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How the International Space Station Plays Dodgeball with Space Junk

The ISS has got moves.
Image: NASA/Wikimedia Commons

Over the weekend, a tumbling hunk of space debris careened toward the International Space Station. Mission Control was not terribly concerned. They took the $150 billion, 200-ton space station and…just moved it out of the way.

It may not look like the graceful swan of the cosmos but, as enormous artificial satellites go, the ISS can tango with the best. And that's a good thing. There are over 500,000 pieces of natural and man-made space junk orbiting our planet right now, and about 20,000 of them are larger than a softball. They can reach speeds of up to 17,500 mph and, when they hit something important, this can happen (more or less):


To prevent such fictional tragedies (and real ones, too!) the ISS must be able to detect and dodge orbiting space junk. It takes a sophisticated tracking system to pull it off and, of course, a relatively maneuverable space station. Here's how it works.

The War on Space Junk

The Department of Defense established the Space Surveillance Network in the 1950s to keep tabs on many of the objects in orbit. Nowadays, that system consists of an extensive, worldwide array of optical, radar, and space-based sensors. Private satellite operators can request access to that information to plan their own launches and skirt celestial trash.

But when it comes to protecting the International Space Station, the stakes are much higher. Besides the billions of dollars of equipment orbiting Earth, there are always at least three astronauts on board, conducting scientific experiments and singing David Bowie songs.

Risk assessment is especially important. The DOD categorizes space junk threats based on the size of the debris.

Objects smaller than half an inch are too tiny to track, but debris shields on the ISS can easily handle those impacts. Trash larger than four inches could blast through the debris shield but the DOD can track these objects and warn the space station crew up to a week in advance. Objects between half an inch and four inches pose the most serious threat. They're big enough to do damage, but small enough to escape the DOD sensors.


ISS: Dancing with the Stars

For small, pebble-sized trash, the ISS debris shields would likely hold up. For larger, unavoidable collisions, the crew can take shelter in the Soyuz spacecraft, which is docked on the ISS. In the event of a truly catastrophic collision, the Soyuz could even serve as a lifeboat to evacuate the crew.

But for everything else, there's debris dodging.

When the DOD picks up a piece of space trash on a collision course with the ISS, Mission Control draws an imaginary, so-called "pizza box" 30 miles deep and 30 miles long around the space station. If the debris is heading for the box, it's considered a serious threat.

Planning the perfect debris dodge takes about 30 hours. If the unmanned Progress supply ships are already docked at the ISS, NASA can coordinate their eight engines to fire simultaneously and push the entire space station out of harm's way. If Progress isn't around, the space station's Russian thrusters can be used the same way, in a pinch.

Image: NASA

Debris dodging is risky business, and the space station only attempts it when the threat is substantial. If the probability of collision is 1 in 100,000 and a dodge might ruin the mission at hand, NASA holds off. When the probability is 1 in 10,000, they typically go for the dodge, regardless of the mission.

Space Junk in the Lab

Near misses in orbit happen more often than you'd think. In the past couple of the years, the ISS has dodged debris on several occasions. To combat the threat of space junk ruining a perfectly good mission, NASA routinely conducts junk collision research. That's right—NASA's got trash-smashing scientists.

Back in 2012, NASA designed DebriSat, a perfect satellite replica, built to be destroyed in the name of science. Recently, scientists smashed DebriSat in a specialized ground-based chamber and recorded the collision data.

Image: J.C. Liou/DebriSat Project

The research may help scientists prevent future collisions in orbit. With all that debris floating around, a better understanding of how junk forms in space could protect expensive equipment, and even save lives.

But for now, debris dodging is the best we've got. Dance on, ISS.