By Philip Hattis, Ph.D., laboratory technical staff, Draper Laboratory (www.draper.com), Cambridge, Mass.
Imagine a world where low Earth orbit (LEO) satellite functions such as hurricane tracking, climate change and natural disaster monitoring or arms treaty verification were no longer viable. That could happen if we don't eliminate orbital debris, which includes dead satellites, spent rocket stages used to launch satellites, pieces of spacecraft shed intentionally during satellite deployment or by accident, and many fragments of satellites and stages.
Recent satellite collisions may be an initial manifestation of the Kessler Syndrome. More than 30 years ago, NASA astrophysicist Donald Kessler, a specialist in space debris, predicted that collisions would occur as Earth's orbital debris population grew, creating more debris and resulting in increased collision frequencies and risk to spacecraft.
The velocity of an LEO object makes its collision with another object potentially lethal. A collision's energy release is like a giant explosion that fragments the colliding objects and creates a spreading cloud of new debris, with each piece ending up in its own unique, altered orbit”often for centuries.
Given the energy of orbiting debris, an object as small as one centimeter can destroy a satellite with which it collides. It's estimated there are more than 500,000 such objects orbiting Earth.
Sophisticated models of the current LEO environment indicate it's unstable. Collision frequency is projected to result in one major accident involving a live satellite about every five years, but the frequency will increase as collisions create more debris.
LimitedÂ ActionsÂ UnderÂ Way
Limited actions are being taken to reduce the likelihood of satellite collisions. Application of standards by some countries now reduces debris generation from newly launched objects. This is accomplished by satellite designs that avoid intentional shedding of objects that remain in orbit.
New, spent rocket boosters now are vented at the end of their mission to prevent their subsequent explosion in orbit. Efforts also are under way to improve surveillance of space objects to better predict potential collisions. This would allow live spacecraft with maneuver capability to better evade objects with which they are at risk of collision.
But none of these measures reduce the number of objects already in orbit, prevent collisions of dead objects or stop generation of debris from intentional anti-satellite weapon tests. Additionally, because objects smaller than 10 centimeters aren't currently seen and tracked, their collisions with spacecraft can't be anticipated and evaded.
Long-TermÂ SolutionsÂ Needed
In the long term, the only solution to assuring that LEO remains safe is to find ways to remove debris, thus addressing the technical and political challenges posed by implementing solutions to the menace. The following recommendations are necessary steps to begin tackling orbital debris issues.
Put new international law and/or agreements into place. Under current international law, debris remains the sovereign property of whoever launched it. Consequently, one nation can't legally remove debris put into orbit by another nation without prior approval.
Develop and demonstrate means to enable debris removal. This must be pursued to the point where the means are proven capabilities widely accepted by all space-faring parties.
Allow international oversight in implementation and monitoring of orbital debris mitigation technology. This is necessary to avoid disputes. Orbital debris removal isn't just a technology problem, because the debris removal systems may also have the potential of being used to harm or dispose of operational satellites.
Fairly share the costs of debris removal. If the cost of debris removal multiplies the net cost of space operations for any single party, then it remains unattractive to address the debris problem. Means must be found both to cover the cost of debris removal and to share equitably that cost among all users of Earth orbit. Even an economical debris removal system will have substantial net cost. Funding this will probably require additional international agreements, which would share the debris removal system costs across all spacecraft operators and maybe across users of services provided by the spacecraft as well.
If international policy issues can be addressed, then necessary technology development and debris removal demonstrations can be pursued. The good news is that awareness within the space operations community of the menace posed by orbital debris has increased greatly in recent years, and many promising mitigation and removal technologies have been proposed.
Editor's Note: This column was adapted from an article that appeared in Livebetter, an e-magazine produced by the Center for a Better Life.