Hypervelocity impacts and protecting spacecraft
The consequences of meteoroid and debris impacts on spacecraft can range from small surface pits due to micrometre-size impactors, via clear hole penetrations for millimetre-size objects, to mission-critical damage for projectiles larger than one centimetre.
Any impact of a 10-cm catalogue object on a spacecraft or orbital stage will most likely entail a catastrophic disintegration of the target.
This destructive energy is a consequence of high impact velocities, which can reach 15 km/second for space debris and 72 km/second for meteoroids.
Effects of hypervelocity impacts
Since only larger space objects can be catalogued and tracked, only these can be avoided through active measures or by evasive manoeuvres. Smaller, uncatalogued objects can only be defeated by passive protection techniques, as used with the International Space Station (ISS).
The effects of hypervelocity impacts are a function of projectile and target material, impact velocity, incident angle and the mass and shape of the projectile.
Beyond 4 km/second (depending on the materials), an impact will lead to a complete break-up and melting of the projectile. Typical impact velocities are around 14km/s for space debris, and significantly higher for meteroids.
At low velocities, plastic deformation prevails
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.
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.
ESA experts have been actively involved in the development and testing of protective shields for the Columbus manned modules of the ISS.
Whipple shields: double-layer protection
Protection is achieved through 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
Structural protection research
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.
Last update: 20 April 2013