The meteoroid (or natural debris environment) has historically been a consideration for spacecraft design. Meteoroids are part of the interplanetary environment and sweep through near Earth orbital space at an average speed of 20 km/s. Observational data indicate that, at any given instant, a total of 200 kg of meteoroid mass is within 2000 km of the Earth's surface. Most of this mass is in meteoroids about 0.01 cm (100 micrometres) in diameter. This natural meteoroid flux is rather stable in the long term but sometimes there is high activity from a shower e.g. the Persied or Leonid populations. The meteoroid environment is well described by current models (Grün, Divine-Staubach).

Figure 1a. Meteoritic particle collected in the Earth’s atmosphere (size: 100x40 microns).		Figure 1b. A typical impact crater produced on an aluminium foil by a small particle.

Man-made space debris (or orbital debris) differs from natural meteoroids because it remains in earth orbit during its lifetime instead of passing through the space around the earth. The estimated mass of man-made orbiting objects within 2000 km of the earth's surface is about 3000000 kg. These objects are in mostly high inclination orbits and pass one another at an average relative velocity of 10 km/s. Most of this mass is contained in about 4000 spent rocket stages, inactive satellites and a comparatively few active satellite. A smaller amount of mass, about 60000 kg, is in the remaining 9000 objects currently being tracked by space surveillance sensors. Information about the current debris environment is obtained through several detecting devices, either located on the Earth's surface or space-borne sensors. Information about the current debris environment is limited by the inability to track and catalogue objects smaller than a few cm.

Space Debris Sources

	Source description	Size Range	Data Sources	Complications in Modeling	Source Database
Launched Objects	From nearly 4000 launches	several 10s of square metres and 1000 kg mass	UN catalog of launches, compendia of engineering data, catalog from US and Russian networks	Incomplete catalog (accurate mass, shape, EOL.) Prediction of future traffic	NASA reference database for historical space traffic
Fragmentation Events	Explosions, collisions, more than 150 events	Several sq metres to millimetres100 kg - mg	Observation of orbiting objects, ground tests	Validation of fragmentation models, shape information , total mass involved	Reference database for historical fragmentations provided by NASA and ESA
Degradation products	AO erosion, UV degradation Thermal cycling	1 cm - 5 µm	Retrieved hardware, simulation	Size distribution,yield rate of paint flakes from thermal cycling	DERA, ONERA, MASTER
West Ford needles	Released in 1963 by Midas 4 & 6 S/C	20 µm x 20 mm needles	US Catalog, X band Goldstone data	needles dispersion of particles	US Catalog, DISCOS database
Ejecta	Primary impacts on solar arrays and satellites or rockets	1 mm - 0.1 µm	Retrieved hardware, simulation	Velocity distribution	Ejecta models, ONERA, MASTER
Solid Rocket Motors	Aluminum oxide particles, by-product of combustion process	micron-sized particles and,by-products 100 µm to 5 cm	Motor manufacturer, few test-firings	Few experimental data, mainly for by-products, ej. velocity	NASA model
NaK Droplets	Leak from 16 RORSAT satellites, produced upon of release of coolant from nuclear reactors	0.1 to 10 mm	Analysis of Haystack radar data, Goldstone data	Core ejection procedure, temperature, evaporation process, min. size, ejection velocity	MASTER Model developed for ESA

Some data pertinent to space debris are summarised in the Table 1.