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
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.
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.
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