How is NASA's ISS protected from space debris?

The movie Gravity showed us what could happen if a space station gets hit by space debris. But how likely is such an event in real life?

Photo by: Facebook ISS Page
Photo by: Facebook ISS Page

The ISS appears vulnerable in the vast expanse of space.

This question originally appeared on QuoraWhat is the likelihood of something like space debris damaging satellites or astronauts like shown in the movie Gravity? What safeguards does NASA provide? Answer by Robert FrostInstructor and Flight Controller at NASA

Impacts with space debris and micrometeorites happen fairly often.  And they become more common as more junk is put into space.  With each collision, a large piece of junk can become several smaller pieces of junk.  Fortunately, so far, the damage has usually been minor.

The two worst incidents, so far, have involved destruction of satellites.  In 2009, a 900 kg failed Russian satellite collided with one of Iridium's communications satellites, destroying them both and creating hundreds of pieces of dangerous debris that now must be tracked.  That was an accident, but shows the importance of designing safe retirement into satellites.

The other incident was just sheer stupidity.  In 2007, the Chinese government deliberately destroyed a satellite with a missile, just to show they could.  That destruction strewed thousands of pieces of debris into a cloud ranging from an altitude of 200 km to 3850 km - effectively all of LEO.  About 1000 pieces of the debris are greater than 10 cm - making them quite deadly.

Most objects in space are either international or commercial property - so NASA has no real control over those to institute safeguards.  The ISS, however, was designed with the threat of small impacts in mind.  Everything in space travels at the speed dictated by its orbit and everything is in an orbit.  Plotting a risk graph over the ISS reveals a butterfly like shape.  The greatest potential for damage occurs when the ISS and the approaching debris are traveling in opposite directions, because their velocities are then combined.  The ISS travels at 7.8 km/s.  An object in the same orbit coming from the other direction would also be traveling at 7.8 km/s so the impact speed would be 15.6 km/s.  However, there isn't anything in the ISS orbit traveling in the opposite direction, because if there were, we would hit it every 45 minutes.

The greatest threat comes from debris approaching from about 2 o'clock or 10 o'clock - for it is clearly in a different orbit but has a very large relative speed.  So, at the front of the station, with those trajectories in mind, the ISS has the greatest shielding.  The ISS has shields called Whipple bumpers.  They are multi-layered with spaces between the layers.  The intent is that impact with a layer will both slow and hopefully break apart the projectile, so that by the time it gets to the bottom layer it is no longer harmful. 

Should a projectile puncture the hull into the pressurized cabin, the crew have patch kits that they can use to plug the hole.

The cabin has not been penetrated, but there are several punctures on the vehicle.  The solar arrays, radiators, some handholds and some trusses have scars from hits.  The crew has occasionally been made aware of a hit by hearing a loud banging noise.

If the object is in LEO and larger than around 10 cm, the ISS can be warned and moved, a few orbits in advance of the potential impact.  For smaller objects, they just accept the risk.

TRTWorld and agencies