Dr. Benford and his brother James were testing a very thin carbon-mesh sail, using microwaves as the energy source for propulsion. Unexpectedly, the sail experienced a force considerably greater than predicted. They theorized that the heat from the microwave beam was causing carbon monoxide gas to escape from the sail's surface; the recoil from the escaping molecules provided what could be a useful adjunct to the propulsive force experienced by light sails.
They believe that by beaming microwave energy up from Earth to boil off volatile molecules from a specially formulated paint applied to the sail will provide enough added force to propel a spacecraft to Mars in record time. "It's a different way of thinking about propulsion," Gregory Benford says. "We leave the engine on the ground." Their research will be published this month in the journal Acta Astronautica.
This is how it would work: a rocket would take the craft to low-Earth orbit, whereupon the craft would unfurl a 100 meter diameter sail. A transmitter on Earth would fire a one-hour burst of microwaves at it to heat it up, accelerating the craft to 60 kilometres per second. This would set an interplanetary speed record for space probes.
However, more work is needed to make this possible. The plan would require a 60-megawatt microwave beam with a similar diameter as the sail that was capable of tracking the craft. The deep-space communications network that NASA uses to communicate with Mars rovers and the Cassini probe now orbiting Saturn can only output half a megawatt.
If this design could work, it would make significant progress in the use of ground-based beam-propulsion designs. As science fiction readers know, this topic was explored in the 1974 novel Mote in God's Eye by Larry Niven and Jerry Pournelle. They used the idea of laser cannon from Robert L. Forward's 1961 paper Ground-Based Lasers For Propulsion In Space to bring an alien spacecraft to our solar system. Readers may also wish to explore early sf looks at solar sails.
Read a bit more about Gregory Benford; the article First Flight By A Laser-Powered Airplane chronicles an earlier effort at ground-based propulsion. See also Solar super-sail could reach Mars in a month and Acceleration of Sails by Thermal Desorption of Coatings (pdf).
(This Science Fiction in the News story used with permission from Technovelgy.com - where science meets fiction.)