Light for Propulsion

11. November 2009

By Patricia Daukantas

Photons have momentum, according to Einstein, and a couple of things that recently crossed my desk reminded me of this small but potentially significant phenomenon.

In yesterday’s New York Times, Dennis Overbye wrote a lyrical article describing an experiment called LightSail-1, which a space advocacy group called the Planetary Society plans to launch into Earth orbit about a year from now (depending on the availability of a launch vehicle, which is still being worked out). The spacecraft will use 32 square meters of Mylar and will go into orbit 800 km above the Earth, where it could avoid the drag from the uppermost layers of our atmosphere.

LightSail-1 is basically a “proof of concept” experiment to see how well sunlight can propel a small craft, but some scientists envision a future era when solar space cruisers use the gentle but constant acceleration of radiation pressure to cruise around the solar system – or beyond it.

The article on the solar-sail project caught my eye because I remember reading a short story about light sailing in elementary school – it might well have been “Sunjammer” by the late Arthur C. Clarke, but it was a long time ago and further details have receded into the mists of time.

On a related note … recently I’ve been getting press releases from a contest called the Space Elevator Games. A “space elevator” – which doesn’t exist yet – would be a tether running vertically from the Earth’s surface up to the level of Earth orbit. According to space-elevator advocates, cargo, and eventually people, could travel up and down the tether to get to space with far less cost and energy expenditure than today’s chemical rockets.

The idea has been around for about five decades, but few took it seriously until the discovery of carbon nanotubes circa 1990. Nanotubes potentially could make a much stronger tether than other materials. Advocates also say that another technology developed roughly five decades ago – the laser – could provide the propulsion to lift cargo into space.

Anyhow, last week three teams of competitors met out in the Mojave Desert of California (U.S.A.) to try to win pots of money by sending a small vehicle up a 1-km-high tether using only “power beaming.” LaserMotive, a startup from Seattle (U.S.A.), won the level-1 prize of $900,000 by getting its 5.2-kg vehicle up the tether at an average speed above 2 m/s (and sometimes at 4 m/s). A second team called the “Kansas City Space Pirates” got its vehicle only about halfway up the tether, and a third group’s climber didn’t really get off the ground.

“Solar sails” don’t work in this type of application. The LaserMotive team turned light into propulsion by beaming an infrared laser array up from the ground and onto solar cells attached to the climber; the resulting energy powered the motor and other apparatus on the vehicle.

No team won the $1.1 million prize for getting their climber to ascend at 5 m/s, but undoubtedly these competitors and possibly more will be back next year for another crack at fame, adventure and cash.

2009-11 November

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