China's Tiangong-1 space station is currently out of control and is expected to fall back to Earth sometime in March this year. It was launched in 2011 as China's first space station. The following year, it was visited by China's first female astronaut Liu Yang. The space station's orbit is decaying as it heads towards a fiery re-entry into earth's atmosphere. The space station weighs about 8.5 tonnes and is currently orbiting 370 km above ground. It is claimed to have fulfilled its mission's objectives.
The Tiangong-1 space station is expected to mostly burn up and unlikely to affect aviation or cause much damage on the ground. A large portion of the space station could melt as it passes through the atmosphere, but some denser parts such as the engines may not burn up. As the Chinese engineers have lost control and cannot fire the thrusters to bring it down in the South Pacific, it is expected to come down, anywhere between Spain and South Australia. It is difficult to be more precise until a few hours before the burn up. However, the Tiangong-1 space station isn't the only one that has problems for its descent.
On July 11, 1979, the US space station, Skylab I, tumbled back to Earth, scattering debris across the southern part of the Indian Ocean and sparsely populated western Australia. It finally struck the Australian coast. Skylab was America's first space station, launched in May 1973. The space station that weighed around 80 tonnes was without crew. The 37-tonne Salyut 7 space station by the Russians came down in South America in 1991. The 140-tonne Russian space station, Mir, that was visited by many teams of cosmonauts was directed down into the South Pacific in 2001, and it was last seen by some fishermen as a fragmenting mass of glowing debris racing across the sky. It survived increased solar activity (unlike Skylab I), lasting 20 years.
One future space station, which is expected to be brought down, is none other than the International Space Station (ISS). It has already been up 15 years and is expected to be decommissioned over the next decade. With a mass of 450 tonnes it will make a spectacular sight on re-entry sometime in 2024 or 2025. Many times a year, supply ships that go to the ISS to replenish food, water and other essentials for the astronauts staying in the station, also come crashing down, with a controlled re-entry into the atmosphere.
Graveyard of satellites
It is interesting that over the years, many large spacecrafts have been brought down over a controlled re-entry path into a region in the South Pacific called the 'oceanic pole of inaccessibility', that is, an area in the ocean furthest away from land. It lies in the south Pacific around 2,000 km from the South Pitcairn island, a literal no man's land between Australia, New Zealand and South America.
Scattered over an area of about 1,000 sq km on the ocean floor, this region is a graveyard of various decommissioned satellites, space stations, and other spacecrafts. At last count, around 300 spacecrafts have crashed here. While smaller satellites will burn up, the big pieces of the larger ones, like space stations, will survive to reach the earth's surface. To avoid crashing in a populated area, they are brought down near the point of oceanic inaccessibility.
However, re-entries done by space stations such as Tiangong-1 face the threat of being hit by some of the man-made trash that are currently orbiting earth. Some are tiny, some are large enough to be seen by telescopes, all pose great risk to orbiting spacecrafts and satellites. The danger is growing as space (around earth) is getting more crowded. Around 25,000 pieces of space junk are big enough to be tracked by the space surveillance network. But most of the debris are under 10 cm in size and cannot be detected.
Even those which are the size of a paper clip can cause great damage. For instance, a loose fleck of paint caused a crack in the window of the ISS. Though such collisions are rare, half of all dusty junk is caused by debris from two events in 2007. China destroyed one of its own satellites with a ballistic missile, shattering it into thousands of pieces. In 2007, an American commercial satellite collided with a defunct Russian satellite. Even last year, debris from that collision forced the crew of the ISS to evacuate to the Soyuz capsule. Such debris will remain in earth's orbit for several years.
More than 7,000 satellites have been put in space, but currently only 1,000 are functioning. Within the next decade, the number of satellites could double to 20,000 with planned launches of mega constellations, a large group of satellites to improve global communication coverage, etc. Objects follow different orbits and can cross paths. Satellite technology is necessary for a whole range - from weather to GPS.
Space junk mission
While Tiangong-1 is falling down, Britain is set to launch the 'Remove Debris: space junk mission'. It will attempt to snare a small satellite with a net and test whether a harpoon is an effective garbage grabber. As stated, over 7,500 tonnes of junk is orbiting Earth, ranging from huge defunct satellites, spent rocket boosters to nuts and belts. Perhaps about a million pieces of such space garbage collisions can cause enormous damage, generating even more pieces of debris, putting spacecrafts and astronauts at risk.
The RemoveDebris mission will first head to ISS on one of the resupply rockets. It has its own space junk on board - small satellites. It will release one of them into space and then will use a net to recapture it. A small harpoon would be fired at a target to see if it can accurately work in a weightless environment. It will finally test future de-orbiting technology. Then, when it descends, it will deploy a large sail to change the spacecraft's speed to ensure that it burns it up. Earlier, a Japanese magnetic space junk remover did not work as expected.
In short, the increasing space activity in the next decade is expected to multiply space debris in orbit posing risk to astronauts and those of us on ground. Space garbage disposal and its detection are pressing problems that need to be looked into to avoid any untoward situation.
(The author is with Indian Institute of Astrophysics, Bengaluru)