellation. Currently large constellations are equipped with autonomous collision avoidance systems
(automatic avoidance AI) to avoid objects including uncooperative objects (dead spacecraft, disposed
spacecraft, and spacecraft without avoidance functions). However, since the only means of avoiding collision
with uncooperative objects areis on the active spacecraft side, thean excessive burdens will be concentrated
onshare of the responsibility lies with the operators of active constellation spacecraft.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Figure 7— North / /South Polarpolar congested regions and densely clouded belts, etc.
Figure 8[Ref. modified from “Space Traffic Management”, Mr. Hisayuki Mukae, 2020-09-04 ]

Figure-8 shows the inclination of large constellations along the periods of 87-110 minutes to 110 minutes
(roughly at altitudes of 130- km to 1200 km), and STARLINK and Kuiper have almost the same period of
95 minutes, with the difference in orbital inclination within 1 degree. The figure also shows the proximity of
STARLINK to Jilin 1 and Hawkeye 360.
Furthermore, Figure 9Figure-9 shows an enlarged polar dense area, and in the narrow range from 97.,2 to
97.,7 degrees in north latitude, multiple constellations with almost the same period and altitude are densely
packed together, raising concerns about collisions. It also appears to be preventing ordinary polar-orbiting
spacecraft from entering at an altitude of around 500 km. It is desirable to ensure that a small number of
constellation operators do not restrict fair access to the greatest number of space users in the world. km.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
P is orbital period (min)
I is inclination (degree)
Ge is Geesat
Hw is Hawkeye 360
J1 is Jilin 1
KP is Kuiper (Amazon)
LM is Lemur
OW is OneWeb
P is Planet Constellations
C
S is Starlink
L
Ot is Other Chinees large constellations
NOTE Data source: US CSpOC/Element Set (ELSET) Data /Low Earth Orbit (LEO) @20250704.
Figure 8 — Interference among large constellations, in the northern congested region, presented by
orbital period and inclination, below altitudes of 1200 1 200 km (period: 110 minutes)
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
[Sources: Orbital spacecraft registered in

Key
P is orbital period (min)
I is inclination (degree)
Hw is Hawkeye 360
J is Jilin 1
LM is Lemur
PC is Planet Constellations
SL is Starlink
NOTE Data source: US CSpOC/Element Set (ELSET) Data/Bulk Download Alternative/Current Catalog Files /Low
Earth Orbit (LEO)/3LE) @20250704].

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Figure 9— Key
P is orbital period (min)
I is inclination (degree)
Hw is Hawkeye 360
J is Jilin 1
LM is Lemur
PC is Planet Constellations
S is Starlink
L
Large constellations around north pole congested region expressed with orbital period and
inclination
[Sources: Orbital spacecraft registered in CSpOC/Element Set (ELSET) Data/Bulk Download
Alternative/Current Catalog Files/Low Earth Orbit (LEO)/3LE @20250704]

In the future, the need for the following measures willare likely to be discussed.:
a) Promotion of traffic management that employs different orbital altitudes for each orbital plane.
— Information sharing among constellation operators who are operating polar orbits.
— Allocation of orbital altitudes for each orbital plane.
— Sharing information of specific events such as disposal or new launch and injection into constellations.
— Coordination among operators and STC coordinators to avoid collisions.
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
b) Control of the timing of polar passes.
— Coordination among operators for the timing of their polar passes to avoid collisions.

5.6 Trend of generation of orbital objects and their decay
Figure 10Figure-10 shows the trend in the number of generated orbital objects (including new launch vehicle
orbital stages, fragments and released objects) and number of re-entries. The recent increase in the number
of re-entries since 2022 is temporary due to the Crona Mass Ejections, but it is easy to imaginepossible that
the number of re-entries will increase in the future according to the disposal of the large constellation
spacecraft. The safety against the threat of reenteringre-entering objects is another issue relating to large
constellations.
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
YL calendar year
NC number of catalogued objects each year
N number of decayed objects each year
D
C catalogued number of fragments and released objects each year
D
CS catalogued number of spacecraft each year
CL catalogued number of launch vehicle orbital stages and assist modules each year
DL decreased number of launch vehicle orbital stages and assist modules each year
DS decreased number of spacecraft each year
NOTE Data source: CSpOC, 2025/07/01.
Figure 10— Number of objects catalogedcatalogued or decayed each year
[Ref: CSPOC @2026-01-05]
6 Purpose, Rationalerationale and feasibility of requirements
6.1 Mission design
6.1.1 Orbit selection of large constellation and maintenance to minimize collision risk
6.1.1.1 Selection of orbit of large constellation considering collision risk with other space   objects
[1]
(See ISO/TS 6434:2024 , 5.1.1.1)
[1]
ISO/TS 6434 ,:2024, 5.1.1.1 requires selectingthe selection of mission orbit in order to minimize the
probability of collision with other space objects. The more hazardous collision with the
[1]
neighboringneighbouring large constellations is mentioned in ISO/TS 6434 ,:2024, 5.1.1.2, particularly.
[1]
Another hazardous intra-constellation collision is mentioned in ISO/TS 6434 ,:2024, 5.1.2.
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
For general spacecraft (such as Earth observation spacecraft), the orbital characteristics are almost fixed
according to the mission requirements, and the degree of freedom in orbit selection is low. Large constellation
program owners are therefore expected to respect their existence.

6.1.1.2 Selection of orbit of large constellation considering collision risk with
neighboringneighbouring large constellations (See
6.1.1.2.1 General
[1]
6.1.1.2 ISO/TS 6434:2024 , 5.1.1.2)
6.1.1.2.1 General
[1]
ISO/TS 6434:2024 ,, 5.1.1.2 requires that constellations be designed and operated to avoid interaction with
neighboringneighbouring large constellations to assure safe operations under both nominal and anomalous
vehicle operating conditions, and notes that "maintaining an appropriate radial distance from other large
constellations is an effective way to minimize collision risks." .

Here, radial separation, though not called out as an explicit requirement, is a very effective way to minimize
collision risk between neighboringneighbouring large constellations.
Figure 11Figure 11 illustrates the rate of close encounter one should expect for a new spacecraft placed in a
circular orbit at the specified altitude and inclination. The entire public catalogcatalogue is used as the
background population. The encounter rate is a strong function of inclination and altitude. As such, peak
encounter rates maycan be observed for the new spacecraft in a polar orbit (i.e., 90 degrees inclination)), even
though the existing resident space object catalogcatalogue has orbits in lower inclination orbits, since the
higher inclination orbit can conjunct with all objects in lower inclination orbits which share its altitude.

© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Key
A  is altitude [km]
i   is inclination [degree]
Ea is annual encounter rate with catalogedcatalogued objects (number of encounters approaching shorter than 1 km)
NOTE The figure is based on the catalogedcatalogued objects registered on 25 March 2025.
Figure 11— Spacecraft encounter rates vs inclination and altitude

Certainly, other operational solutions maycan also be effective, including (1):
— assessments of cumulative collision probability to demonstrate that risk is controlled, (2);
— ensuring the close and reliable cooperation between neighboringneighbouring operators of the individual
constellations, or (3);
— careful interleaving of two or more coexisting constellations to limit collision risk.
However, considering the congested region mentioned in 5.5sub-clause 5.5,, it maycan be difficult to ensure
the safety of the operation with keeping the multiple large constellations in same radius of orbit only by the
operational efforts, without radial separation.

As explained in 5.5sub-clause 5.5,, the risk of inter-constellation collisions is of particular concern in the
following three areas:
a) Collision in the “Polar Congested Region”polar congested region among polar orbital constellations with
different orbital planes at the same altitude.
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
b) Collision in the “Northern /and Southern densely clouded Belts”belts among constellations with adjacent
orbital planes, at the same altitude and inclination, and.
c) Collisions in the "Midmid-latitude congested area” among intersections where constellations in adjacent
orbital planes at the same altitude. Also, the intersection of the congested region in the altitude direction
of large constellations and the congested orbital planes of SSO is an unavoidable danger zone and requires
special management.
These collision avoidance measures are coordinated by the relevant constellation operators and STC service
providers. However, collision avoidance maneuversmanoeuvres can result in mission interruptions (or
additional work to avoid service interruptions), losses such as propellant consumption, and potential new
collisions at the collision avoidance point in congested areas. Preventing collisions in dangerous areas is the
most desirable approach. While possible solutions to reduce the probability of collision include mean radial
separation, radial separation at eccentricity-based epochs, orbital inclination adjustments, and orbital plane
adjustments, radial separation is perhaps the most basic approach.

Countries or companies planning to deploy large constellations in the future maycan argue that using radial
separation as the sole criterion would introduce a first-come, first-served approach, potentially inconsistent
with the free access to space guaranteed by the Outer Space Treaty. However, the key issue here is preventing
large-scale fragmentation that couldcan trigger Kessler Syndrome. The monopolization of vast swaths of space
by large constellations for the benefit of a few companies in a few countries poses a significant threat to world
space users, who feel that free access to space is already being restricted by large constellation operators. The
increased threat posed by additional constellations couldcan push the Kessler Syndrome beyond its critical
[1]
point. ISO/TS 6434:2024 exists to help prevent this.
As explained in 5.5 sub-clause 5.5,, to reduce the probability of collision with other constellations and general
spacecraft, the optimal selection should be made, considering all intersections in the “Polar Congested
Regions”,polar congested regions, the “Northern /and Southern densely clouded Belts”belts and the “Midmid-
latitude congested area”. including radial separation, orbital inclination, eccentricity, orbital plane, and
coordination among operators of relating constellations.
Among these, radial separation is probably the most basic measure. Considering all possible factors, such as
loss of function due to unexpected deterioration in quality, errors in workmanship during manufacturing,
space weather, or other unforeseen circumstances, such as the failure of operators to make appropriate
adjustments or take appropriate action in the case of large constellations that do not inherently have collision
avoidance capabilities, radial separation is one of the safest and most reliable methods.

6.1.1.2.2 Abnormal situation
As long as we cannot deny that there are potential errors of the automatic collision avoidance system, defects
in the SSA system, or workmanship errors of the coordination system, collisions can occur. Collisions can occur
by human error regardless of manual operations or automated operations for collision avoidance. Operating
a large constellation becomes more reliantreliable through automation, but nobody can say that it will be
perfect in front of suchis imperfect due to potential uncooperative objects.
Concerning the addition of new constellations, the passage of many disposed spacecraft, and securing
transportation of additional spacecraft and carrier vehicles, greater burden will pose onlie with operators in
the future. It will be essential to ensure, at least, theThe separation of altitude must be ensured in order to
reduce the risk of collision.
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
6.1.1.2.3 Situations in other frameworks
Requiring a large radial separation distance would limitlimits the number of constellations in congested
altitude zones. Some constellation program owners will possibly not welcome the introduction of such
restrictions. However, if necessary for safe space utilization, they will consider it.
The radial separation is mentioned favorablyfavourably in both the IADC statement and The French Space
Operations Act (FSOA) Technical Regulation.

The IADC statement recommends radial separation as follows:
a) It is recommended that sufficient radial separation be considered between all parts of the large
constellation to minimize collision risks between large constellation members.
b) Radial separation at the intersection between the orbital planes of the large constellations has been shown
to have a positive impact on the environment if it is large enough to avoid interplane mergers.
c) To minimize the collision risk, it is recommended to consider sufficient radial separation with respect to
other large constellations, frequently used orbits, and orbits with a high concentration of other space
debris.
[ [13] ]
The IADC Debris Mitigation Guidelines , , version 4, issued in January 2025, state that "sufficient separation
distances are recommended" and provide simpler guidelines on separation distances, as follows:
1) Spacingspacing between all components of a large constellation to minimize potential collision risks
between members of the large constellation,;
2) Intersectionsintersections of orbital planes to avoid close contact between the orbital planes of the large
constellation,;
3) Withwith respect to other large constellations and spacecraft in highly utilized orbits and other densely
populated orbits to minimize potential collision risks.
The French Space Operations Act (FSOA)FSOA Technical Regulation also requires considering the risk of
interference between adjacent large constellations by either guaranteeing adequate radial separation
between these large constellations, or if not possible, by demonstrating robustness regarding the risk of
collision between both large constellation spacecraft.
While it has been acknowledged that ensuring a radial separation is an effective measure to minimize collision
risks between large constellations, some more measures couldcan be considered to mitigate the risks
associated with adjacent large constellations. For instance, considerations of cumulative probability of
collision couldcan also demonstrate the risk is controlled, or a coordination with the operator of the other
constellation maycan be sufficient in some specific cases.
Another aspect is that eccentricities are generated unintentionally in real operations.
The overlap among several constellation programs can be observed at altitudes of 350- km to 570 km among
Aleph, Kepler, Kuiper and Flock. (Note that mostMost Flocks have only differential drag
maneuverability.)manoeuvrability. We can also see that their mean altitudes are significantly different. Even
if each constellation was planned at a separate altitude, coarse altitude control couldcan cause interference
due to variations in mean altitude and eccentricity.
© ISO #### 2026 – All rights reserved
ISO/DTR 25775:(en)
Taking these factors into consideration, planning for sufficient orbital separation from the outset is important.

[1]
6.1.2 Intra-constellation collision avoidance (ISO/TS 6434:2024 ,5.1.2)
[1]
6.1.2.1 Allocation of orbit of member spacecraft to minimize collision risk (ISO/TS 6434:2024 ,
5.1.2.1)
AsISO/TS 6434:2024, 5.1.2.1, as mentioned in 5.5sub-clause 5.5,, outlines that there are concerns about
collisions within the constellation, besides the cross points in the Midmid-latitude congested area, in more
critical area, the North/ and South polerpolar congested region and in the Northern /and Southern densely
clouded Belts.  belts.
These can be avoided by allocating orbital planes, separating altitude, or adding eccentricity. Intersections in
the mid-latitudes are also an area to be aware of. An automatic collision avoidance system is being
implemented for the operation of the constellation.

When commissioning the launch of additional spacecraft to a constellation,:
a) Conditionsconditions must be met to avoid immediate entry into congested areas in the North/ and South
polerpolar congested region or Northern /and Southern densely clouded Beltsbel
...