Slaloming through Space

Only two types of object can fall to the Earth from Space: ‘natural’ bodies such as asteroids, meteorites or comets and waste items from the Space industry: parts of launch vehicles (stages, boosters, fairings etc.), satellites at end-of-life, nuts, bolts and tools ‘dropped’ by astronauts and so on. Although objects are naturally cleared from Space by orbit decay in the long term, this is no longer sufficient when confronted with the proliferation of waste created by humans. Since the launch of Sputnik in 1957, more than 18,000 objects have been placed in orbit. As a result, the quantity of Space debris has multiplied: each collision creates more by causing further fragmentation that disperses on new orbits, endangering operating satellites and increasing the risk of further collisions exponentially.

A tangled skein of orbits

The numerous pieces of debris that may float alongside an orbiting Space station are of no danger to it, as they follow the same orbit at very low relative velocities. However, the laws of celestial mechanics are such that any slight perturbation can cause their orbital planes to change and move apart. Two objects that were originally on neighbouring orbits can find themselves on diverging orbits that can eventually lead to a collision course, even meeting each other head-on. Fortunately, the probability for any structure to encounter debris on a collision orbit is much lower than that for encountering debris on its own orbit: despite the quantity of debris, the probability of collision therefore remains low.

The International Space Station (ISS) therefore runs a negligible risk, especially considering that it is very carefully monitored and often performs avoidance manoeuvres.

Risks in orbit

In Space, debris moves about in all directions. At the same time, the fantastic speeds of certain items mean that such projectiles are potentially very dangerous for operating satellites or, worse, for the ISS and the astronauts it contains. For example, an aluminium sphere with a diameter of 1 mm travelling at a velocity of 10 km/s can perforate a 4 mm sheet of the same material. Such a sphere possesses the same kinetic energy as a metal French ‘pétanque’ ball moving at 100 km/h. In these conditions, the risks increase proportionally with the size of the debris, which are considered as highly dangerous from 10 cm upwards.

The most recent collision, between an (abandoned) Russian satellite and an American telecommunications satellite, brought it home to satellite operators that an impact on this scale can generate thousands of new items of debris that now have to be taken into account

Risks on the ground

Except for the very largest pieces, most items burn up during re-entry as they fall back to Earth. Indeed, the objects we call Shooting Stars are simply tiny celestial bodies burning up in the atmosphere. The risk on the ground is therefore minimal. One reason is that we are able to control the way the larger items return to Earth and guide them to uninhabited regions, preferably the oceans. Examples include the de-orbiting of the famous MIR Space Station and also of the Automated Transfer Vehicle (ATV). Another reason is that, concerning the smaller items, we should remember that the oceans cover 70% of the Earth’s surface. Up to now, some 20,000 objects have fallen back to Earth, without causing injury to anyone. Finally, the danger from falling man-made objects from Space is less than the danger from meteorites, which is already considered very low.

In the booklet on the subject, "Pollution spatiale sous surveillance" [Space pollution under surveillance], we learn that solar activity can influence the fall-back of debris. When the Sun is more active than normal, the atmospheric layer around our planet dilates, increasing aerodynamic resistance at high altitudes. As a consequence, in periods of high solar activity, more Space debris burns up in the atmosphere or falls back to Earth.

Launchers

The launch phase of any rocket is marked by the jettisoning of stages that have completed their mission, meaning that large objects or debris fall back into the sea. The final stage, however, and the structure carrying the payloads remain on the satellite’s injection orbit after separation, thus becoming debris that will also one day fall back to Earth.

Design rules require that the launcher generates no more than one item of debris per payload placed in orbit. If an accident occurs during launch, the launch vehicle is destroyed, generating a quantity of debris that fall back. The Range Safety rules ensure that, should this occur, populated areas are preserved from any debris or toxic effects.

Satellites