How Can Risk Be Controlled?
Risk is best controlled by limiting the creation of debris through mitigation. Unfortunately, debris mitigation usually increases mission cost. Some debris mitigation procedures have minimal impact on mission cost if they are specified early in the development phase. For example, deployment procedures can be designed to prevent ejection of objects. Tethered lens caps and bolt catchers for explosive bolts are examples of preventive design.
To prevent explosions, satellite components that store energy can be passivated at the end of useful life. For example, propellant in upper stages and satellites can be eliminated by either venting or burning to depletion. Batteries can be designed to reduce risk of explosion. Passivation may entail moderate cost during the nonrecurring phase of the mission, but cost during operation should be low.
To prevent debris accumulation from collisions in common orbits, satellites and other objects must be removed from the orbit at the end of life before collisions are likely to occur. This practice is called post-mission or end-of-life disposal. International guidelines for limiting orbital debris recommend that objects in low orbit be moved to a lower orbit when near their end of life such that they will reenter within 25 years.
Satellites, upper stages, and deployed objects in low Earth orbit can take advantage of Earth’s atmosphere to reduce time spent on orbit. At sufficiently low altitudes, atmospheric drag on the object will cause the object’s orbit to decay and result in reentry within 25 years. Vehicles at higher altitudes can perform post-mission maneuvers to drop the perigee (the point closest to Earth) further down into the atmosphere. Propellant must be reserved for this maneuver. Hence, the cost to satellites is reduced mission life and reduced performance to upper stages. If atmospheric decay is exploited to remove an object from orbit, then the risk posed to the ground by reentry of the object must be considered. Current guidelines call for a risk of no more than 1 in 10,000.
At altitudes above 2,000 km, it is often not feasible to force reentry within 25 years using current space technology. At this time, it is generally recommended to place vehicles in disposal (or “graveyard”) orbits. Spacecraft in geosynchronous orbits are typically boosted into a higher disposal orbit at the end of their mission life.
For many missions, it may be necessary to perform collision avoidance. The space station frequently maneuvers to avoid collisions with other objects. Many satellite operators routinely monitor future close approaches and sometimes maneuver to lower the risk of a collision.
Satellite operators can also manage risk by increasing redundancy in their designs, particularly relative fragile components such as solar arrays. Extra solar cells and alterations in the wiring of the arrays can reduce the effect on a mission if there is a small debris strike. Aerospace studies indicate that for most constellations of multiple spacecraft, it would be prudent to plan for a spare spacecraft.
In the future, it may be necessary to completely remove all non-operational satellites and upper stages from orbit.