Sailing to Halley's Comet

Long before the invention of the rocket, man dreamed of hoisting sail and traveling through space in wind-blown ships. In The True History, a tale written in the 2nd century A.D. by the satirist and onetime lawyer, Lucian of Samosata, a ship with a 50-man crew is caught in an Atlantic storm, carried aloft and sent, sail billowing, on a journey to the moon. Later storytellers launched ships with sails on even more fanciful space trips. But none of these fictional voyages was as remarkable as the mission now being planned for NASA by scientists at Pasadena's Jet Propulsion Laboratory. If all goes well, they will launch an unmanned spacecraft guided with a giant sail to rendezvous with Halley's comet when it next approaches the sun, in 1986.

Free and Inexhaustible. The fantastic voyage was proposed by a group commissioned by J.P.L. Director Bruce Murray to consider imaginative concepts for interplanetary exploration. A mission to Halley's comet, which returns every 74 to 79 years, has long been one of NASA's goals. But using conventional space-flight techniques to rendezvous and keep up with the glowing visitor--which reaches speeds of 198,000 kilometers (124,000 miles) an hour as it approaches the sun--would require enormous amounts of fuel and an impractically large and expensive rocket.

Instead, the J.P.L. scientists proposed taking advantage of a free and virtually inexhaustible source of power: the pressure of sunlight. Moving at 300,000 kilometers (186,000 miles) a second, the photons from the sun would exert force on the large sail--just as a handful of sand, thrown against the sail of a toy boat, can push it through the water.

The J.P.L. project, which has a $5.5 million NASA grant, calls for a kitelike framework covered by a square of plastic film measuring a huge 800 meters (2,600 ft.) on a side. The thin sail (ordinary plastic kitchen wrap is five times thicker) would be coated with an aluminum reflecting layer on the side that will face the sun, and painted a heat-absorbing black on the other side. The total weight of the sail and the instrument-packed ship mounted in a hole at its center will be only 5,000 kilograms (11,000 lbs.)--a payload that could easily be launched into earth orbit by a rocket.

Still, the huge sailer poses a problem. The sail must be carried aloft furled (folded, it will fit into a package of only one cubic meter) and the framework assembled far beyond the atmosphere. Luckily, NASA is readying a suitable ferry: the space shuttle. Capable of carrying the sail and framework in its large equipment bay, the shuttle should be in regular use by the proposed launch date for the sailing ship: January 1982.

Once the package has been hauled by the shuttle into near-earth orbit, a small rocket will push it to escape velocity. At about 100,000 kilometers (60,000 miles) above the earth, the framework will be assembled and the sail deployed automatically.

Ground controllers will then begin navigating the craft into closer and closer orbits of the sun by properly trimming the sail. Then they can put the ship--moving at a top speed of 198,000 kilometers (124,000 miles) an hour--on a course to intercept Halley's comet in March 1986. Jettisoning the sail, and "flying station" just two kilometers above the comet's head, the ship will take TV pictures and readings to determine the visitor's composition and origin. Says J.P.L.'s Murray: "We don't have a clue about comets. The space sailer could help provide some."

It might also effectively open up the rest of the solar system to manned spaceflights that cannot be considered now because of tremendous costs. J.P.L.'s Louis Friedman thinks that a flotilla of sunjammers could embark on a manned Mars mission by the end of the century, and foresees a day when fleets of huge kites shuttle through space--as the East Indiamen plied the oceans three centuries ago--making regular stops at Mercury, Venus, Mars or the asteroids.

Should plans for the space sailer hit a snag, earthlings could still get their first closeup view of Halley's comet in 1986. Another group at J.P.L. is working on the design of a spaceship that would be propelled by an ion engine; a small, continuous amount of thrust would be provided by the engine's ejecting ions produced when a beam of electrons (generated by electric current from solar cells) is sent through vaporized mercury. Such a low-thrust ion engine could, like the sunjammer's sail, maneuver a ship to a rendezvous with the comet. NASA is scheduled to decide next August which craft, if either, will make the mission. Until it decides, there will be fierce but friendly competition at J.P.L., where employees last week identified their allegiance by wearing buttons reading either TRUCKING WITH ION DRIVE or I'M A SOLAR SAILOR.

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