Propulsion

A peopled mission using ion propulsion to escape LEO for an inner-solar-system destination might not be practical. The astronauts might accumulate a dangerously high radiation dose as their craft slowly spirals up through Earth's Van Allen radiation belts. However, SEP may be applied to peopled interplanetary missions departing from higher orbits.

As of this writing, several interplanetary probes have been launched utilizing SEp for post-launch, or post-Earth-escape prime propulsion. The first of these was NASA's Deep Space 1 (Figure 11.3), which was launched from Cape Canaveral in October 1998 on board a Delta 2 rocket.

Deep Space 1, with a launch mass of 489 kilograms, was propelled after Earth escape by a xenon-fueled solar-electric rocket with an exhaust velocity of about 30 kilometers per second. As part of the NASA New Millennium Program, Deep Space 1 was primarily a technology demonstrator of 12 new technologies, including SEP.

The solar arrays of the probe provided a maximum of 2.4 kilowatts of electricity, which was used to power scientific items including imagers, particle monitors, and an infrared spectrometer as well as the ion engine.

After Earth escape, the ion drive was turned on. The SEP on board Deep Space 1 operated successfully for several hundred days in space. The drive was operated continually for 2.5 months, expelling about 20

FIGURE 11.3 NASA's Deep Space 1 approaches comet Borrelly. (Courtesy NASA)

kilograms of xenon propellant and altering the craft's heliocentric velocity by approximately 1 kilometer per second.

During its three-year mission, Deep Space 1 was redirected to approach several inner-solar-system objects. It returned excellent images of Comet Borrelly and Asteroid Braille during its close-up exploration of these bodies.

After the successful demonstration of SEP technology on board Deep Space 1, other ion-propelled probes were launched in rapid succession. Europe's SMART-1 demonstrated the utility of Hall thrusters in performing long-duration Earth-escape maneuvers. This probe required approximately 13 months to spiral from LEO to the vicinity of the Moon. Although much more time was required for cis-lunar maneuvers than would have been necessary by chemical rockets, ion propulsion is much more fuel-efficient.

Another SEP-propelled interplanetary venture, Japan's Hayabusa (Muses-C), was launched in May 2003. This craft is currently performing a sample return mission to Asteroid 25143 Itokawa. If all continues to go well, samples should parachute to Earth near Woomera, Australia, in June 2007.

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