Solar-sail mission reflects past and future

James Oberg NBC News space analyst

The fanciful idea of sailing through space on a beam of light — the premise for this week's test flight of the Cosmos 1 solar sail — has been around for centuries. But it's only been in the last few years that dreamers have gained the technology to try it out, and the boldness to risk failure after failure until it works.

When it finally does work — if not this time, then certainly someday — all previous methods of space propulsion may seem a lot more primitive and a lot less elegant.

The privately funded $4 million mission is due to begin Tuesday, when a Russian submarine fires a missile bearing the folded-up Cosmos 1 payload into orbit. Cosmos 1 is billed as a totally new kind of spacecraft, but the project's precedents go back almost 400 years, when Johannes Kepler and Galileo Galilei were building the foundations of modern astronomy.

"Provide ship or sails adapted to the heavenly breezes, and there will be some who will not fear even that [outer-space] void," Kepler told Galileo in a letter dated April 19, 1610. Kepler justified the study of astronomy for the benefit of “those who will come shortly to attempt this journey” — a clear vision that was centuries ahead of its time.

If Cosmos 1 works as hoped, it will mark a significant watershed in space propulsion technology first imagined by Kepler, who noted that the glowing tails of comets were blown by what he saw as a powerful wind. Later, astronomers realized that there were actually two distinct "winds" flowing from the sun: the variable stream of electrically charged particles making up the solar wind; and the constant stream of photons, or particles of light.

Contrary to some of the popular conceptions about solar sailing, Cosmos 1 is designed to ride the force of the photons rather than the solar wind. In an effort to head off the confusion over which wind is involved, futurist Eric Drexler proposed 30 years ago that the term "light sail" be used instead — a double entendre that refers to the fact that such sails uses light for power and also must be very light to be effective. Sadly, the poetic and proper term hasn't yet caught on.

The modern concept of using photon pressure to gently but inexorably push spacecraft to high speeds was fleshed out early in the 20th century. However, flexible materials that were light enough, mechanically strong enough, and physically resistant enough to the disintegrating effects of solar radiation just weren’t available until recent decades. As far back as 1960, photon pressure played orbital soccer with the Echo 1 thin-film balloon in orbit, pushing its orbit around with astonishing force until the balloon’s skin shattered. The shards were then flung far and wide by sunlight.

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