Marshall Space Flight Center, Alabama, and the Arnold Engineering and Development Center (Siebold et al., 1993). These medium-sized particles, which have lower characteristic ejection velocities and smaller area-to-mass ratios than the smaller particles, may also be longer-lived than the small particles and could pose a long-term hazard to other Earth-orbiting space objects.
Fragmentation debris—the single largest element of the cataloged Earth-orbiting space object population—consists of space objects created during breakups and the products of deterioration. Breakups are typically destructive events that generate numerous smaller objects with a wide range of initial velocities. Breakups may be accidental (e.g., due to a propulsion system malfunction) or the result of intentional actions (e.g., space weapons tests). They may be caused by internal explosions or by an unplanned or deliberate collision with another orbiting object.
Since 1961, more than 120 known breakups have resulted in approximately 8,100 cataloged items of fragmentation debris, more than 3,100 of which remain in orbit. Fragmentation debris thus currently makes up more than 40 percent of the U.S. space object catalog (and undoubtedly represents an even larger fraction of uncataloged objects). The most intensive breakup on record was the 1987 breakup of the Soviet Kosmos 1813, which generated approximately 850 fragments detectable from the Earth. The fragmentation debris released from a breakup will be ejected at a variety of initial velocities. As a result of their varying velocities, the fragments will spread out into a toroidal cloud that will eventually expand until it is bounded only by the limits of the maximum inclinations and altitudes of the debris. This process is illustrated in Figure 1-5. The rate at which the toroidal cloud evolves depends on both the original spacecraft's orbital characteristics and the velocity imparted to the fragments; in general, the greater the spread of the initial velocity of the fragments, the faster will the evolution occur.
In contrast, debris fragments that are the product of deterioration usually separate at low relative velocity from a spacecraft or rocket body that remains essentially intact. Products of deterioration large enough to be detected from Earth are occasionally seen—probably such items as thermal blankets, protective shields, or solar panels. Most such deterioration is believed to be the result of harsh environmental factors, such as atomic oxygen, radiation, and thermal cycling. During 1993 the still-functional COBE (Cosmic Background Explorer) spacecraft released at least 40 objects detectable from Earth—possibly debonded thermal blanket segments—in a nine-month period, perhaps as a result of thermal shock.