Space Debris Basics

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. Cost during operation should be low.

To prevent debris accumulation in preferred mission orbits due to collisions, satellites and other objects must be removed from the mission orbit at the end of life before collisions are likely to occur. NASA's guidelines for limiting orbital debris recommend that an object not remain in its mission orbit for more than 25 years.

The Aerospace Corporation's Collision Vision workstation is used to determine and visualize potential close-approach situations. This tool helps mission designers and controllers assess risk during launch and on-orbit operations.

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 (orbital 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 to upper stages it is reduced performance. If atmospheric decay is exploited to remove an object from orbit, then the risk posed to the ground by re-entry of the object must be considered.

At altitudes above 2,000 km, it is 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. Many spacecraft in geosynchronous orbit are already boosted into a higher disposal orbit at end of mission life.

For some missions, it may be necessary to perform collision avoidance. The space shuttle orbiter has maneuvered to avoid collisions with other objects on several occasions. Regarding satellite constellations, if a potential collision will lead to the creation of a debris cloud that may result in damage to other constellation members, it may be worthwhile to perform a collision avoidance maneuver.

In the more distant future, it may be necessary to completely remove all satellites and upper stages from orbit. This removal will not be feasible until new technology is developed.