Page last updated at 21:35 GMT, Friday, 26 March 2010

Tiny cube will tackle space junk

By Jonathan Amos
Science correspondent, BBC News


The CubeSail mission would expect to operate for about a year

UK researchers have developed a device to drag space junk out of orbit.

They plan to launch a demonstration of their "CubeSail" next year. It is a small satellite cube that deploys a thin, 25-sq-m plastic sheet.

Residual air molecules still present in the spacecraft's low-Earth orbit will catch the sheet and pull the object out of the sky much faster than is normal.

The Surrey Space Centre team says the concept could be fitted to larger satellites and even rocket stages.

The group also envisages that a mature system would even be sent to rendezvous and dock with redundant spacecraft to clean them from orbit.

"Our system is simple and very low cost; but we need to demonstrate that it can be done," said Dr Vaios Lappas, lead researcher on the project and senior lecturer in space vehicle control.

"It would help make space a sustainable business. We want to be able to keep on launching satellites to provide new services; but unless we do something, the amount of junk up there is going to grow exponentially."

Simplicity of approach

It is thought more than 5,500 tonnes of junk now clutters the region of space just a few hundred km above our heads.

Last year, two satellites even collided, showering their orbit with tiny fragments that now pose additional risk to operational spacecraft.

Cubesat dimensions (SSC)
The controlling spacecraft is a box that is 30cm in its longest dimension

International agencies have agreed that retired hardware - old satellites or spent rocket stages - should be removed from space within 25 years of the end of service.

Using large deployable surfaces to increase the drag on these objects so they fall to Earth rapidly is one possible solution to the space litter problem.

CubeSail, unveiled on Friday, is a 3kg (6.6lb), 10cm x 10cm x 30cm (4in x 4in x 12in) nanosatellite.

It incorporates within its tiny frame a polymer sheet that is folded for launch to be unfurled once in space.

The simple deployment mechanism features four metal strips that are wound under tension and will snap into a straight line when let go, pulling the sheet flat in the process.

The team hopes to launch its demonstrator at the end of next year, riding piggy-back on another mission or as part of a cluster of small research satellites that are sometimes lofted en mass atop a single rocket.

Force of sunlight

The nanosat will then circle the Earth, going from pole to pole at an altitude of about 700km (435 miles), testing its systems and assessing the drag principle.

If successful, CubeSail could become a regular add-on system to satellites and rocket stages, opening up a new space business akin to the daily refuse services here on Earth.

"We would be looking to put it on our own satellites and to put it on other people's spacecraft as well," said Sir Martin Sweeting, the chairman of SSTL, the world-leading small-satellite manufacturer, which is supporting the research.

A demo of how the stowed sail would deploy

"We want this to be a standard, essential bolt-on item for a spacecraft; and that's why it's very important to make it small, because if it's too big it will interfere with the rest of the spacecraft," he told BBC News.

The researchers hope to develop the project as a propulsion system as well. The pressure of sunlight falling on such a large structure would also move it. The force is tiny but continuous.

This "solar sailing" technique has long been touted as a means of moving spacecraft around the Solar System, or even just helping conventional satellites to maintain their orbits more efficiently.

Indeed, some of the large geostationary satellites, for example, already use solar-sail flaps to maintain their attitude without firing their thrusters. This saves valuable chemical propellant and extends mission lifetime.

Delicate control

CubeSail will endeavour to demonstrate this "propellantless propulsion" by trying to shift the path it takes across the surface of the Earth by just a few degrees over the course of a year.

To do this though, the nanosatellite will have to carefully control the angle of the sail with respect to the Sun, just as an ocean vessel has to play with its sails to catch the wind.

Sail deployed in lab (SSC)
The sail will be made from an extremely fine polymer

"We're going to control our sail with a very novel geometric technique; we're not going to use any thrusters," explained Dr Lappas.

"We have developed a tilting mechanism that uses very tiny motors. It's able to move in two directions. This enables you to change the centre of mass of the sail. We're also going to be using small magnets to control the sail because they will interact with the Earth's magnetic field."

Once its mission is complete, CubeSail will be instructed to take itself out of orbit.

The project is a private venture within the Surrey Space Centre, which is based at the University of Surrey, Guildford.

CubeSail has been funded by Europe's largest space company, EADS Astrium, which is one of the world's biggest manufacturers of satellites. It also produces Europe's heavy-lift rocket, the Ariane 5, which launches about half of the world's commercial satellite platforms.

The entire cost of the project is expected to be no more than £1m ($1.5m).

Other groups around the world are expected to launch solar sail demonstrators soon. The US space agency has been working on a project with The Planetary Society, a long-time proponent of the technology.

The Japanese, too, have work in progress. And even Astrium is sponsoring other space junk mitigation strategies within its own division.

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