If you think it is hard to keep your bedroom tidy, spare a thought for the space around our Earth. There are over 100 million particles of space junk out there, and humans are responsible for all of it!
Space junk – or orbital debris – is a term used for all of the objects orbiting the Earth that were created by humans but no longer serve a useful purpose. These objects range in size from quite large – spent launch vehicles and obsolete satellites – to tiny paint flecks. The US Space Surveillance Network routinely tracks about 21,000 objects larger than 10 cm. It does this using ground-based radar, optical telescopes and space-based telescopes including the Hubble.
Sources of space junk
For many decades, much of the space junk came from the explosions of old rocket engines left in orbit and from the testing of anti-satellite weapons in the 1960s and 70s. In 2007, China used a missile to intentionally destroy its own Fengyun-1C polar orbit weather satellite. This created more than 37,000 pieces of debris 1 cm or larger. Another significant source of space junk came from the 2009 collision of the Iridium 33 communications satellite and the non-operational Kosmos 2251 military satellite. These two incidents represent about a third of all catalogued objects.
Dangers from space junk
The greatest concentrations of space junk are found in low Earth orbits near 750–800 km. The speed of these orbiting objects is 7–10 km/s, so collisions with even small pieces of debris involve substantial energy. As the amount of space junk increases, so does its potential to cause harm to other objects in space. For example, chips in the Space Shuttle’s windows and thermal tiles (similar to what happens to cars on unsealed highways) were becoming a common problem in the 1990s. To combat this, the shuttle would fly tail first once it reached orbit.
The International Space Station (ISS) manoeuvres away from any object expected to pass inside its safety zone of 25 km on both sides and around 1 km above and below it. As an added precaution, the ISS is the most heavily shielded spacecraft ever flown. Its habitable compartments and high-pressure tanks should be able to withstand an impact with debris up to 1 cm in diameter. That’s about the size of a sugar cube.
If space junk is a risk to astronauts, is it also a risk to people on Earth? Believe it or not, in the last 50 years, an average of one piece of catalogued junk falls to the Earth each day! No serious injury or substantial property damage has been confirmed by falling debris. This is partly due to the debris burning up during re-entry. It also helps that oceans and other uninhabitable terrain covers much of the Earth’s surface.
Mitigating the risks from space junk
Satellites have a life span of 5–15 years. However, the time it takes for their orbits to decay is quite long. Above 1000 km, a non-operational satellite continues to orbit for a century or more. Telecommunication and weather satellites in geosynchronous Earth orbit can remain up there for millions of years! So when a satellite comes to the end of its useful life, it is often moved into what is known as a graveyard orbit – an orbit not used for functioning satellites.
Cleaning up space junk is a big challenge. The Swiss have made a prototype satellite called CleanSpace One. Once launched into space, its mission is to grab hold of and stabilise a piece of space junk at high speed. It will then guide itself and the debris back into Earth’s atmosphere where both objects would burn on re-entry. This might be one way to clean up space but comes with big technical and economic challenges.