The ISS is 450 tons of spacecraft that’s mostly intended to stay put. Unfortunately, orbit is a little bit messy…much of it our fault…and every so often, the ISS crosses orbits with a piece of space junk.
The junk, of course, is on a fixed trajectory.
This means that a couple of times a year, the ISS has to engage in an “avoidance maneuver.” Which is a fancy way of saying that a 450 ton spacecraft has to dodge.
How does the ISS Avoid Debris?
The ISS is in a set orbit, but it is not in a 100% stable orbit. For the most part it handles station keeping by the use of 220-pound gyroscopes that keep the spacecraft balanced, stop it from tumbling, and secure its orbit.
To dodge, however, the ISS has to shift to a slightly different orbit. So, how do they do this? It depends on who’s at home.
If there is a Progress supply ship docked, then the supply ship uses its eight engines in a specific pattern that applies thrust safely to the station.
If not, then the Zvezda service module does have thrusters, although they are not as powerful and generally they try to use the Progress.
The ISS moves every time a ship docks to optimize the orbital transfer. They also have plans to raise the orbit because there is still trace atmosphere at that height and it applies drag to the solar panels which would eventually pull the station down.
As for dodging debris, the ISS needs to boost 135 minutes (1.5 orbits) before the point of collision, and can move either over or under. This is the last possible minute to alter orbit, and they wait so as to be sure they have to. Most of the time, they don’t.
What do the Astronauts Do?
The astronauts on the station don’t decide whether the station needs to dodge; that’s the responsibility of ground control at Cheyenne Mountain (which does the U.S. orbital tracking) and/or in Russia. They also don’t perform the actual maneuver. Again, that’s on flight control.
What the astronauts do is move to the Russian side of the station so that if something goes catastrophically wrong, they’re close to a Soyuz which is kept docked at all times to use as a lifeboat. Hopefully they never have to bail.
In the future it’s possible that a space station will be controlled by its crew. More likely, however, debris avoidance will be in the hands of the station’s AI, which can make the needed calculations much faster than any human.
Space debris is a growing problem, however, and what we really need is ways to get rid of it so the station…and any future orbital stations…don’t have to dodge quite so often.