the shuttle operates. The speed at which objects in low Earth orbit (LEO) can collide makes these objects dangerous—at typical impact velocities of 10 kilometers per second (km/s), even millimeter-sized objects can cause considerable damage. Only the very largest objects are tracked and monitored from Earth; the locations and trajectories of the vast majority are unknown.
Because it was designed to be launched into space and return safely to Earth 100 times, the shuttle orbiter is fairly rugged. However, because it was not designed with the meteoroid and orbital debris hazard in mind, some orbiter components are vulnerable to impact damage. This can include damage that does not affect a mission but increases refurbishment costs (such as damage to window surfaces); damage that might force the crew to abort a mission (such as the penetration of a radiator pipe); damage that would prevent the orbiter from successfully returning to Earth (such as a large hole in the leading edge of a wing or the nose cap); and damage that would result in the loss of life or the vehicle (such as a collision with a large fragment from the breakup of a spacecraft).
For years, the space shuttle program has had the ability to move the orbiter out of the path of pieces of debris that are large enough to be tracked by ground-based sensors. More recently, the shuttle program office has planned missions so that, whenever possible, the orbiter maintains orientations that protect its most vulnerable components from the greater part of the meteoroid and orbital debris flux. In the near future, the program plans to shield some of the orbiter’s most vulnerable components against meteoroids and orbital debris.
The shuttle orbiter circles the Earth at a velocity of about 7.5 kilometers per second (about 17,000 miles per hour) a few hundred kilometers above the surface of the Earth. Its orbit is inclined to the equator, usually by 28.5 degrees (for maximum payload mass) or 51.6 degrees (typically to rendezvous with a space station).
Debris orbits the Earth in a tremendous variety of circular and elliptical orbits at different altitudes and with different inclinations. When the orbiter’s trajectory intersects the orbit of a piece of debris, the two objects are generally heading in different directions at high relative velocities.
When the shuttle is in a 51.6 degree inclination 400 kilometer altitude orbit, NASA’s model of the debris environment predicts an average collision velocity of 9 kilometers per second for orbital debris with a diameter of 1 centimeter or more.