Collision & re-entry risk control

Hypervelocity impact sample

At low velocities, plastic deformation normally prevails. With increasing velocities the impactor will leave a crater on the target. Beyond 4 km/second (depending on the materials), an impact will lead to a complete break-up and melting of the projectile, and an ejection of crater material to a depth of typically two to five times the diameter of the projectile.
 
 

	Hypervelocity impact testing in early 1990s

In hypervelocity impacts, the projectile velocity exceeds the speed of sound within the target material. The resulting shock wave that propagates across the material is reflected by the surfaces of the target, and reverses its direction of travel. The superimposition of progressing and reflected waves can lead to local stress levels that exceed the material's strength, thus causing cracks and/or the separation of spalls at significant velocities.

With decreasing target thickness, the effects range from cratering, via internal cracks, to spall detachment, and finally to clear hole perforations.

ESA's space projects use damage assessment tools in combination with debris and meteoroid environment models to predict potential damage from hypervelocity impacts, and to define effective protection measures through shielding and design.
 
 

	Giotto in 1985, with Whipple shield at bottom

Whipple shielding
 
ESA experts have been actively involved in the development and testing of protective shields for the Columbus manned modules of the ISS.

Protection is achieved through stuffed Whipple shields with aluminium and Nextel-Kevlar bumper layers. The shields are composed of an external, thin bumper shield that is exposed to the debris flux and causes the impactors to completely disintegrate during impact. The resulting cloud of liquid projectile and target material that forms behind the bumper leads to a much wider spatial and temporal distribution of momentum, allowing the back wall of the shield to withstand the impact pressure.

Intermediate fabric layers further slow down the cloud particles. Today, these shields have reached a mature state of development
 
 
Today, ESA's impact protection research activities concentrate on quantifying the expected failure rates and failure characteristics of unmanned spacecraft due to space debris and meteoroid impacts. The aim is to reduce the design margins required for no structural perforation, as required by manned modules.

Material models for composite materials under very high strain rates have been developed for Nextel and Kevlar. These models have been used to verify the structural protection of several ESA spacecraft, including Columbus and ATV.
 
 
Best protection: avoid creating debris
 
Apart from protection and shielding, the effects of debris impacts can be best mitigated by avoiding their occurrence in the first place. 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 US Space Surveillance Network catalogue is sufficient to predict all close fly-bys (conjunctions) of a target satellite with any of the 13,000 catalogue objects.
 
 

	Avoiding impacts: ESA's daily conjunction bulletin

ESA's Space Debris Office routine screens close conjunctions between the Agency's LEO spacecraft (currently ERS-2 and Envisat) and all known catalogue objects. Conjunction event predictions are performed every day, for seven days ahead, using automatically retrieved catalogue data, operational orbit files and environmental data for the orbit propagation.

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.

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-defined collision risk level is exceeded, an alert message is automatically issued.

In such cases, improved orbit information of the chaser object is generated from dedicated radar campaigns (e.g. using the TIRA radar). The resulting orbit uncertainties are mostly reduced by two orders of magnitude. As a consequence, in most cases, an avoidance manoeuvre is not necessary even if the fly-by distance remains small (e.g. within 300m). Typically ten high-risk warnings are generated for the ERS-2 and Envisat spacecraft per year, whereas, on average, less than one avoidance manoeuvre has to be performed annually.
 
 
Re-entry events
 
Every day satellites, rocket stages or fragments thereof re-enter into the denser layers of the atmosphere, where they usually burn up. Shortly before re-entry, at about 120 km altitude, spacecraft have velocities of typically 28 000 km/hour.
 
 

	First images of Jules Verne ATV re-entry over the Pacific Ocean

In the last 10 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 re-entry events were Skylab (74 tonnes, July 1979), Salyut-7/Kosmos-1686 (40 tonnes, February 1991) and Mir (135 tonnes, March 2001). In such cases, up to 20 to 40 percent of the spacecraft mass may impact the surface.

ESA's ATV (Automated Transport Vehicle) performed a controlled and safe re-entry into an uninhabited area in the South Pacific Ocean on 29 September 2008. The re-entry break-up process was monitored from two observation aircraft.
 
 
ESA's re-entry prediction capabilities
 
For people and property on the ground, the hazards posed by re-entering spacecraft or debris are extremely small. So far, there has been only one reported injury and no fatality (except for crew fatalities during manned vehicle re-entry).

The controlled or uncontrolled re-entry of space systems is, however, associated with a number of legal and safety aspects that must be considered. This risk due to re-entries 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.

Re-entry 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 re-entry).

In the case of uncontrolled re-entries, the re-entry 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 re-entries. 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 re-entry 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 re-entry data exchange service that is used by 11 members of the Inter-Agency Debris Coordination Committee (IADC) to monitor the re-entry of risk objects and to exchange orbit determination and re-entry prediction results.
 
 
Last update: 19 February 2009