There are hundreds of thousands of tiny fragments in space. These are the remnants of third stages of rocket carriers or retired satellites. Their momentum is enormous, so they can damage the ISS shell. Objects larger than one centimeter can be detected by USAF radars. Based on their warnings, instructions are then issued for maneuvering the ISS or recommendations not to perform EVA. Among the various debris in orbit are also about 44 retired plutonium thermoelectric generators.
Plutonium generators used Soviet radar satellites to track the Western fleet. The satellites had to fly low so that the radar could gather as much data as possible. At that time, the solar panels did not have sufficient power to operate the radar. Secondly, friction with the atmosphere would also significantly reduce the satellite’s orbit, which would shorten its life. The method of lowering the path by solar panels was used, for example, by the Mars Global Surveyor probe. At the end of its life, the Russians shot the core of the orbital reactor into a higher orbit (at least in those where the launch system did not fail), where it will last for hundreds of years. On the other hand, however, radioactive liquid from the primary circuit of the reactor was released into the space.
Instead of plutonium reactors, radioisotope cells can also be used, for example with radioactive plutonium-238. Although it emits only alpha particles that shield ordinary paper or a few centimeters of air, it is poisonous. It is actually the most toxic inorganic substance. In April 1964, the Transit-SB navigation satellite perished in the atmosphere. Only a kilogram of plutonium-238 was dispersed in the atmosphere, but even that was enough for NASA to use radioisotope cells only when necessary only on interplanetary probes for political reasons.
The biggest problem was caused by China when it demolished a satellite with a cruise missile. On 11 January 2007, over 2,300 golf ball-sized fragments and 30,000 coin-sized fragments were created. Shortly thereafter, the United States also shot down its satellite USA 193, but because it was at the end of its life, the fragments were in a low orbit, so they had nothing to threaten.
The tanks of the last stages of launch vehicles are space time bombs. They contain fuel residues, the volume of which can reach tens of kilograms. As the tank expands and contracts under sunlight, it gradually loses strength until one day it explodes. This creates one large object that is easy to track into a series of smaller objects that are difficult to track. Newer tanks have valves that open slightly after carrying the payload to allow the fuel to flow freely into the space or solidify in the tank.
The first uncontrolled collision of two satellites occurred in the history of our planet on January 12, 2009, when two telecommunications satellites collided over Siberia. The active civilian satellite Iridium 33 collided with the non-functional military satellite Kosmos 2251. Although the orbits of all active and passive satellites are monitored so that the active ones can maneuver and prevent a collision, the calculated orbit of Kosmo 2251 was not accurate. The collision created 600 pieces of debris that can damage nearby satellites. The track where the collision took place is quite busy.
The polar orbit is the tightest, because the entire planet can be imaged from it. The vast majority of military satellites and therefore the most waste are on it. If the satellite does not fail for unexpected reasons, it is transferred to a higher orbit just before the end of its life so that its orbit does not collide with the orbits of active satellites. This is done because a change in speed of 1,500 m·s⁻¹ is needed to re-enter the atmosphere, while only 11 m·s⁻¹ is needed to re-enter the atmosphere. However, this is also a great place to park a military satellite and pretend it’s no longer active.
In March 2009, the ISS crew had to take their place in their Soyuz evacuation seat as a precaution. A cluster of debris approached the station unnoticed. By the time they were discovered, it was too late to maneuver the Station. Therefore, the crew prepared to evacuate in case of hitting the station and subsequent decompression.
So far, the largest passive satellite to enter the atmosphere uncontrolled has been the 43-ton Salyut 7. The Skylab and Mir stations were larger, but their demise was controlled. The 20-ton Salyut 7 was launched by a Proton rocket. It was an important link in the development of modular stations. It was used to practice connecting other modules. In 1985, it was expanded with the twenty-ton Kosmos 1686, which was a modified TKS, i.e. a supply ship for Almaz military stations. Since 1986, the station has not been inhabited and in February 1991 it entered the atmosphere. The remains that did not burn in it fell on the territory of Argentina.