Flying with Ion Power

By LEON JAROFF

One of the most daring deep-space missions NASA has ever planned is turning out to be one of the least publicized. The target is a large asteroid named 1992KD, which orbits the sun millions of miles from Earth. But that destination is almost incidental to the performance of the spacecraft that will make the trip. Though it looks little different from countless other unmanned probes NASA has launched, the ship will be navigated by an electronic brain that has been likened to HAL, the independent-minded computer in the film 2001, and will move through space under power of a system that has long been the stuff of technological fantasies: an ion propulsion engine.

If all goes as planned, Deep Space 1, scheduled for launch later this month, will be the forerunner of a new generation of spacecraft. While flight planners hope the ship will make some interesting observations about the target asteroid, including its composition and the structure of its surface, DS1's prime assignment is to validate a host of new technologies NASA had always considered too risky to try on a high-profile mission. Says Marc Rayman of Jet Propulsion Laboratory, DS1's chief engineer: "We have an unproven propulsion system, powered by an unproven solar array, commanded by an unproven navigation system."

What is most remarkable about the spacecraft is how it gets from place to place. After being launched by an ordinary rocket, DS1 will be pushed through space by an engine that works by firing electrons into atoms of xenon gas, stripping each of an electron and giving the atoms an electric charge--ionizing them. The ions are then accelerated through an electric field and emitted from thrusters at 65,000 m.p.h. Despite that speed, the particles produce little thrust, comparable to the weight of a piece of paper.

Still, that constant push will add 15 to 20 m.p.h. daily to DS1's speed. "It takes three days to get to 60 m.p.h.," says Rayman, "but if you thrust for 300 days, you're up to 6,000 m.p.h. I like to call it 'acceleration with patience.'"

That patience pays off. DS1 will carry a mere 180 lbs. of xenon fuel, about one-tenth the fuel needed for a conventional craft. Electricity required to power thrusters and other equipment will come from a new solar panel equipped with 720 lenses that focus sunlight down to a strip of solar cells.

Just as innovative is DS1's navigation system. By scanning stars and asteroids, the spacecraft will know precisely where it is and will make its own maneuvers, perhaps even during its asteroid rendezvous. Programmed to fly six miles above the giant rock, DS1 will also have the option of swooping down to half that altitude.

DS1's use of ion propulsion and self-navigation, Rayman says, "is analogous to having your car find its own way from L.A. to Washington, arrive at a designated parking place, and do it all while getting 300 m.p.g." By the same analogy, could the car then drive on to other cities too? Apparently. Barring problems, DS1 will have enough fuel and navigational smarts to proceed to a burned-out comet called Wilson-Harrington in January 2001 and, as its grand finale, to Comet Borrelly later that year.

--By Leon Jaroff