Science: Trade Wind in Space

In his years at Brown University, Dr. Theodore P. Cotter of Los Alamos Scientific Laboratory learned to love sailing in New England waters, and he still sails a folding kayak on Colorado lakes. At Los Alamos he was assigned to N Division, which works on the knotty problem of providing nuclear propulsion for spaceships. He began to think about the great solar “wind,” the sun’s radiation blowing outward through the solar system, and how this solar wind might be used to drive a space vessel.

The force of the sun’s light is extremely small—9X10-5 dynes per square centimeter, or about the weight of four cigarettes per acre of surface at the distance of the earth. But it is free and unfailing, and in the weightless, placid vacuum of space, large, frail sails might be spread to intercept it. For a starter, Dr. Cotter would like to try a 50-lb. space sailer. Once launched in the usual way to an orbit around the earth, the satellite would sprout a circular sail of thin plastic coated with shiny aluminum. If the satellite is spinning, the sail would spread itself by centrifugal force. Another method would be to construct a sail with inflatable tubes connected by fragile membranes on the model of an insect’s wing. At the proper moment, plastic foam would be injected into the hollow tubes, distending them and spreading the sail. Later, the foam would harden to act as supporting ribs.

Outward Bound. A sail 50 yds. in diameter, Dr. Cotter figures, should weigh only 25 lbs., leaving 25 lbs. for the hull, instruments and controls. This gossamer structure, more delicate than a firefly’s wing, would be strong enough for sailing in space. Meteors would punch small holes in it but do no serious damage. It ought to remain spaceworthy for many years.

The simplest maneuver for a sailing spaceship, says Dr. Cotter, will be escape from the earth. The satellite will be placed in an orbit in the plane of the earth’s orbit around the sun (see diagram). After spreading its sail, the satellite will be designed to have a slow turning motion, rotating once during every two trips around the earth. When it is moving away from the sun, its sail will be at right angles to the sun’s light, and it will get the maximum push in a forward direction. By the time it gets to the other side of its orbit and is moving toward the sun, the sail will have turned 90°. Its thin edge will point toward the sunlight and will be little affected by it.

The result of this feathering action will be to push the satellite into an elliptical orbit that grows longer and longer until the earth is so far away that its gravitation is negligible, and the satellite can break loose. Dr. Cotter estimates that a 50-lb. space sailer could escape from the earth in about six months.

Orbit to Orbit. Once free from the earth, the space sailer would fall into a solar orbit, use sunlight to waft it almost anywhere in the solar system. For such maneuvering it would need a way to change its sail’s angle to the sunlight; Dr. Cotter believes that this can be done by gyroscopic devices that act in response to radio signals from the earth. With its sail broadside to the light, it will be pushed farther and farther from the sun in wider and wider orbits. Eventually it will reach the orbits of Mars or the outer planets and can take a look at them. A fragile space sailer could not land on any planet; even a brush with the fringe of an atmosphere would destroy the sail. But it could be maneuvered to approach smaller bodies in space that have no atmosphere.

To bring the space sailer back to the earth’s orbit, the operator on earth could reset the sail at such an angle that sunlight bouncing off would tend to reduce its orbital speed. As the speed slowly diminished, the space sailer would spiral inward toward the sun, eventually returning to the earth’s orbit.