“You plan a safe move or hunker down and duck”
Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics explains the dangers posed to Space Station astronauts by orbiting junk
What happened?
In June 2011 a piece of space junk flew past the International Space Station at 29,000 miles per hour. While it missed the ISS by 335 metres (1,099 feet) it was spotted too late to perform any manoeuvres, and so the six crew members had to seek safety in the two docked Soyuz capsules until the danger had passed. The probability of a hit was calculated at 1 in 360 – far below the 1 in 10,000 risk at which NASA usually calls for precautions.
What mission were they on?
The incident took place during the Expedition 28 mission. Russian cosmonaut and ISS commander Andrei Borisenko was with flight engineers Aleksandr Samokutyayev and Sergey Volkov, as well as NASA astronauts Mike Fossum and Ron Garan, and Japanese Aerospace Exploration Agency astronaut Satoshi Furukawa. “In the US, the military is responsible for tracking space junk, so the US strategic command goes through a catalogue of 18,000 objects every day to work out how close each piece is going to get to the International Space Station. They are almost all false positives because they cannot tell for sure whether a piece of debris will hit: the junk is generally calculated as being somewhere between a mile and zero, but it's usually half a mile or so.
“Because it might be zero, however, alerts are issued, and it means a lot of work goes on to improve the tracking and prediction capabilities.
The more accurate they are the fewer alarms there will be, and as long as there is enough warning action can be taken. With enough time, NASA can fire the rocket engines on one of the cargo ships to boost the orbit slightly. They can also change the timing of the orbit so that all that's needed is to be in a different place at a time when a piece of space junk is going to strike.
“But it's also a little more complicated than that because you don't want to move the ISS to a place where some other piece of space junk will collide with it instead. It's certainly not a case of spotting something with five minutes to go and saying: ‘alarm, alarm! We've got a close conjunction so we're going to move the station.’ It takes a day or so to figure out. In the case of 2011, there was no time. The warning came too late so they had to go to a red conjunction, and this indicates that a piece of debris is close enough to pose a threat."
“What they do – as they did in this case – is tell the astronauts, ten minutes before the close approach, to go to their rescue ships and close the hatch. In this case it was the Soyuz ferry ships, and they provide a safe compartment if the ISS happened to get holed on this occasion. Now, if the ferry ship happens to be the thing that gets holed, well, it's a bad day. But the idea is that the Soyuz are much smaller targets.
“From that point the astronauts could just ride it out, and Houston calls up and says: ‘Ok, it's passed, no worries, go back into the space station.’ So those are basically the two scenarios: you either have enough time to plan a safe move of the orbit or you hunker down and duck. In 2011, this was a case of ducking and hoping their head would still be attached an hour and a half from now.
“If the danger to the ISS was real, however, the capsules can allow for a quick undock, returning the astronauts to Earth. There's also a little bit of micrometeorite protection on the space station, so really small pieces will get soaked up. But anything big and there's a problem.
“For example, a classic case of a space collision happened in 2009 when two communication satellites smashed into each other. One of them was an old, dead Russian military device while the other was an active US commercial Iridium satellite. That was two half-ton satellites smashing into each other at 42,117 kilometres (26,170 miles) per hour, and the energy in the collision was about 50 gigajoules. Now, a megajoule is the energy you get if you were hit by a one-ton truck travelling at a 160 kilometres (100 miles) an hour. And this collision is 54,000times the energy of that. That's the sort of energy we're talking about in a space junk collision, so there's not a lot you can do except not get hit.
“It can happen, though. In 1997, an unmanned supply vessel crashed into the Mir space station, and despite it being at a low speed of probably a few miles an hour, it was enough to hole the module. The astronauts on board heard a hissing sound and a bang, and they had to immediately close of the door to the module. It wasn't catastrophic, but it was certainly near-fatal. When you consider that some random piece of space junk will be going at tens of thousands of miles an hour then you realise how screwed you'd be. We could get to a point where it's not possible to have something like the ISS because there's just too much space junk.”