LDEF in its 28.5 degree-inclination LEO orbit, based on the same calculations as Figure 4-9 and with the assumption that the relative velocities in that figure are due to circular orbits. For the same reasons that typical collision velocities change with inclination, the distribution of probable impact angles will be more tightly grouped around the direction of travel for spacecraft in higher-inclination LEO orbits and will be more widely distributed in GEO, where inclination differences between space object orbits are typically small. Debris in highly elliptical orbits may impact the sides and rear of a spacecraft more frequently than debris in circular orbits; such impacts were detected on the rear surfaces of LDEF.
Certain high-energy collisions may not just incapacitate a spacecraft, but actually fragment it into many small pieces. Although this distinction may not be important to a spacecraft designer, it is (as discussed in Chapter 8) very important for the future evolution of the debris population. As discussed in Chapter 2, models of such breakups are based on sparse data and contain large uncertainties. Current estimates indicate that complete breakup will occur 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 (McKnight, 1993). For example, a 0.1-kg piece of debris impacting at 10 km/s would probably not completely fragment the Japanese Astro-D spacecraft (420 kg), but a 0.5-kg piece of debris impacting at the same velocity probably would. Of course, the particular geometry of
BOX 4-3 Breakup of Space Objects Containing Radioactive Materials
Approximately 55 space objects containing radioactive materials currently orbit the Earth. Although no new spacecraft with nuclear power sources are currently planned for Earth orbit, it is possible that nuclear-powered spacecraft already in orbit may break up as a result of debris impact. Although many such spacecraft were designed so that their nuclear fuel would survive a launch pad explosion relatively intact, all are vulnerable to breakup if involved in a sufficiently high-energy collision.
Radioactive fragments from such a breakup would not interfere with observations by astronomers (only operating reactors produce detectable amounts of x rays or gamma rays), but might reenter the atmosphere sooner than they would have if the spacecraft had remained intact. These small fragments would burn up in the atmosphere, potentially resulting in a slight increase in the background health risk.