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Detecting Orbital Debris
NASA goes to great measures in preventing damage to the International Space Station and its Space Shuttles from the millions of pieces of debris hurtling through space.

There are two categories of objects that can collide with spacecraft: orbital debris and meteoroids. Orbital debris refers to the man-made items, such as fragments of rocket bodies, stray nuts and bolts lost on a space walk, paint chips, and other leftovers from human activity. Meteoroids are natural substances: bits of comets and fragments of asteroids that remain in space.

Both can impact spacecraft in Earth orbit. Orbital debris averages 9 kilometers per second and meteoroids average 20 kilometers per second, which is much faster than a rifle bullet in both cases. At these speeds, both have a probability of causing impact damage to the Space Station, Space Shuttle, a satellite, or other spacecraft.

Orbital debris streaking across the sky.Image to right: Orbital debris as small as a grain of salt can move at speeds of 27,535 kilometers per hour. Credit: NASA

There are approximately 9,000 items larger than a softball orbiting the Earth, all of which are tracked and monitored by the Department of Defense. If one should be headed on a collision course with either the Space Station or the Shuttle, mission control is informed and the crew takes evasive action. However, the orbits of the smaller bits can't be accurately tracked, which makes avoiding collisions more difficult.

Much of the Space Station's pressurized modules are made of aluminum; Space Shuttles are constructed of tile and ceramic fabric panels and thin composite leading edges only 1/4-inch thick. All of these materials are susceptible to damage. So how have researchers helped reduce risk?

In space, the Shuttle flies backward and upside down most of the time, so that it faces away from most debris. There are more than 200 different kinds of shields protecting the Space Station from impacts, and as a group, they're referred to as "Whipple shields." They are named after Fred Whipple, who came up with the idea of protecting spacecraft from high-speed collisions with pieces that are ejected from comets as they circle the Sun. The Whipple shields on the Station have been enhanced with ceramic and Kevlar materials, the same types used in "bullet-proof" vests, to improve impact protection.

Hubble Telescope was damaged by orbital debris.Image to right: Orbital debris created a hole in the Hubble Space Telescope. Credit: NASA

If debris caused a penetrating hole in the pressure shell of a Space Station module or Shuttle crew cabin, the depressurization alarm would sound and the trained crew would locate, patch or isolate the leak. So far, the Shuttle and Station haven't experienced that type of situation. In the Shuttle's case, evidence of contact with debris has been found from high-resolution photo surveys while in orbit and inspections after the vehicle is safely home on the ground.

When the Space Shuttle returns to Earth, the vehicle is carefully inspected for impacts. Most that occur are spotted on the windows of the Shuttle, as well as the radiators. So far, the largest crater found on the Shuttle was 17 mm, approximately the size of a dime.

Some damage is expected and acceptable on space missions. Calculating and evaluating the risk from impact collisions by meteoroids and orbital debris is a complex procedure. NASA employs a combination of techniques to reduce the risk to spacecraft from impact by these high speed particles including collision avoidance from ground-trackable debris, operational procedures and state-of-the-art shielding.

NASAexplores and NASA's John F. Kennedy Space Center