Dealing with Envisat
As unlikely as it seems, one of the biggest known threats to a stretch of low Earth orbit heavily used for weather forecasting and environmental monitoring is a civilian craft whose mission has been so successful it recently was extended three years. Equally surprising might be the fact that the satellite’s owner is the 18-nation European Space Agency (), which takes very seriously its responsibility to preserve the orbital environment.
According to space debris experts, Envisat, ESA’s 8,000-kilogram Earth-observing flagship, will become an orbital time bomb upon retirement in 2013. With its fuel expected to be exhausted by then, the expansive craft — measuring 26 meters by 10 meters by 5 meters — will linger in its near-polar orbit for some 150 years, unable to move out of the path of space junk. The satellite recently maneuvered to avoid a catastrophic encounter with a 1,500-kilogram Chinese rocket stage, an event that likely would have created 10 times as much space junk as last year’s collision between an operational communications satellite and a spent Russian craft.
Experts say there is a 15 to 30 percent chance of a collision involving Envisat in the next 150 years, but that’s only if no debris is created during the period — a completely unrealistic scenario — so the odds are actually higher. NASA estimates that even if all space launch activity were to come to an immediate halt, the amount of orbital debris would continue to rise for 50 or so years due to collisions and break-ups among stuff that’s already up there.
Envisat is problematic not just because of its size but also because it is bristling with antennas and other gear prone to splintering if hit even by a relatively small piece of debris. As Envisat ages in the harsh space environment, this hardware will become more brittle, magnifying that effect should a collision occur.
ESA officials say steering Envisat into the atmosphere over an unpopulated area is not an option. For reasons that include industrial policy and the desire to keep costs down, the satellite was outfitted with a copy of the fuel tank used on the much smaller Spot 4 optical Earth observation satellite; preserving enough fuel for a controlled re-entry would have meant limiting the $2.9 billion mission to just a few months. Also out of the question is putting Envisat in an orbit that over the next 25 or so years would decay to the point that the satellite would re-enter on its own — large pieces of the craft likely would survive re-entry and crash into the ground at locations that are impossible to predict.
Could ESA have planned better? Absolutely. Experts were aware of the growing space junk problem in the 1990s when the mission was being designed; minimizing Envisat’s potential as a debris hazard clearly didn’t get the priority it deserved.
That said, the magnitude of the problem was not nearly as well appreciated then as it is today. It wasn’t until mid-2007 that the United Nations Committee on the Peaceful Uses of Outer Space endorsed debris mitigation guidelines developed five years earlier by the Inter-Agency Space Debris Coordination Committee, whose members include the leading spacefaring nations. In 2006, ESA and several national space agencies adopted a European Code of Conduct that tracks closely with the voluntary U.N. guidelines. ESA also came out with specific debris mitigation requirements that took effect in 2008 and apply to all agency space missions and procurements. If Envisat were proposed today with the same design and mission profile, it probably would not be approved.
Knowing how not to design future missions, while obviously important, won’t help mitigate the danger at hand. One option available to ESA and the rest of the spacefaring world — Envisat is ESA’s responsibility but everyone’s problem — is to simply live with the hazard and hope for the best.
But there might be a better alternative. Several designs exist today for orbital tugs that would serve as surrogate propulsion systems for aging spacecraft, extending their lives or moving them to locations where they can do no harm. NASA has examined sending a propulsion module to de-orbit the massive Hubble Space Telescope, which otherwise will re-enter the atmosphere uncontrolled in the not-too-distant future. Industry has designed systems with an eye toward making a business of extending the lives of on-orbit craft. Space robotics specialist MacDonald, Dettwiler and Associates, for example, is designing a spacecraft that could perform multiple on-orbit service missions over a five-year period, but the company recently disclosed it would drop the effort if a customer is not found soon. The just-released U.S. National Space Policy, meanwhile, calls for developing “technologies and techniques” for removing space debris or otherwise reducing hazards; Envisat certainly qualifies in both cases.
A robotic mission to extend the life of Envisat or guide it to a safe atmospheric re-entry — or both — is by no means far-fetched. With all the on-orbit servicing design and testing activity that has taken place in recent years, ESA — perhaps in cooperation with NASA and others — has both commercial and government-led options to consider. It’s not too early to begin thinking seriously about the technical and economic feasibility of such a mission, but when it’s too late, it really will be too late.