Don't panic but space junk on Earth collision course

THIS time it splashed down in the Atlantic Ocean — but what about next time?

The European Space Agency says one of its research satellites re-entered the Earth’s atmosphere early yesterday on an orbit that passed over Siberia, the western Pacific Ocean, the eastern Indian Ocean, and Antarctica.

The 1,100kg satellite disintegrated in the atmosphere but about 25% of it — about 275kg of “space junk” — slammed into the Atlantic between Antarctica and South America, a few hundred kilometres from the Falkland Islands, ESA said. It caused no known damage.

The satellite — called the GOCE, for Gravity field and Ocean Circulation Explorer — was launched in 2009 to map the Earth’s gravitational field. The information is being used to understand ocean circulation, sea levels, ice dynamics, and the Earth’s interior. The satellite had been gradually descending in orbit over the last three weeks after running out of fuel on Oct 21.

But how much space junk is out there? About 6,600 satellites have been launched. Some 3,600 remain in space but only about 1,000 are still operational, according to ESA. Not all are still intact, and the US Space Surveillance Network tracks roughly 23,000 space objects, ESA said. A lot of junk comes down unnoticed, said ESA space debris office deputy head Holger Krag. Statistically, he said, “roughly every week you have a re-entry like GOCE”.

About 100 to 150 metric tonnes of space junk re-enters Earth’s atmosphere each year, according to Heiner Klinkrad, the head of ESA’s space debris office. In 56 years of spaceflight, a total of 15,000 metric tonnes of human-made space objects have re-entered the atmosphere.

Space junk — mostly satellites and rocket stages or fragments — typically travels at about 28,000km/h shortly before re-entry at about 120km above the Earth, according to ESA. It starts to slow down and heat up in the dense atmosphere. In the last 10 minutes, it hits a travelling speed roughly equal to that of a Formula One racing car — between 200km/h to 300km/h.

There have been no known human injuries or significant property damage caused by space junk, according to ESA. Unlike meteorites, which hurl into the Earth as a solid chunks travelling about three times faster, space junk typically falls as fragments and is distributed over a fallout zone up to 1,000km long. Krag says fragments from a satellite came down in 2011 over the Netherlands, Germany, and Czech Republic but no pieces were ever found.

A common question is whether or not the junk can be redirected.

When systems are still functioning, spacecraft can be manoeuvred to try and direct them to land in areas where there would be minimal impact, like into an ocean. In the case of uncontrolled re-entries, scientists are able to predict where the impact will be — but that can be a very wide swath. The GOCE satellite’s systems kept on working much longer than expected, providing data that Krag said will be invaluable in helping scientists figure out prediction models for future space junk descents.

One of the best-known cases is Nasa’s Skylab space station, which re-entered in 1979. About 74 metric tonnes hit the Earth — some of it in Australia and the rest falling into the Indian Ocean. Fragments of Russia’s Mir space station weighing about 135 metric tonnes came down in 2001 in a controlled dive into the Pacific Ocean. In 2011, Nasa’s UARS satellite crashed into the Pacific and Germany’s ROSAT satellite landed in the Bay of Bengal. None caused any damage.


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