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The micro-meteoroid and space debris environment are often considered together since they are fast moving pieces of matter. Behaving like projectiles, they can penetrate material easily. The energy they possess is very high and the impact can vaporise the primary particle, generate fragments and leave a crater or hole on surfaces.
The amount of damage depends on the mass of the particle and the relative velocity of the impact. Many small imacts are observed on the surfaces returned from LDEF, EURECA, and the Hubble Space Telescope (HST) Solar Array.
Man-made space debris and natural micro-meteoroid particles can damage satellites and constitute a serious hazard to manned spaceflight. The International Space Station, with its large surface area and long planned lifetime has multi-wall design to protect it. Environment models have been built up, mainly at NASA/JSC by Kessler for space debris and by Gruen for micro-meteoroids for impact risk assessment. Nowadays more sophisticated micro-particle prediction models like NASA's orbital debris engineering model (ORDEM2000) and ESA's Meteoroid and Space Debris Terrestrial Reference (MASTER-2005) are being used.
In evaluating space debris and micro-meteoroid effects it is necessary to know what size and velocity of particle can penetrate a given shield design. The shield may comprise a single aluminium wall or multiple walls with spacing. Design equations give the particle size which just penetrates (or causes some defined damage) as a function of particle velocity for a given shield; there are other forms for the design equation but the principle is the same. The design equation can then be used together with the environment model, which provides particle fluxes as a function of size and velocity, to predict penetration or damage probability over a certain time. Here again we see the necessity of having good test results as a prerequisite to a reliable analysis. Furthermore, since current technology impact tests cannot reach the extreme velocities of the debris population, hydrodynamic computer codes need to be used to augment the test data in establishing design equations.
In the past, it has often been adequate to assume random orientation for the shield. However, the International Space Station has a preferred orientation with respect to the velocity vector. A body will encounter debris coming primarily from a forward direction, with peaks in the flux 30 to 70 degrees away from the velocity vector. Fluxes are also predominantly from directions parallel to the Earth's surface. Therefore the spacecraft geometry and orientation need to be taken into account. This is the case for micro-meteoroids too, because of the relative velocity of the spacecraft through the micro-meteoroid environment. LDEF surfaces show this effect clearly. For the geometrical/directional effects a new PC-based micro-meteoroid and space debris impact risk assessment tool (ESABASE2/Debris) has been developed. With this tool, the number of impacts per square metre over time, the number of failures and the probability of no failure can be computed for user-defined mission parameters, spacecraft geometry, attitude and shield design.
The environment remains uncertain, especially for particle sizes just below the trackability limit. It is also expected to evolve significantly through increased activity and debris-debris collisions. More flight data are clearly required.
Last update: 2 October 2007
Related sites:Space Environment Information SystemGeant4 for spaceMULASSISSSATGRAS (PDF)ESABASE2/DebrisMASTER-2005Martian Climate DatabaseCOMOVASPISSpacegrid