Microwave Beaming and the Cosmos Sail

by Paul Gilster on May 31, 2005

We’re closing in on the launch of the Cosmos 1 solar sail, the first free-flying spacecraft to be powered by the momentum of solar photons. Yes, there have been deployment experiments before this, such as the Russian Znamya missions and the Japanese deployment of a thin film just last summer. But Cosmos 1 will be a functioning spacecraft capable of returning data, and its launch thus marks an important first in sail development. The launch window opens on June 21. The spacecraft is to be launched into a near polar and circular 800 kilometer orbit, with sail deployment four days later.

Cosmos 1 has been moved from the test facility in Moscow to Severomorsk; the plan calls for it to be launched by a converted Russian ICBM from a submerged submarine. If successful, the mission will be a landmark not only for sail propulsion but also for commercial space development. The vehicle was funded by Cosmos Studios, with donations from members of the Planetary Society, whose latest update on the mission can be read here.

Photon propulsion works because although photons have no mass, they do impart momentum. We’re talking about forces that are vanishingly small at Earth’s distance from the Sun, where the solar flux is roughly 1.4 kilowatts per square meter, some nine orders of magnitude weaker than the wind on Earth’s surface. But keep pushing on the sail in a zero-g environment and it will move, each push adding up, gradually gaining enough velocity to make missions within the inner Solar System practical. Missions to the nearby interstellar medium may also be in range, as witness the Oort Cloud Explorer we looked at in these pages in April.

Cosmos 1 will not, of course, attempt anything so dramatic. It’s an experiment in sail technology to gather information on how a sail performs under actual flight conditions. But particularly interesting for interstellar theorists will be its performance under microwave beam. James Benford of Microwave Sciences and his brother Greg (of the University of California-Irvine, and one of the most highly regarded of all practicing science fiction writers) will lead a team using NASA’s Goldstone antenna to send the beam to Cosmos 1. That will occur only when and if all other flight objectives have been accomplished.

But let’s hope that it happens, because laboratory data also compiled by Microwave Sciences indicates that beamed propulsion ought to work. The experiments used a 10-kilowatt microwave beam pushing a 10-square centimeter sail in a vacuum chamber. Exactly what effect a microwave beam from Earth will have is something Cosmos 1 should be able to measure with its onboard accelerometer and GPS equipment.

Ultimately, beamed technology will be necessary as we look toward missions to nearby stars, since the dropoff of solar radiation beyond Jupiter’s orbit makes solar sails less practical beyond that range (although ‘sundiver’ maneuvers close to the solar disc can push a sail-driven device to high enough velocities for missions to the nearby interstellar medium, furling the sail when it is no longer effective for propulsion).

James Benford’s paper on the microwave experiments is “Flight and Spin of Microwave-Driven Sails: First Experiments,” in Proceedings Pulsed Power Plasma Science 2001, IEEE 01CH37251, p. 548 (available at the Microwave Sciences site. Also available at the site: J. Benford, G. Benford et al., “Microwave Beam-Driven Sail Flight Experiments,” Space Technology and Applications International Forum, AIP Conference Proceedings 552, ISBN 1-56396-980-7STAIF, pg. 540 (2001).