May 4, 2009

‘Sails’ Could Return Objects Rapidly To Earth

With space becoming increasingly more crowded, scientists are turning to "sails" to help satellites and other orbiting space objects return more rapidly to Earth. 

Researchers hope the approach will help minimize the number of redundant objects in orbit, thereby reducing collision threats to operational missions.

Extending a sail on an old spacecraft would increase its drag, pulling the vehicle back in to the Earth's atmosphere to burn up.

It's a concept with great potential, said European space company EADS Astrium.

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

Santerre and his colleague, Max Cerf, have been collaborating on something they call the Innovative DEorbiting Aerobrake System (IDEAS).

The idea of the sails involves extending sheeting and booms from space objects to increase the amount of drag they experience from the residual air molecules present at altitudes up to 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 told BBC News.

"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," he said.

Serf and Santerre have been developing the aerobraking sail concept for the upcoming French Microscope satellite, a science mission that will study the force of gravity and the behavior of free-falling objects in a test of the "equivalence principle".

The satellite will take about a year to obtain its measurements, but will serve no purpose beyond that.

In a perfect world, it would be removed from orbit once its mission was complete, particularly since it will be orbiting at an altitude in which many crucial 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," Mr. Santerre said.

By utilizing their boom and membrane mechanism, Serf and Santerre believe the satellite could return to Earth in less than 25 years, in keeping with international orbital junk mitigation guidelines.

Astrium, which leads the production of Europe's premier launcher, the Ariane 5, is now looking in to how the IDEAS concept could be applied to spent rocket stages.

Although the Ariane's main core stage and solid boosters fall quickly out of orbit at the conclusion of a flight, it may take as long as a century before the upper-stage falls naturally to Earth.

Once the Ariane has ejected its satellite payload, the upper-stage continues in a large elliptical orbit that extends more than 22,000 miles from Earth and comes as close as about 155 miles.

"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," said Mr. Santerre.

"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 solution is to give the Ariane 5 upper-stage the ability to take a powered dive into the Earth's atmosphere. This was done last year for the first time after the launch of the Jules Verne space station freighter.  The rapid removal of the Jules Verne from orbit  was considered critical because of the number of manned missions that typically follow station's orbit. After Jules Verne was released from the rocket, the upper-stage reignited its engine and made a controlled burn-up over the Pacific.

While this type of de-orbiting has clear advantages, the added fuel requirements and overall complexity  adds cost to an already expensive endeavor.

In contrast, aerobraking sails are lightweight and simple, and could even be controlled by a pre-set timer programmed to deploy a certain number of minutes after a flight's conclusion.

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


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