Commentary | Rethinking Space Debris Mitigation
We should reconsider the scope and application of measures to mitigate space debris. The same constraints need not and probably cannot be imposed on all satellites.
Space debris is a serious environmental threat. The consequences of collisions between satellites may be unacceptable. We must mitigate low-probability, high-consequence events.
However, not all mitigation guidelines may be required for all satellites, and some classes of satellites cannot satisfy all of the guidelines.
Voluntary debris mitigation guidelines were published in 2002 by the Inter-Agency Space Debris Coordination Committee (IADC), an advisory body composed of representatives of the 12 principal governmental space agencies. After five years of deliberation, most of these achieved consensus among the 69 (now 77) nations represented in the U.N. Committee on the Peaceful Uses of Outer Space. These led to a voluntary international standard for debris mitigation (ISO 24113) and several standards and technical reports, describing what should be done but not how to do it, vetted by the 18 national standards bodies represented in International Organization for Standardization (ISO) space operations subcommittees. These cover avoiding collisions, estimating orbit lifetime, depleting stored energy, and disposing of debris from low and high Earth orbit.
It has taken a long time to achieve consensus in any of these bodies.
The preponderance of satellites are civil, commercial and military, but these sectors had almost no voice, even though they would be affected most. Small satellites cannot meet many of the guidelines. Cubesats have been placed in long-lived orbits such as geostationary transfer orbit. If a 1,000-cubic-centimeter cubesat were all high-energy-density cryogenic hydrogen propellant, it would contain only about half of the energy required for deorbit. The surface area of a cubesat is hardly sufficient to provide enough solar electric energy for reasonable deorbit with electric thrusters.
Other guidelines are difficult to enforce even if they are codified in national laws, as France has done. Propagating orbits far into the future is notoriously imprecise. Compounding this with great uncertainty in the strength and occurrence of solar maxima and minima makes orbit lifetime estimates over a couple of decades notoriously imprecise.
ISO’s attempts to establish a uniformly understood lifetime estimation analysis have failed. Everyone has his own, few are willing to accept any other, and estimates vary by many years.
Disposing of spent boosters is similarly hampered because few have both the residual propellants and enduring guidance and control to achieve controlled re-entry. More so for the emerging generation of small boosters for small satellites.
There have also been well-motivated and diligently considered decisions arguably contravening the guidelines. Satellite lifetimes have been extended beyond planned mission end. This risks insufficient propellant for safe re-entry, and failure of subsystems beyond their planned life can disable satellites, making them space debris.
All satellites do not pose the same collision risk, and all do not produce the same consequences. Collisions are avoided and debris prevented by choosing orbits wisely, accommodating the disparate risks of diverse satellites, and disposing safely at end of mission — not waiting 25 years or gaming what 25 years is.
One size does not fit all, but IADC guidelines are indiscriminate.
Dave Finkleman is chief engineer for SkySentry LLC and chairman of the ISO Space Operations Working Group. He is an academician of both the International Academy of Astronautics and the International Institute of Space Law.