where D is the average diameter in centimeters. The average relative velocity is usually assumed to be lOkm/s. The requirement to withstand such impacts is obviously very challenging.
Although local shielding of certain critical components or areas is possible, as is done, for example, on the International Space Station, completely armoring a spacecraft is not practical from a mass standpoint and in some cases may not even be possible. At this point, the most practical strategy may be to avoid high-probability orbits. As mentioned, the problem is expected to increase in severity for some years before greater awareness and increased use of various mitigation strategies begins to reverse the trend. A spate of antisatellite (ASAT) vehicle tests of the type conducted by the USSR on several occasions, and by the United States in September 1985, could greatly aggravate the problem.
As an illustrative example of the effects of hypervelocity impact on orbital clutter, the September 1985 test, in which the P78-1 SOLWIND satellite was destroyed by an air-launched ASAT rocket, was estimated to have created approximately 106 fragments between 1 mm and 1 cm in diameter. This event alone thus produced, at an altitude sufficient to yield long-lived orbits, a debris environment in excess of the natural micrometeoroid background.
It is possible to conduct such tests in a more suitable fashion. In September 1986, the U.S. Department of Defense (DoD) Strategic Defense Initiative Organization conducted a boost-phase intercept test involving a collision between an experimental interceptor and a Delta 3920 second-stage rocket in powered flight. A direct hit at a relative velocity of approximately 3 km/s ensued. The chosen intercept altitude of about 220 km, which then became the highest possible perigee point of any collision debris, ensured that the residue from the collision remained in orbit for at most a few months.22
Numerous national and international efforts have been undertaken to increase the level of awareness of the space debris problem and to develop and promulgate mitigation strategies for the future.19 Among the recommended approaches are (1) cessation of deliberate spacecraft breakups producing debris in long-lived orbits; (2) minimization of mission-related debris generation; (3) passivation of spacecraft and rocket bodies remaining in orbit after mission completion, i.e., expending residual propellants, discharging batteries, venting tanks, etc.; (4) selection of transfer orbit parameters to ensure reentry of spent transfer stages within 25 years; and (5) boosting separated apogee kick motors, other transfer stages used for geostationary spacecraft circularization, and defunct geostationary satellites to an altitude at least 300 km above the geostationary ring.
Mitigation measures such as these obviously place an additional burden on space vehicle design and operation not present in earlier years. For this reason, while international cooperation over debris mitigation has increased in recent years, full compliance continues to elude the space community. Space operations and plans must increasingly take into account strategies for avoiding, or coping with, orbital debris. For example, in the five years between 1989 and 1994, the space shuttle received four collision-avoidance warnings and acted upon three of them.23 It has been estimated that the International Space Station can expect to receive about 10 collision avoidance warnings per year of sufficient concern that an avoidance maneuver could be required.24
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.