Page last updated at 14:28 GMT, Sunday, 3 May 2009 15:28 UK

How satellites could 'sail' home

By Jonathan Amos
Science reporter, BBC News

Aerobraking prototype (EADS Astrium)
The booms can be inflated with gas but need then to be made rigid

Satellites and spent rocket stages could soon deploy "sails" to guide them back to Earth much faster than they would otherwise fall out of the sky.

With space becoming ever more crowded, there is a need to remove redundant objects that could pose a collision threat to operational missions.

Extending a sail on an old spacecraft would increase drag and pull it into the Earth's atmosphere to burn up.

Major European space firm EADS Astrium says the scheme has great potential.

"It is an interesting solution, especially for the satellite that has no propulsion system at the end of its life," Brice Santerre told BBC News.

Santerre and colleague Max Cerf have been working on the Innovative DEorbiting Aerobrake System (IDEAS).

The concept involves extending booms and sheeting from spacecraft to increase the amount of drag they experience from the residual air molecules still present at altitudes up to even 750km (470 miles)

"The principle of aerobraking is to increase the surface over mass ratio of an orbital object, to accelerate the fall-out by increasing the drag on the system," Mr Santerre said.

"To do that, we need to deploy a very light structure. That's why we chose to use 'gossamer structures'. These are composed of booms and very thin membranes."

Microscope (CNES)
Microscope will investigate the behaviour of free-falling objects

Astrium has been developing an aerobraking sail concept for the forthcoming French Microscope satellite.

Microscope is a science mission that will investigate the force of gravity and the behaviour of free-falling objects in a test of what has become known as the equivalence principle.

The satellite will take about a year to make its measurements and will then have no further purpose.

Ideally, such a spacecraft would be removed from orbit, especially since it will be circling at an altitude where many important Earth observation satellites also operate.

"Microscope has no propulsion system so it cannot de-orbit by itself. If we were to do nothing, the fall-out duration would be between 50 and 100 years," said Mr Santerre.

By erecting their boom and membrane mechanism, Santerre and Serf believe Microscope could be brought out of the sky in less than 25 years, which meets international orbital junk mitigation guidelines.

Astrium is now investigating how the IDEAS concept could be applied to spent rocket stages.

The company leads the production of Europe's premier launcher, the Ariane 5.

Microscope system (EADS Astrium)
The concept developed for Microscope would bring it back inside 25 years

Much of the Ariane's structure - its main core stage and solid boosters - fall rapidly out of the sky at the end of a flight; but the upper-stage is much longer lived in orbit.

Once it has ejected its satellite payload, the stage continues to circle the Earth in a large ellipse, running out to more than 35,000km from the Earth and coming as close as about 250km.

It may take 100 years before an upper-stage falls naturally from the sky.

"Our study shows that if we want to apply the aerobraking concept to an Ariane-class upper-stage then we need a system with booms, or masts, of about 12m and a deployed surface of about 250 sq m.

"This is possible with our current technologies. We need now to check that this is the best solution. We are also thinking whether this type of system can be applied to other launchers as well."

One alternative, of course, is to give the Ariane 5 upper-stage the capability to take a powered dive into the Earth's atmosphere.

Ariane 5 upper stage (EADS Astrium)
The Ariane 5 upper-stage continues to circle the Earth for decades

This was done for the first time last year at the end of the launch of the Jules Verne space station freighter. This was considered essential because of the number of manned missions that routinely follow the station's orbit.

Once Jules Verne was released from the rocket, the upper-stage reignited its engine to make a controlled burn-up over the Pacific.

The advantages of de-orbiting in this way are clear, but the extra fuel requirements and complexity of re-ignitable engines add cost to what is already a very expensive endeavour.

Aerobraking sails, on the other hand, are lightweight and extremely simple. Their operation could even be controlled by a pre-set timer, fixed to deploy a certain number of minutes after the end of a flight.

This means that even an upper-stage that is out of control can still be guaranteed to return to Earth in a timely fashion.

Santerre and Serf presented their latest research at the recent European Conference on Space Debris in Darmstadt, Germany.

The meeting closed with a statement from its organisers saying that effective measures to clean up space debris needed to be devised and implemented.

Artist's impression of ATV separation (CNES)
The upper-stage that launched Jules Verne took itself into a controlled dive

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