In the 1970s, when the space shuttle was being developed, orbital debris was not recognized as a significant threat to spacecraft. During the 1980s and 1990s, however, extensive data gathering and analysis greatly improved understanding of this growing hazard. The National Aeronautics and Space Administration’s (NASA) models of the space environment now suggest that meteoroids and orbital debris pose a significant threat to the shuttle orbiter.
The shuttle orbiter has already been struck many times by small meteoroids and orbital debris, but it has not been damaged severely (although NASA now replaces pitted orbiter windows after most flights). The potential exists, however, for more serious damage. Objects ranging from paint chips to fragments of exploded rocket upper stages to intact spacecraft orbit through the regions in which
Meteoroids are created from the breakup of asteroids and comets. They orbit the Sun and rain steadily on the Earth and on objects in Earth orbit.
Orbital debris is human-generated and orbits the Earth. All nonfunctional objects in Earth orbit are considered to be debris (NRC, 1995). This debris can be anything from a piece of paint that has flaked off of a rocket or a spacecraft to fragments of an exploded rocket upper stage to an entire derelict spacecraft.
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.
In this report, the committee examines NASA’s strategy for protecting the shuttle orbiter from meteoroids and debris and recommends new strategies where appropriate. Chapter 2 examines the hazard to the orbiter and crew from meteoroids and orbital debris. Chapter 3 reviews the shuttle program’s risk assessment and risk management strategies, and Chapter 4 looks at the tools NASA uses to assess the risk. Chapter 5 explores the use of collision warning and avoidance systems, and Chapter 6 describes steps that can be taken to improve the shuttle’s survivability.
NRC (National Research Council). 1995. Orbital Debris: A Technical Assessment. Committee on Space Debris, Aeronautics and Space Engineering Board. Washington, D.C.: National Academy Press.