Deep Space I

This small, low-budget spacecraft, Deep Space 1, now speeding on its way toward a July 1999 encounter with an asteroid, could have a big effect on the future of space exploration. Launched on a Delta II rocket from Kennedy Space Center on October 25, it marks the first salvo in the National Aeronautics and Space Administration's New Millennium Program to create a future generation of spacecraft that will be less expensive and less reliant on large ground crews to manage operations.

Deep Space 1 cost a mere $152 million, a bargain-basement price: many space missions cost at least several times that amount. But during its mission the spacecraft will test a suite of new technologies. After its boost away from Earth's gravity on a chemical rocket, Deep Space 1 will rely on an ion engine, the first time one of these devices has been employed for propulsion. Sophisticated navigation systems and artificial intelligence built into the craft will enable it to make decisions about its mission, a capability that has been likened to the fictional computer HAL 9000, from Arthur C. Clarke's novel 2001: A Space Odyssey.

The ion drive, a staple of Star Trek episodes and other science fiction venues, works by first bombarding xenon gas (an inert element commonly used in photographic flash units) with electrons from a cathode; the process strips electrons from the xenon atoms. The resulting charged atoms (called ions) are then accelerated by a high voltage applied to fine metal grids on one side of the engine; the ions shoot through the grids and out into space.

The xenon jets out of the drive at more than 100,000 kilometers per hour, producing a faint blue glow. Electrons are fired into the exiting stream of xenon ions to neutralize them, thus keeping them from being attracted back to the spacecraft by any build-up of negative charge. Electricity to energize the drive is provided by solar panels, which on Deep Space 1 are also of innovative design: they incorporate lenses that concentrate the sun's light, leading to greater efficiency.

Ion drive

XENON DRIVE

The acceleration provided by the xenon is barely a nudge: even at maximum thrust, Deep Space 1's ion drive would produce less than 10 grams' weight of force. (And it will not be run at that level during the mission, as the craft does not have enough electrical power.) But because the drive uses xenon at a low rate -- less than three milligrams per second -- it can be operated continuously for days or weeks. Through its staying power, the ion drive should boost Deep Space 1's speed by almost 13,000 kilometers per hour. Chemical rockets might achieve that result in mere minutes but would require up to 10 times more fuel than the 82 kilograms of xenon carried by Deep Space 1.

Ion drives have been tested extensively in the laboratory. Early versions employed mercury or cesium rather then xenon, but these materials caused problems because they tended to condense on the outside of the engine. Xenon, a gas at normal temperatures, does not react chemically, so it is much cleaner. A small ion drive is in use now on a satellite, PanAmSat 5, but it serves only to keep the satellite in its proper orbit.

The larger ion drive on Deep Space 1 is the first to be used for long-term propulsion. If it performs as well as engineers hope, future solar system missions powered by ion drives will have to carry less propellant than they would otherwise and so will be smaller and less expensive to build.

Some source of electricity other than solar panels may be required for missions very far from the sun, but that should not be a major obstacle. Generators that rely on the heat generated by the decay of plutonium-238 have been used in the past to supply electrical power for spacecraft traveling to remote regions, such as Cassini, now on its way to Saturn. New forms of generators based on plutonium-238 are now in development.

Deep Space 1's other major innovation, its use of artificial intelligence, could also reduce the cost of future missions, because it means fewer engineers have to track and monitor every detail of the spacecraft's systems. The craft supports a software "remote agent" that monitors the overall state of the vehicle and issues commands to various systems as it deems appropriate, in accordance with high-level goals provided by the ground. The craft thus decides for itself when to transmit data to Earth, when to make course corrections and when to image different objects. It signals its general state of health to earth-bound controllers but does not routinely send readouts from all sensors.

Supporting the remote agent is a sophisticated automatic navigation system that identifies the craft's position by observing the stars and the asteroids. The craft has the orbits of 250 asteroids stored in its computer memory. It will image several asteroids a few times each week and use the information to calculate and possibly modify its own trajectory, by altering thrust levels from either the ion drive or from separate chemical thrusters (which burn hydrazine and are standard issue on spacecraft).

If all goes according to plan, Deep Space 1 will fly within 10 kilometers of the asteroid 1992 KD in July 1999 and send back images taken in ultraviolet, infrared and visible light. The mission could then be extended to approach a solar system body known as Wilson-Harrington, a comet thought to be changing into an asteroid (it has burned off nearly all its volatile components), and the comet called Borelly.

But Deep Space 1's scientific promise takes a back seat to its innovative technology. By making complex missions to distant bodies more affordable, systems like those to be tested during the flight could open up new vistas in solar system exploration--and beyond.


Images: NASA