New Scientist is covering Gregory and James Benford’s intriguing sail concept that would get a spacecraft up to 60 kilometers per second. That’s faster than any spacecraft we’ve ever launched; by comparison, the fastest vehicle out there is Voyager 1, now pushing toward the heliopause at some 17.5 kilometers per second. The brothers Benford (Gregory from the University of California — Irvine and James of Microwave Sciences in Lafayette, CA), talk about beamed microwaves driving a sail design with a difference.

At play here is an effect James Benford discovered when testing a thin, carbon-mesh sail with beamed microwaves. The forces exerted on the sail turned out to be stronger than expected, because the heat from the microwave beam was causing outgassing from material in the sail itself. It was the push from these unexpected gas molecules that gave the sail the extra push. You can read the New Scientist story here.

I haven’t talked to James Benford since 2003, but even then he was zeroing in on this phenomenon, which is called ‘desorption’. It seemed clear that desorption could be used purposefully.

“You want to use desorption as a tool,” Benford said then. “You could paint on materials that will come off at the right temperature, and plan a mission to use that effect… The engine is basically the beam from earth. The fuel is the material that’s on the sail.

“All of this comes from our experiments, where we found materials coming out of the sail. We expected this from some materials; if you just have an ordinary surface and put it in vacuum, the surface will have weakly absorbed molecules of air and water, the sort of stuff that clings to any surface. Water is very clingy in vacuum, but if you heat it up to a few hundred degrees for a little while, you burn all that off.

“But when you go up to much higher temperatures, you start to bring out things that are actually embedded in the lattice of the carbon, things that have been in there like carbon dioxide. It takes high temperatures to get those out. We found we were getting a lot of gas out of the material, and that gas, being very hot when it comes out, gives you quite a bit of thrust.”

The Benfords’ concept will be described in an upcoming issue of Acta Astronautica. In it, they speculate that such a sail could be covered with a paint designed to emit gas to harness the desorption effect. They believe a one-hour microwave burst could bring the vehicle up to speeds that would take a spacecraft to Mars in a month’s time. Pushing the vehicle would be a 60-megawatt microwave beam, currently beyond our capabilities, but surely not for long.

Sources: Interview with James Benford (16 February 2003); Celeste Biever Boston, “Solar super-sail could reach Mars in a month,” New Scientist 29 January 2005. Details on the Acta Astronautica article when they become available.