Space sail could bring used rockets back to Earth

THE risk to spacecraft from a collision with space debris could be reduced by equipping launchers with a gossamer-thin "sail". The idea is to deploy the sail after the rocket has released its payload to amplify the drag of the last vestiges of the atmosphere, and so force the rocket out of orbit.

Rocket stages are a particular risk to spacecraft because they often contain large amounts of unused fuel, which can explode when sunlight heats the tank. Leaking fuel can also act like a mini-thruster, pushing the rocket into an orbit where it may cause a collision. One way to tackle the problem is to vent unused fuel in a controlled way, and drain power from the battery, but this is unlikely to eliminate all collisions.

Now space-flight engineers Max Cerf and Brice Santerre at the European aerospace firm EADS Astrium are devising ways to build a sail that would quickly remove a spent rocket from orbit. The sail or "aerobrake" would be deployed after a rocket has delivered its satellite into low-Earth orbit, slowing it down by friction with the thin atmosphere so that burns up in around 25 years, much earlier than conventional rocket stages, some of which are expected to survive for at least 100 years.

The aerobrake would be deployed after the rocket has delivered its satellite into low-Earth orbit

For the final stage of an Ariane 5 launcher, the conical sail would need to have an area of about 350 square metres and be supported by an inflatable mast 12 metres long. Cerf and Santerre propose a number of possible ways to build the mast. The simplest envisages a woven polymer and aluminium tube that is kept inflated by nitrogen gas. Another uses a tube made of polymer composite, which after being inflated with nitrogen is set hard by the sun's ultraviolet rays. A third design uses epoxy resin that is set hard by solvent evaporation.

The pair revealed their designs at this month's Fifth European Conference on Space Debris in Darmstadt, Germany, organised by the European Space Agency.

It's a good idea, says Peter Roberts, a space-flight engineer at Cranfield University in the UK, who is working on similar technology for small satellites. "The risk of fragmentation of end-of-life spacecraft due to impacts from other debris can be greatly reduced by deploying a drag sail."

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