It’s been some time since Centauri Dreams looked at the work Gregory Benford (University of California at Irvine) and his brother James (Microwave Sciences) are doing with solar sail concepts. But I just noted, in paging through a back issue of the Journal of the British Interplanetary Society, that their proposal for a microwave-beamed sail was written up there, based on a talk at the 2005 IAA symposium in Aosta, Italy. And because I want to keep sail concepts visible in a time when funding constraints have all but driven them from the news, let’s revisit that work.

What got the Benfords headlines not so long ago was the speeds they were proposing. Five years or less to Pluto? That’s almost a halving of New Horizons’ travel time, and it makes for some intriguing conjecture indeed. The Benfords learned from earlier laboratory experiments that heating up ultralight carbon sail materials causes accelerations greater than would be expected from the pressure of photons alone. Apparently causing the effect are molecules evaporating from the hot side of the sail.

So here’s a solar sail scenario that a science fiction writer (especially one named Benford) could put to good use: ‘Paint’ various compounds onto a sail to take advantage of these effects, which vary depending on how they’re managed. The duo envisage a ‘Sundiver’ mission that uses microwave beaming, a close Solar pass and a second boost from the Earth-orbiting microwave transmitter to achieve maximum speed.

The solar sail could be deployed in low Earth orbit by conventional rocket. It would then be launched by microwave beam, the heat of which would cause a polymer layer to desorp from the sail (think of desorption as the opposite of absorption — some of the ‘paint’ material is released to provide propulsion).

The microwave beaming cancels most of the sail’s orbital velocity around the Sun, causing it to fall toward it. The craft approaches edge-on but at perihelion, a few solar radii out, it rotates to face the Sun. Now a second layer of polymers desorps away under the intense sunlight, and the craft gets a 50 kilometer per second boost, departing the area as a conventional reflecting solar sail with its final layer of aluminum now exposed. Mission speed is approximately 10 AU per year. A parting boost from a microwave transmitter in Earth orbit could add still more delta-V.

The numbers in this paper are quite encouraging. The authors explore, for example, Mars travel times of roughly one month (New Scientist liked that headline) though they pass on the question of how to slow down once the vehicle arrives. And it’s useful indeed to know that beamed power from Earth can heat sail temperatures enough to simulate the conditions of a near Solar pass, allowing a nearby ‘laboratory’ to pursue materials research on a space-based sail.

The paper is Gregory Benford and James Benford, “Power-Beaming Concepts for Future Deep Space Exploration,” in the Journal of the British Interplanetary Society Vol. 59 No. 3/4 (March/April 2006), pp. 104-107. It outlines a hybrid concept that draws on the best features of solar and laser-driven sails, adapting the beaming strategy to the microwave region for maximum efficiency. And it’s the kind of thinking that could push sails out past the heliopause.