At the furthest point from any landmass on earth, and 4km under the sea, lies the space cemetery.
When their outer space journeys come to an end, old satellites, rocket parts and space stations are sent to this desolate spot in the Pacific Ocean to rest on the dark seabed forever.
The technical name for this stretch of water is the “ocean point of inaccessibility” because it lies about 2,700km from any land. But it is more commonly known as the space cemetery, or Point Nemo – named for the fictional submarine captain in Jules Verne’s 20,000 Leagues Under the Sea.
It’s here that the International Space Station, the football-field-sized laboratory orbiting Earth, is likely to end up. Reports emerged this week of cracks in the ISS and while the fissures may not spell its imminent demise, it is certainly in its twilight years.
When spacecraft die, they become a danger to everything else in orbit. Space debris is rapidly clogging up space, and at orbital speeds of up to 17,500km/h even tiny flecks of paint can cause serious damage to other spacecraft.
According to Nasa, there are thousands of bits of space junk out there.
“There is so much junk that we are worried one tiny collision could trigger a big chain reaction. This possibility is called the ‘Kessler Effect’,” Nasa says.
这 Kessler Effect, or Kessler Syndrome, is the potential for the amount of debris in orbit to reach a critical mass where each collision creates more pieces of debris in a cascading way, to the point where the orbit is no longer usable.
“To prevent such a disaster, anyone launching something into orbit these days has to have a plan to either send it into a graveyard orbit, or send it back down to burn up in Earth’s atmosphere,” Nasa says.
Very high satellites can be blasted further into space, out of harm’s way, with the last of their fuel – that’s the “graveyard orbit”. Closer satellites can be nudged out of orbit, and the smaller ones will burn up entirely on re-entry. Those that don’t burn can crash to Earth in an “unplanned” trajectory (喜欢 China’s Long March 5B rocket, or the Skylab space station, 哪一个 hit Western Australia). But it is generally preferable that, instead of potentially careening into inhabited land, the debris is carefully guided to splash down at Point Nemo.
As the European Space Agency explains, modelling is used to pick the point at which a craft will hit the upper atmosphere, and doing that at a calculated and steep angle ensures debris will fall within a certain zone.
在 2001, the Russian space station Mir reached the end of its useful life. A cargo ship docked to the craft fired its engines to take Mir out of orbit and back to Earth. Parts burned up on re-entry, while up to 25 tonnes survived, and plummeted to its watery grave at Point Nemo.
自那时候起, Mir has been joined by defunct satellites, rocket parts and even an automated transfer vehicle that delivered cargo to the International Space Station – an ATV called the Jules Verne.
The spacecraft that have survived space, and the fiery descent into Earth’s atmosphere, are hardy enough to resist the crushing pressure 4km (one league) down.
One day in the not-too-distant future, the International Space Station is likely to join them. The ISS has been orbiting Earth since 1998, when Russia, 美国, 加拿大, Japan and several European countries began the joint venture. It has been home to astronauts since 2000.
最初, it was only expected to last for 15 年. 现在, it’s authorised to operate until 至少 2024. But the ISS is showing its age.
Just over a week ago, the Nasa spokeswoman Angela Hart told CNBC that although Nasa is still “actively working to continue to do science and research … the ISS at some point will have its end of life”.
This week the Russian official Vladimir Solovyov said most of its in-flight systems were past their expiry dates, which could lead to “irreparable failures”.
The BBC reported a range of other issues, from air leaks to malfunctions and “structural fatigue”.
The space archaeologist Alice Gorman, from Flinders University, says the ISS has done well to last this long, but it is getting on.
Luckily, humans have learned a lot about de-orbiting spacecraft. European Space Agency engineers have observed supply modules de-orbiting and re-entering, and built up data about how to ensure a controlled re-entry.
“People involved would acknowledge that at some point it will come to an end,” she says of the ISS. “But it has been planned for.”
One of the tricky things, Gorman says, is that the ISS is a beast of many parts. There are various modules where the six astronauts live, and others that function as laboratories. There are solar arrays and robot arms on the outside. Nasa says it is as big as an American football field, with bathrooms, a gymnasium and a big bay window.
“If they do have to separate some of the modules from each other, that’s likely to create some debris,” Gorman says. “People will be watching that process. There’s a keener sensibility about leaving stuff in orbit these days.
“As it enters the atmosphere it will start to break up – one reason Point Nemo is a good place is that the debris footprint, from the first bit to the last bit, can be kilometres and kilometres long.
“But there will be fears. What if something goes wrong? What if they don’t predict it accurately?”
When Long March 5B made its out-of-control descent, there were fears not just about the dangers of it hitting someone, 但那 there might be toxic propellants still on board.
Gorman says that could be the case even with a controlled descent, but that “by the time it comes screaming in through the atmosphere all the volatile fuels have gone”. And what survives to settle in the space cemetery will be safe materials such as stainless steel, titanium alloys and ceramics.
And she paints a peaceful ending to that hectic journey from outer space to the deep blue sea.
“We send it down to the bottom of the ocean,“ 她说. “And like shipwrecks the world over it becomes a habitat, a coral reef. A whole new life.”