ISO 20991:2025

International
Standard
ISO 20991
First edition
Space systems — Requirements for
2025-10
small spacecraft
Systèmes spatiaux — Exigences relatives aux petits engins
spatiaux
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 2
5 Requirements . 2
5.1 Launch interface . .2
5.2 Safety.2
5.3 Main payload, adjacent payload(s), and launch vehicle harmlessness .2
5.3.1 Separation .2
5.3.2 Outgassing.3
5.3.3 Dummy specimen .3
5.3.4 Power state, radio transmission and deployable mechanism . .3
5.3.5 Radio frequency compatibility .3
5.4 Debris mitigation .3
5.5 Use of radio frequencies .3
5.6 UN registration .3
5.7 Verification for design and manufacturing .4
5.8 CubeSat .4
5.9 Release from ISS .4
5.10 Collision avoidance .4
5.11 Management . . .4
6 Verification . 4
Bibliography . 6

iii
Foreword
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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.
This first edition cancels and replaces ISO/TS 20991:2018, which has been technically revised.
The main changes are as follows:
— reorganized Introduction and Scope to clarify the target;
— changed the wordings referring to contractual requirement, or legal or statutory requirements in 5.1,
5.3.3 and 5.5;
— changed the wordings to focus on the specification process of the applicable minimum requirements in
the project, in 5.1, 5.2, 5.3.2, 5.3.3, 5.3.4 and 5.3.5;
— added 5.10 and 5.11;
— removed the wording of “In the event of any conflict regarding requirements between ISO 17770 and this
document, the requirements in this document supersedes.” from 5.8;
— added ISO 17981 as a normative reference.
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
Introduction
Since 2013, the number of small spacecraft built and launched has shown explosive growth. These small
spacecraft are often built by universities, or by newcomers to the space sector, employing untraditional
development and management philosophy that manages risks in cost and time effective manner to achieve
low-cost and fast-delivery. Because of the risk taking approach, small spacecraft often fails in orbit. But
the replacement spacecraft is quickly built and launched reflecting the lessons obtained in the previous
spacecraft. As the launch cost depends on either the spacecraft size or mass, or both, the spacecraft size
becomes “small”.
These spacecraft projects are usually budget-limited or mass-limited, which makes a single (exclusive)
launch unaffordable. Therefore, small spacecraft is very often launched either with a main payload or in a
grapple, which implies specific requirements.
Besides requirements regarding mission success or other goals, which are beyond the scope of this document,
there are minimum requirements that apply to every small spacecraft regardless of its size, mission, value,
capability or any other nature. The purpose of this document is to clearly state those minimum requirements.
In that sense, this document serves as the top document to cover the minimum requirements for various
stages of small spacecraft system life-cycle – with emphasis on design, launch, deployment, operation, and
disposal phases. In this way, the following aspects are ensured:
a) safety;
b) harmlessness to co-passengers and launch vehicle;
c) debris mitigation.
This document provides references to existing standards and documents that elaborate on those
requirements, especially for the benefit of those who are entering the space sector through small spa
...