functional lifetime—if possible, in a manner that moves the spacecraft into an orbit that reduces its long-term contribution to the debris hazard.

Ending or reducing deliberate spacecraft breakups would also, of course, reduce the spacecraft fragment population. Historically, spacecraft have been broken up deliberately for structural testing, to destroy sensitive equipment so that it would not be recovered by hostile forces, and in antisatellite weapons tests (Johnson and McKnight, 1991). Deliberate breakups are believed to account for slightly more than one-third of all spacecraft breakups. Another 20 percent of all spacecraft breakups may be due to the unintentional detonation of on-board self-destruction systems. Combined, these types of breakups are the source of approximately 6 percent of the current cataloged space object population. Deliberate breakups of spacecraft about to reenter the atmosphere do not contribute greatly to the debris hazard; the debris created in such events is typically ejected into orbits that decay rapidly. Fragments from intentional breakups at high altitudes (>600 km) can, however, remain in orbit for thousands of years or more. Ensuring that any future deliberate spacecraft breakups are not carried out in high orbits would help contain the future debris hazard.

Passivation of Rocket Bodies

Debris generated through the explosive breakup of liquid-fueled rocket bodies after they have completed their missions makes up 25 percent of the cataloged space object population, and probably a large fraction of the uncataloged large and medium-sized debris population. Rocket body breakups are believed to be caused most often by the residual propellant (as much as several hundred liters) that may remain in the rocket body's fuel and oxidizer tanks at the end of a mission. Explosions that break up rocket bodies are caused most often by accidental mixing of the components of this residual propellant or by physical factors such as overpressure.

Accidental mixing occurs most commonly in rocket bodies that store fuel and oxidizer in thin tanks with a common bulkhead. During ground handling and launch, a positive pressure difference exists between the oxidizer tank and the fuel tank, but after spacecraft separation, this pressure difference can vanish due to leaks in pipes and valves, resulting in damage to the bulkhead. Fuel and oxidizer are then able to mix through the damaged bulkhead, leading to an explosion. The bulkhead also can rupture from corrosion or thermal stress; thermal stressing of a fuel tank bulkhead may have led to the breakup of seven Delta rocket bodies. Fragmentations caused by accidental fuel mixing can be extremely energetic, because of the large amount of fuel that may be involved.



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