A Japanese rocket unfurled a 300-metre-long ribbon in space on Monday, testing technology that could one day allow spacecraft to navigate by surfing Earth's magnetic field.
Conventional spacecraft have to burn fuel to manoeuvre in orbit. But the fuel adds weight and cost to the launch and eventually gets used up, limiting the probes' lifetime.
In principle, it is possible to propel an orbiting spacecraft without fuel by using a long piece of metal to interact with the magnetic field surrounding our planet. "You're essentially pushing against the Earth's magnetic field," says Les Johnson of NASA's Marshall Space Flight centre in Huntsville, Alabama.
On Monday, the Japan Aerospace Exploration Agency (JAXA) launched a spacecraft to test the idea.
Called T-Rex, short for Tether Technologies Rocket Experiment, the mission launched from the Uchinoura Space Center in Japan at 2000 GMT (5 am on Tuesday, local time) on a suborbital flight that lasted about 10 minutes and reached a maximum altitude of 309 kilometres.
While in space, the spacecraft unfurled the 300-metre-long "tether" – a 2.5-centimetre-wide metallic ribbon. It also successfully turned on its cathode, a device at one end of the tether designed to expel electrons into space.
The tether was meant to sweep up electrons floating in space and channel them along its length, creating an electric current. It would then expel them into space from the cathode.
In practice, the electric current would interact with Earth's magnetic field to drag the spacecraft to a lower orbit. Using power from solar panels, however, a spacecraft could also drive current in the opposite direction, which would raise its orbit.Impact hazard
It looks like the mission generated a current, which should have produced some thrust, says Johnson. He is a member of the mission team, which is led by Hironori Fujii of the Kanagawa Institute of Technology in Tokyo.
But there is no way to know for sure because the mission was not equipped to measure thrust, Johnson says. It was meant instead to return data on how efficiently the tether collected electrons, to help engineers design future tethers for propulsion. It is not yet clear what data the mission was able to return, Johnson says.
Some previous tether designs have used a thin wire, but these can be severed if hit by fast-moving bits of space debris or micrometeorites. The flat, ribbon-like tether used by T-Rex is less vulnerable to such impacts, which would merely punch a hole in the ribbon rather than break it, says Johnson.Short circuit
Johnson is part of a team planning a mission to demonstrate tether-based propulsion using NASA funds. This mission will have to compete for funding with other advanced technology demonstrations, such as solar sail propulsion, however. If selected, the tether demonstration could be ready to fly in 2013 or 2014, Johnson says.
Tethers have been launched into space before, but with mixed success. One was deployed from the space shuttle in 1996, but broke before it reached its full extent. An investigation determined that a short circuit burned through the tether, Johnson says.
More recently, in 2007, two attempted tether missions went awry. On one, called Multi-Application Survivable Tether (MAST), the tether failed to deploy. On the other, called Young Engineers Satellite 2 (YES2), the tether probably deployed fully but then snapped.
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