Re-entry and collision avoidance

Avoiding impacts: ESA's daily conjunction bulletin
Avoiding impacts: ESA's daily conjunction bulletin

Apart from protection and shielding, avoiding their occurrence in the first place can best mitigate the effects of debris impacts.

This, however, can only be done if the orbits of the debris and target object are known with sufficient accuracy. For initial assessments, the information provided by the USSTRATCOM catalogue is sufficient to predict all close fly-bys (conjunctions) of a target satellite with any of the catalogued objects. The collision risk is determined as a function of the object sizes, the predicted miss distance, the fly-by geometry and the orbit uncertainties of the two objects involved.

ESA provides services

ESA's Space Debris Office offers conjunction event predictions and estimation of collision risks as a service to ESA missions and third parties. Predictions are updated every day, typically covering seven days ahead, using automatically retrieved catalogue data, operational orbit files and environmental data for the orbit propagation.

ESA's Space Debris Office offers conjunction event predictions and collision risk estimation as a service.

Results of this process are provided daily, by email, in the form of automatically generated conjunction event bulletins, indicating all relevant data for the assessment of the 10 top-ranking risk events. This includes approach geometry, miss distance, collision probability, detailed information on the chaser object (geometry, mass, origin, type) and information on orbits and orbit uncertainties.

If a customer’s designated collision risk level is exceeded, an alert message is automatically issued. In such cases, improved orbit information of the impending object may be generated by conducting dedicated radar campaigns, e.g., using the TIRA radar.

Such tracking improves the orbit information and reduces the uncertainties usually by two orders of magnitude. Very often a collision avoidance manoeuvre is then no longer necessary, even if the fly-by distance remains small.

The provided service also covers processing externally provided information, such as Close Approach Warnings and Conjunction Summary Messages from the US Joint Space Command (JSpOC).
Due to the recent changes of the space debris environment, ESA missions must execute several collision avoidance manoeuvres per year.

Reentry events

Every day satellites, rocket stages or fragments thereof reenter into the denser layers of the atmosphere, where they usually burn up. Shortly before reentry, at about 120 km altitude, spacecraft have velocities of, typically, 28 000 km/hour.

ATV Jules Verne reentry
Jules Verne ATV re-entry

In the last ten minutes before reaching ground, the dense atmosphere starts to heat up and decelerate the spacecraft. In the case of very compact and massive spacecraft, and if a large amount of high-melting material is involved (e.g. stainless steel or titanium), fragments of the vehicle may reach the Earth's surface.

Well-known examples of large-scale reentry events were Skylab (74 tonnes, July 1979), Salyut-7/Kosmos-1686 (40 tonnes, February 1991) and Mir (135 tonnes, March 2001). In such cases, as much as 20-40% of the spacecraft mass would have impacted the surface.

Recent examples of uncontrolled reentries that gained a lot of public interest are the US Upper Atmosphere Research Satellite (UARS), the German Röntgensatellit ROSAT and the Russian Phobos-Grunt.

ESA's ATV (Automated Transport Vehicle) missions all perform a controlled and safe reentry into an uninhabited area in the South Pacific Ocean. The reentry break-up process of the first ATV mission on 29 September 2008 was monitored from two observation aircraft.

ESA's reentry prediction capabilities

For people and property on the ground, the hazards posed by reentering spacecraft or debris are extremely small. So far, there has been no fatality (except for crew fatalities during manned vehicle re-entries).

The controlled or uncontrolled reentry of space systems is, however, associated with a number of legal and safety aspects that must be considered.

Controlled and uncontrolled reentries can be assessed. A 1-in-10 000 limit for the casualty risk for a single uncontrolled reentry event is widely accepted.

The risk due to reentries can be determined through analysis of surviving fragments (if any), their dispersion across a ground swath, and the resulting casualty risk for the underlying ground population distribution. The Space Debris Office coordinates communication with local national air traffic authorities for ATV reentries.

Reentry manoeuvres can be optimised to control the impact footprint (ideally over an ocean area), and thus maintain the casualty probability below an acceptable risk threshold (e.g. less than 1 in 10 000 for a single reentry).

In the case of uncontrolled reentries, the reentry time window and impact footprint can be predicted and monitored. The quality of this process can be improved through tracking data and sophisticated orbit prediction tools. ESA has all necessary capabilities to provide analysis of both controlled and uncontrolled reentries. This includes detailed simulations of the aero-thermal and structural break-up of satellites or orbital stages, the prediction of the orbit and attitude of each reentry fragment, the identification of objects reaching ground and the analysis of associated risk potentials for the population in the entry ground swath.

These tools have been used, for instance, for re-entry assessments for ATV, Beppo SAX, TerraSAR, GOCE, Ariane-4 and Ariane-5.

ESA's Space Debris Office also maintains a Web-based reentry data exchange service that is used by 12 members of the Inter-Agency Debris Coordination Committee (IADC) to monitor the reentry of risk objects and to exchange orbit determination and reentry prediction results.

Last update: 19 April 2013

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