Avoiding debris, removing mass

The most effective short-term means of reducing the space debris growth rate is through the prevention of in-orbit explosions. The only effective long-term means of stabilising the space debris environment at a safe level is through the removal of mass from regions with high object densities.

Both types of mitigation measures need to be applied broadly and in a timely manner to avoid uncontrolled growth of the debris environment. If mitigation concepts are applied insufficiently, or too late, some orbit regions, particularly at 800- to 1400-km altitude, may experience a collisional cascading process that could render these regions too dangerous for space activities within a few decades.

End-of-life disposal

In order to remove mass from densely populated orbits, it is recommended that spacecraft and orbital stages are commanded to re-enter the Earth's atmosphere within 25 years of mission completion, if their deployment orbit altitude is below 2000 km (i.e. in the LEO region). Alternatively, they may be re-orbited above 2000 km.

For spacecraft and orbital stages in or near the geostationary ring, re-orbiting after mission completion to a 'graveyard orbit' is the only viable option. The recommended re-orbit altitude is about 300 km above the GEO ring. This guarantees that the re-orbited object will never interfere with operational GEO spacecraft. Both the LEO and GEO regions are denoted as 'protected regions', due to their commercial and scientific value.

Passivation

An important part of the end-of-life disposal of a space system is passivation.

During this activity, all latent energy reservoirs of a spacecraft or orbital stage are depleted to prevent an accidental post-mission explosion. Such passivation measures may include depletion burns, fuel and/or pressurant venting, the discharging of batteries and the inhibiting of pyro devices.

ESA is playing a leading role in the implementation of space debris mitigation measures. Since 1997, Ariane orbital stages have performed a controlled fuel/pressurant venting and battery discharging. No explosive break-ups have occurred for Ariane stages launched after 1997. ESA has also re-orbited all GEO spacecraft controlled by the Agency (several of them well before the existence of any international guidelines).

Furthermore, since the mid-1990s, ESA has performed collision avoidance for their LEO spacecraft, monitoring high-risk conjunctions with 13 000 known objects, and performing avoidance manoeuvres, if required.

Mitigation guidelines at ESA

ESA Space Debris Mitigation guidelines, April 2008

In 2002, the Inter-Agency Debris Coordination Committee published the "IADC Space Debris Mitigation Guidelines," and presented these to the UNCOPUOS Scientific & Technical Subcommittee (STSC), where they served as a baseline for the "UN Space Debris Mitigation Guidelines."

In 2007 these guidelines were approved by the 63 STSC member nations as voluntary high-level mitigation measures. Since the mid-1990s, space agencies in Europe have developed more technically oriented guidelines as a "European Code of Conduct," which was signed by ASI, BNSC, CNES, DLR and ESA in 2006.

The core elements of this Code of Conduct are in line with the IADC and UN guidelines. In order to tailor the Code of Conduct to the needs of ESA projects, ESA developed their own "Requirements on Space Debris Mitigation for Agency Projects" (ESA/ADMIN/IPOL(2008)2 Annex 1). These instructions came into force on 1 April 2008. They are applicable to all future procurements of space systems (launchers, satellites and inhabited objects).

International debris mitigation standards

Space debris mitigation guidelines provide a framework for 'what' needs to be done. The way 'how' mitigation measures must be implemented is specified in a more formal manner, via international standards - or via binding national requirements for the design and operation of space systems. Such common standards guarantee a level field for industrial competition and for safe access to space into the future. International debris mitigation standards are presently being developed at ISO.

Experts from ESA regularly support these developments and their harmonisation with existing guidelines and requirements. The ultimate ISO standards on space debris mitigation, however, will remain non-binding (as is true for any ISO standard).

Responsibilities of launching states

According to the Outer Space Treaty of the United Nations, ESA is a 'launching state', since we launch objects or procure the launch of objects into space.

As a consequence, ESA can be held liable for damages caused by a launched object or its components in space and on ground (article VII of the United Nations Outer Space Treaty, 1967). Another UN convention (Law of the Sea, 1982) regulates sovereignty aspects over sea areas and the dumping of space objects into such areas.