NASA's first New Millennium mission, Deep Space 1, will be propelled through interplanetary space by an ion drive engine.

Tim Furniss/LONDON

AS A PART OF SEVERAL new space-engine initiatives by the US space agency NASA, the prototype of a xenon-ion engine is undergoing a year-long test firing at the agency's Jet Propulsion Laboratory (JPL), in preparation for a mission to be launched in 1998 to explore a comet and an asteroid.

The spacecraft, called the Deep Space 1 - being built by Spectrum Astro at a cost of $30 million - is the first in NASA's $100 million-a-year New Millennium programme. The New Millennium project is part of NASA's "smaller, faster, better and cheaper" way of doing things, directed especially at creating and validating the essential technologies - including ion drive - required for interplanetary and Earth science missions next century (Flight International, 10-16 January).

The 100kg Deep Space 1 will fly past the comet West-Kohoutek-Ikemura and an asteroid named after teacher-astronaut Christa McAuliffe, who died in the 1986 NASA Challenger Space Shuttle accident. The craft will carry a miniaturised spectrometer to produce chemical maps of the asteroid and comet. "The prime objective of the mission, however, is not the science, but the technology," says Richard Howard, of NASA's Space Science office.

The 260mm-diameter ion-drive thruster, developed by NASA and the Ballistic Missile Defense Organization (BMDO), expels high-velocity beams of xenon gas, ionised by using 2kW of electricity from the craft's BMDO-built solar arrays. The efficiency, compared with brute-force chemical systems, comes from expelling the molecules at a much higher speed than the 16,640km/h rate of chemical engines.

The ion engine's 1.1 x 10-6 (0.00024lb) of thrust, visible only by its blue, glowing, exhaust, will increase the spacecraft's speed very gradually during its interplanetary journey, to 35,200km/h for the fly-bys. This is a slow speed compared with the fastest recorded post-boost velocity of 54,600km/h by the European Space Agency-NASA Ulysses solar polar orbiter, when it was despatched from Earth.

Whereas the Ulysses relied on the powerful initial boost of its solid-propellant upper stage to give it the momentum for the journey, the Deep Space 1, after an initial boost by the Delta 2 expendable launch vehicle, will rely on the continuous, efficient, firing of its lightweight ion engine, consuming just 45kg of xenon, to maintain the pace. While atoms -charged by an electric arc which removes electrons around its nucleus - are fired in vast numbers out of the thruster at more than 112,000km/s, their accumulative mass is so low that the spacecraft accelerates only millimetres per second per second in its early stages.

Many other space agencies - including the UK's AEA Technology's Culham Laboratory at Abingdon and the Defence Evaluation and Research Agency at Farnborough - have been studying ion drive for deep-space missions, while ion thrusters have already been flown on some satellites. AEA-DERA UK-10 thrusters will be flown on the European Space Agency's Artemis satellite. Like the UK Atomic Energy Establishment - the commercial arm of which is the AEA - NASA has "-been experimenting with ion-drive engines for 30 years", says Jack Stocky, manager of the ion-propulsion-system project. "This test, however, will be the most extensive instrument-endurance test of an ion engine ever performed," he adds.

Built at NASA's Lewis Research Center in Cleveland, Ohio, the engines will be tested for 8,000h (330 days) in the vacuum chamber. "Ion engines have such a low thrust that they cannot operate in the atmosphere and have to be tested in a vacuum," says Dr John Brophy, one of the project managers of the test. This will involve running at full power for two days at a time, shutting down for 1h and restarting, the process being repeated during the full duration of the evaluation.

Source: Flight International