The largest (in terms of occupied space) rubbish dump is located just over our heads. 300 km above the Earth's surface, where conventionally space begins, thousands of garbage are circling.
Since the beginning of the space age 50 years ago, more than 5000 rockets were launched, approximately 6300 tons of equipment in total. Satellites carried into Earth orbit after a certain time stop to function: they exhaust the fuel needed to maneuver, they end their mission or they break down. They remain in orbit and form a population of space debris together with the upper stages of rockets, shields rejected, the lost parts and remnants of the explosions and collisions. Those of space debris that are low over the surface of the Earth (up to 300 km) have a chance to burn in the atmosphere after a short time. But the majority is a threat – for already working instruments, as well as for future missions. One dangerous event has already occurred – in 2009, Iridium 33 communications satellite collided with an inactive Russian satellite Cosmos-2251. Satellite maneuvers to avoid collisions with space debris are already performed frequently.
The problem was identified quite early and relevant procedures were implemented such as programmed passivation: before the death of the satellite all the remaining fuel is used and all power and supply systems are turned off to minimize the risk of explosion and (if it is possible) the vehicle is set on the trajectory leading to the burning in atmosphere (it is called deorbitation). Space debris are also observed with the help of specialized terrestrial radar stations. At present, the catalog contains more than 23,000 objects tracked. Objects larger than 5-10 cm can be detected at low orbits and larger than 30-100 cm at geosynchronous orbits.
But these activities are not enough - already dangerous objects are larger than 1 mm - in 1983 the damage of window of the space shuttle Challenger was caused by a fleck of paint. So small objects arise mainly as a result of collisions within the debris. It has been calculated that if all human activities in the space are suspended the amount of space debris is still expected to grow exponentially - just as a result of collisions. Other calculations indicate that removal of 5-10 high collision risk objects per year would stabilize the amount of debris at today's levels. High risk of a collision is assigned mainly to large inactive satellites at high inclination (near-polar) orbits – due to the favorable conditions for Earth observation these orbits are quite crowded.
To prevent littering the space environment and to enhance safety of future missions (both scientific and commercial) the CleanSpace initiative was established. It’s lead by the European Space Agency (ESA). Within this initiative, among the others, studies are conducted on the technical capabilities of the active removal of space debris (Active Debris Removal - ADR). An autonomous vehicle is being designed – kind of "space garbage truck", whose aim will be to find the piece of debris, capture it and perform its controlled deorbitation, through appropriate modification of its orbit.
One of the techniques seriously considered for object capturing is to use the net launched by the vehicle, which would wrap around the target allowing to tow it. The big advantage of such a net as compared to other solutions considered (robotic arm among the others) is its large tolerance to the shape of the target and its movement relative to the vehicle. All approaching maneuvers in space are realized basing on telemetry data transmitted by the both approaching objects. In the case of space junk vehicle will have to rely solely on its own sensors (radars, cameras) so closing up to a small distance would be technically difficult. The net will be able to be shot from a distance of 50-100 m, and the velocities (linear and rotational) doesn’t need to be precisely synchronized. It will make whole the task much easier for control computer.
The target of the first ADR mission may be Envisat – ESA observation satellite, inactive since 2012. It is the largest civil satellite in history – it weighs over 8 tons, and with solar panels it is 26 m long. It is located at polar orbit at an altitude of about 770 km. Time to its natural deorbitation (due to slowing down by the residual atmosphere) is estimated to be about 150 years.