April 27, 2009 We’ve recently examined the danger posed to future space missions by the ever increasing collection of space junk orbiting the Earth. Now a plan by a pair of space engineers to use a sail to take out the trash – or rather, bring it back to Earth - may help to stop future space missions adding to the problem of space junk.

Recent near misses between orbital debris and the space shuttle on its mission to service the Hubble Space Telescope only serve to highlight the problem. In fact, NASA’s Steve Stich believes the risk of a micrometeroid or orbital debris (MMOD) hit is the greatest risk to the shuttle program. According to the Hypervelocity Impact Database obtained by Wired Science, in the 54 missions from STS-50 through STS-114, space junk and meteoroids hit shuttle windows 1,634 times, which required 92 window replacements, while the shuttle's radiator was hit 317 times, punching holes in the radiator's facesheet 53 times.

While none of those impacts was enough to endanger the crew or the shuttle, the frequency of such incidents suggests it is only a matter of time before sufficient damage is inflicted to threaten the lives of the crew. As reported recently in New Scientist, the plan, hatched by space-flight engineers Max Cerf and Brice Santerre at the European aerospace firm EADS Astrium, involves deploying a sail or “aerobrake” from spent rockets after they delivered their satellite into low-Earth orbit.

Friction between the sail and the thin atmosphere would slow the rocket down so that it burns up in about 25 years. That may sound like a long time, but it is much shorter than the lifespan of many conventional rocket stages, which can expect to survive for more than 100 years.

The sail's design for the final stage of an Ariane 5 launcher, revealed at the Fifth European Conference on Space Debris in Darmstadt, Germany, is conical with an area of about 350 square meters supported by an inflatable 12-meter mast. Cerf and Santerre propose a number of ways to build the mast. The simplest would be to use a woven polymer and aluminum tube that is kept inflated by nitrogen gas. Another method involves the uses a tube made of polymer composite, which would be set hard by the sun's ultraviolet rays after being inflated with nitrogen. A third design uses epoxy resin that is set hard by solvent evaporation.

Unfortunately, debris mitigation systems, such as the sail proposed by Cerf and Santerre, only address half of the problem. The amount of debris would still continue to increase, even if humans were to stop contributing to the existing amount of orbital debris tomorrow. This is because a collision between existing debris results in more debris that can then collide with other debris, causing more debris, and so on.

To fully address the problem, mitigation systems that stem the rise in debris would need to be used in conjunction with debris remediation systems that remove existing debris. At the moment, there aren't any viable solutions, so we must hope a solution is found in the near future, before the escalating amount of debris makes space exploration, and even the use of satellites, extremely dangerous, if not unfeasible, for generations to come.

Darren Quick