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and below the breakup altitude. This increase in spatial density can persist for long periods of time; the higher the altitude, the longer will the spatial density remain elevated. Since explosions can produce a considerable amount of large and medium-sized debris with potentially long orbital lifetimes, reducing the creation of debris from explosions will clearly have a major effect in containing future growth in the debris hazard. A reduction in the frequency of explosions can be achieved by passivating spacecraft and rocket bodies.
Passivation of Spacecraft
Debris from spacecraft explosions makes up about 12.5 percent of the cataloged space object population. Spacecraft can explode both during and after their functional lifetime for a wide variety of reasons, including propellant tank explosions, thruster malfunctions, tank failures due to the impact of small debris, battery ruptures, accidentally induced high rotation rates, other degradations of the structure, or deliberate explosions. There are correspondingly many possible measures to prevent spacecraft breakups. There is no one single remedy, and there is probably no possible way to avoid all future spacecraft breakups: despite safeguards, a residual number of spacecraft breakups will continue to generate debris, if at a reduced level.
However, spacecraft designers can take a general system-level approach to prevent accidental spacecraft breakups. The approach is (1) to determine all potential sources of stored energy remaining on a spacecraft late in its active life; (2) for each source, to provide a method of dissipating the stored energy in a benign manner; and (3) to activate these means at the end of the spacecraft's functional lifetime (i.e., "passivate" the spacecraft). Protecting the spacecraft from debris impact damage, as well as other methods to increase spacecraft survivability, can help ensure that the spacecraft is capable of carrying out passivation measures at its EOL.
The "passivation" approach described above can be applied to numerous spacecraft subsystems. For example, spacecraft batteries are sources of stored energy believed responsible for a few breakups. To prevent such breakups, designers can implement a battery management system that ensures that the batteries will be left in a completely discharged state at the end of the spacecraft's functional lifetime and will be short-circuited to prevent recharging. Implementation of this system would prevent inadvertent overcharging, which can lead to battery rupture and potentially break up spacecraft. Another example of this approach would be to ensure that all residual propellants and stored pressurized gas in the spacecraft are vented at the end of the spacecraft's