Today’s man-made space-debris environment has been created by the space activities that have taken place since Sputnik’s launch in 1957. There have been more than 4000 rocket launches since then, as well as many other related debris-generating occurrences such as more than 150 in-orbit fragmentation events. tracked, with estimates for the number of objects larger than 1 cm ranging from 100 000 to 200 000.

The sources of this debris are normal launch operations (Fig. 2), certain operations in space, fragmentations as a result of explosions and collisions in space, firings of satellite solidrocket motors, material ageing effects, and leaking thermal-control systems. Solid-rocket motors use aluminium as a catalyst (about 15% by mass) and when burning they emit aluminium-oxide particles typically 1 to 10 microns in size. In addition, centimetre-sized objects are formed by metallic aluminium melts, called ‘slag’. They typically amount to 1% of the propellant mass and leave the motor with low velocities at the end of the burn. There is evidence from ground-based radar measurements that 16 of a total of 31 nuclear reactors used by Russian RORSATs (Radar Ocean Reconnaissance Satellites) have lost their sodium-potassium (NaK) coolant, following their reorbiting and subsequent core ejection in disposal orbits at between 700 and 950 km altitude. The size of the NaK droplets observed ranges from 6 mm to 4.5 cm. The NaK population is assumed to consist of about 60 000 objects, with a total mass of about 50 kg.