Dream of solar sailing in space lives on in new project

The US-based Planetary Society this week announced its second attempt to launch a small spacecraft with sails propelled by sunlight.

An artists rendering of a solar sail is shown in this undated publicity image from The Planetary Society released to Reuters. Backers of a failed mission to launch the world's first solar-sail spacecraft unveiled plans on Monday to try again five years later with a smaller, swifter satellite to test the limits of sunlight propulsion.

Its designers call it LightSail-1. And if it works as advertised, the solar sail project would represent a baby step toward humanity's first starship.

This week, the California-based Planetary Society announced a new project to launch a small spacecraft propelled by a solar sail. In principle, the idea is simple: Use the sail to intercept sunlight, which presses on the sail much like wind on canvas. (The same pressure keeps the sun from collapsing under its own gravity.)

Initially a solar-sail craft builds momentum almost imperceptibly. But with no friction in space to resist its motion, the craft in theory should go faster and faster, as long as there is enough light to propel it. It requires little on-board fuel for course corrections, since the sails can be "trimmed" to change course, just as a sailboat's canvas is trimmed.

This is the one known technology that can get up to the speeds as you leave the solar system "that make interstellar flight not completely ridiculous,” says Bruce Betts, director of projects for the Planetary Society, which is undertaking the $2 million demonstration effort.

First, however, the craft needs to reach space – the toughest part of any solar-sail project.

This is the Planetary Society's second try. Its first attempt in 2005 ended up in the Pacific Ocean when the first stage of a Russian submarine-launched missile carrying the group's Cosmos 1 solar-sail satellite misbehaved. Last year, the National Aeronautics and Space Administration (NASA) attempted to launch its version of a solar-sail demonstration satellite. It hitched its satellite to SpaceX's – the American space transport company – Falcon 1 rocket. That rocket also failed to deliver the craft to orbit.

Undaunted, the Planetary Society is pressing ahead. And while LightSail-1 is the group's main focus, it envisions this as a three-stage program.

LightSail-1 is designed to orbit some 800 kilometers (roughly 500 miles) above Earth. At that altitude, the craft orbits above the last vestiges of Earth's atmosphere, which introduces friction and so can slow the craft faster than sunlight can accelerate it. The LightSail-1 team hopes to test a solar sail's ability to steer the craft by changing the trim to adjust the craft's orbit.

Assuming all goes well with LightSail-1, and there is additional fundraising, LightSail-2 would operate at a higher orbit and for longer. LightSail-3 would be designed to leave earth orbit and would head to a spot between Earth and the sun some one million miles away. There, the sun's and Earth's gravity virtually cancel each other, allowing a spacecraft to stay put with only occasional corrections to its orbit. A solar-sail craft could make those corrections without having to carry fuel for steering motors.

Planners have high hopes for LightSail-1, which the Planetary Society would like to launch toward the end of 2010. It's half the price of Cosmos 1. Its sail is smaller and the deployment mechanisms less complicated. And because it's far lighter – about 4-1/2 kilograms (10 pounds) versus 100 kilograms for Cosmos 1 – the sail is expected to give the craft more thrust, despite a smaller surface area.

The solar-sail propulsion technique could find some nearer-term uses, Mr. Betts says. One possibility is to use solar sails for a new generation of craft that would give Earth early warning of solar storms. Currently, the lead time for tipping off satellite owners and electric utilities to a looming solar storm is about 10 minutes, Betts says. By stationing a craft between the Earth and the sun, or sending it closer to the sun (yes, it would have to tack its way there, sailboat-like), space-weather forecasters could give astronauts on the moon or in earth orbit, as well as other interested parties, at least an hour's advance notice.

Now, where's Capt. Jack Aubrey when you need him?


See also:

Russians to ride a nuclear-powered spacecraft to Mars.

Solar-sailing era begins in space.


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[Editor's note: The original version of this article misstated the orbit of LightSail-1. It will be 800 kilometers above Earth (roughly 500 miles).]

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