Carl Wiley, the prescient engineer who offered an early description of solar sails in “Clipper Ships of Space” (Astounding Science Fiction (May, 1951), was not the first to look into sail propulsion, but he was one of the more visible. Konstantin Tsiolkovsky’s thinking on the matter in the 1920s was not widely circulated, and it may be that John Desmond Bernal, a political activist and professor at Cambridge and, later, the University of London, was Wiley’s primary forerunner as far as public awareness of sail ideas is concerned. In The World, the Flesh & the Devil (1929), Bernal looked at the propulsive possibilities in light:

However it is effected, the first leaving of the earth will have provided its with the means of traveling through space with considerable acceleration and, therefore, the possibility of obtaining great velocities – even if the acceleration can only be maintained for a short time. If the problem of the utilization of solar energy has by that time been solved, the movement of these space vessels can he maintained indefinitely. Failing this, a form of space sailing might be developed which used the repulsive effect of the sun’s rays instead of wind. A space vessel spreading its large, metallic wings, acres in extent, to the full, might be blown to the limit of Neptune’s orbit. Then, to increase its speed, it would tack, close-hauled, down the gravitational field, spreading full sail again as it rushed past the sun.

Image: Artwork for Carl Wiley’s article “Clipper Ships of Space,” which ran in the May, 1951 issue of Astounding Science Fiction. Credit: Orban.

My friend Adam Crowl, whose research skills are all but preternatural, has also called my attention to J. B. S. Haldane’s “The Last Judgement,” a look at future human history that was an influence on Olaf Stapledon’s Last and First Men. Haldane, a well-known British biologist and geneticist, has solar sails worked into an interplanetary infrastructure that eventually is considered for an interstellar crossing. The essay covers a Clarke-ian 40 million years, and it’s interesting to note the exchange of letters between Arthur C. Clarke and Haldane. Clarke would go on to edit The Coming of the Space Age: Famous Accounts of Man’s Probing of the Universe (London, 1970), in which “The Last Judgement” was reprinted.

I could spend a lot of time on Haldane and his relation to Clarke, and especially on his paper “Daedalus, or Science and the Future, delivered at Cambridge in 1923, but I’d quickly be digressing to the point of absurdity. So back to sails: We can say that by 1951, when Carl Wiley wrote his essay on the matter for John Campbell’s magazine, he was probably introducing the subject to most of his readers, and certainly looking at it with a level of detail that no previous writer had offered to the public. Its influence would be felt later in the decade.

A Sail Mission Design

For it was in 1958 that Ted Cotter, then working at Los Alamos and later himself an influence on ‘Medusa’ creator Johndale Solem, put together “An Encomium on Solar Sailing.” The memo — and it was little more than that, assembled for internal circulation at Los Alamos — set about to flesh out details of solar sail ideas by describing a design for an unmanned sail mission to Mars. ‘Russell Saunders,’ who as we saw yesterday was Wiley’s pseudonym for the “Clipper Ships of Space” article, appears in one of two footnotes, the other name being that of Richard Garwin, the third of our 1950’s engineers with a sail bent, about whom I’ll be speaking tomorrow.

Cotter’s memorandum was influential only within the realm of Los Alamos, but it makes for absorbing reading nonetheless, and it offers at least one new wrinkle:

The present note contains little new beyond the observations of the previous authors, except the notion of spinning the sail. Its intent is to advertise the considerable merits of solar sailing by filling in more details of the scheme. In order to expose the problems and indicate some technical possibilities for their solution, I will presently describe the construction, operation and flight of an unmanned instrumented solar sailing vehicle on a round trip of exploration to the neighborhood of Mars under command guidance from the earth. Before presenting this particular body of circumstantial evidence in favor of feasibility it seems worthwhile to provide some incentive for the task by emphasizing some of the implications of the properties of solar-sailing vehicles in general.

What follows is a backgrounder on the advantages of using solar photon momentum and the issues sails raise in relation to conventional rocketry. Remember that this was being written just as the first satellites had reached orbit, and Cotter speculates that now that placing objects in orbit had been accomplished, it should be possible to experiment with solar sail designs (it’s probably a good thing he didn’t know just how long it would be before the IKAROS deployment). He is at pains to stress the key advantage of all sails — they require no propellant. No wonder he assumed we’d have a working sail in orbital testing within a few years of his writing.

The Cotter design is a circular disk 500 meters in diameter and 10^-4 centimeters thick, made out of a plastic film coated on one side with 20-30 micrograms per square centimeter of aluminum. Cotter envisioned reinforcing the sail panel seams to prevent tears. He assumed a 250 kilogram payload and introduced the idea of spinning the sail, which aids deployment and holds the sail flat, with stress nowhere exceeding 0.1 percent of the breaking strength of the plastic film. Here’s an essential part of the description:

The capsule consists of two parts connected through a universal joint, and provided with a motor which now causes the sail package and the main part of the load to start counter-rotating. At the appropriate times the collapsed structural backbone is extended, the capsule cases are jettisoned and the vehicle blossoms forth under centrifugal force… The sail is spinning at the rate of one revolution in two minutes. The load, two-thirds of which is suspended in three pods by wires at 50 meters from the axis of spin, is counter-rotating at a rate of 10 revolutions per minute. The vehicle as a whole has zero net angular momentum.

Cotter was already anticipating the many issues that sails would raise, including the deterioration of plastic films in strong ultraviolet radiation, and the problem of sail damage due to micro-meteorites — on the latter point, basing his thoughts on estimates of interplanetary dust and particles current at the time, he concluded that the half-life for decay of the sail’s reflectivity would be measured in thousands of years. Of course, as we look ahead to interstellar applications and vastly ramped up speeds, these issues become much more acute.

And I love this statement, which concludes the piece:

Any remarks on the effects of interplanetary electromagnetic fields would be purely conjectural. These are typical of questions which are perhaps best answered by sending out a solar sail to see.

Indeed. And we are finally in the process of deploying our early sails to make such measurements, long after the first speculations about actual missions were produced. But it’s fascinating to see how space technologies develop in our thinking, particularly when we trace them back to the first engineering concepts. If you’d like to see Cotter’s text, it’s now available online, a restricted access paper long since declassified and made available through the website of the Federation of American Scientists. Tomorrow: Sails get into the scientific literature, and the man who made it happen.