removal to as much as 5 percent of the exposed paint areas (Coombs et al., 1992). Such loss of thermal control area is probably a minor issue, however, because it can be handled easily through oversizing in design. (Oversizing and other operational protection schemes are discussed in Chapter 6.) Perforation of thermal control blankets can also damage thermal control systems by delaminating layers and exposing protected components to the space environment (Allbrooks and Atkinson, 1992; Meshishnek et al., 1992).

Finally, small debris impacts can damage spacecraft solar power systems. Effects of debris impact can range from localized damage to cover glasses and solar cells to failure of strings of cells. Impacts can perforate or break exposed spacecraft cabling (including power cables), causing short circuits or failures. In addition, even small debris impacts can create plasmas, which can couple into solar arrays, causing failures (Krueger, 1993). Because of the phenomena associated with perforation through thin (compared to the impacting particle's diameter) films, however, the newer thin-film solar cell technologies are less susceptible to large-scale damage from small impacts.

Finding 1: The probability that a spacecraft will be struck by debris is dependent on the spacecraft's orbital altitude and, to a lesser extent, its orbital inclination. The orbital regions where impact with medium or large debris is most likely are those between about 750- and 1,000-km altitude and those around 1,500-km altitude. Spacecraft in semisynchronous orbits or GEO are, on average, probably about 100 times less likely to be struck by debris than most LEO spacecraft, and spacecraft not in any of the heavily used orbital regions (LEO, semi-synchronous orbit, or GEO) are even less likely to collide with debris.

Finding 2: Current models indicate that a collision in orbit will result in complete breakup if the ratio of the impactor's relative kinetic energy to the mass of the object with which it collides is greater than about 40 J/g. In LEO, debris as small as 0.1% of a space object's mass can cause the object to break up into many fragments. A typical LEO catastrophic collision involving a spacecraft may eject tens or hundreds of fragments large enough to cause a breakup if they collide with another spacecraft. At higher altitudes, where collision velocities are slower, a much larger impactor would be needed to cause catastrophic breakup.

Finding 3: Impacting space objects not large enough to break up a spacecraft can still cause significant damage through a variety of mechanisms,