SMART Technology: Moon Mission to Test Solar Engine With No Limits

Wed Jul 31,10:01 AM ET

By Heather Sparks
Staff Writer,

Electric power and ion propulsion are about to combine for only the second time to propel a spacecraft beyond Earth orbit. Next April the European Space Agency's SMART-1 lunar satellite heads off to explore the Moon from an orbital position.

Electric propulsion has been used in low-Earth telecommunications satellites for years, and a high-tech ion engine powered NASA ( news - web sites)'s Deep Space 1 test craft from 1998 until late last year. But before this technology is ready for prime-time missions to other planets and beyond, more tests are needed.

While SMART-1 is slated to perform science experiments at the Moon, its main mission is to test this technology.

The engine being built for SMART-1 was originally designed by the Soviet space program in the 1970s. It does not burn fuel like chemical rockets; instead solar panels convert sunlight into electricity, which powers the atomic-scale propulsion system.

"There is no limit to where this technology may take us," said Denis Estublier, the propulsion engineer for the mission.

Most spacecraft use rockets for propulsion, but the rocket fuel hydrazine is enormously heavy. Burdensome fuel tanks limit payloads, from how many scientific instruments can be carried to the number of astronauts. It's virtually impossible to embark on any voyage beyond the Moon without the help of gravity assist -- looping around a planet for a slingshot effect -- which limits launch windows and creates long, meandering trajectories.

Electric propulsion avoids these problems.

"To get rid of gravity assist, a...more efficient propulsion system is needed, so that a craft can fly directly to Saturn for example, and not have to spend the time it takes go from Earth and then to Venus and then back again, before it finally travels out to Saturn," as the Cassini mission did, said John Brophy, a propulsion engineer at the Jet Propulsion Laboratory. The limits so far will be less than those imposed by chemical rockets.

Electric-ion propulsion is more efficient because its energy source (the solar panels) and the propulsion chemical (xenon) are separate, Estublier said. This way, the xenon atoms are used at an atomic scale, not on a massive scale like rocket fuel, which is both the fuel and energy source.

Instead of the blazing chemical fire of solid hydrazine rocket fuel creating momentum, on SMART-1 highly efficient lithium solar panels will power what's known as a Hall-effect thruster.

The thruster is a 6-inch (10-cm) ceramic chamber ringed with magnets. On one end of the chamber a cathode emits electrons generated by the solar panels. On the other end of the chamber a positively charged anode attracts the electrons.

As xenon gas is released into the chamber, electrons rush in with a momentum caused by the magnets and knock electrons off the xenon atoms. This creates positively charged xenon ions, and even more electrons. The electrons continue to bombard the xenon, creating even more positive xenon ions.

The cathode then pushes the xenon ions out of the chamber in a glowing blue ion beam.

The atomic scale of the mechanism means the thrust on SMART-1 will be very small, equivalent to the pressure a sheet of paper applies to the palm of your hand. But because the xenon will be energized so much more efficiently than in a rocket, the ion engine will run much longer, Estublier said.

Over time the momentum of the craft will increase, eventually gaining speeds faster than a rocket would be able to produce.

"Imagine SMART-1 is a car that uses 10 times less gas," Estublier said. "The car may also go very slow when it starts, but it gains a certain velocity every hour that it runs."

The ability to run continuously, all the while gaining velocity, will aid energetically demanding missions such as the upcoming ESA trip to Mercury. This journey will demand more energy output than a trip to Mars or Venus, Giuseppe Racca, the SMART-1 project manager, said.

Racca reported last year in the science journal Planetary and Space Science that an electric propulsion spacecraft would take two-and-a-half years to reach Mercury, beating a conventional spacecraft by a year and four months. The extra weight of the rocket fuel would require gravity assists from both Earth and Venus, he said.

The low thrust of electric propulsion has another advantage: maneuverability. The tiny push of the thruster, pointed in different directions, will make for more precise orientation of the craft.

NASA's Deep Space 1 mission tested the maneuvering capability of its thruster after an onboard star-tracking device broke down last year. The thruster's soft touch was able to control pitch and roll with more stability than the original steering mechanisms. Brophy said the images taken at that time were less shaky because of it.

Ion propulsion was first developed in the 1960s, but it wasn't taken seriously until recent changes at NASA forced planners to look for cheaper alternatives to space travel. Brophy and others say similar propulsion systems could one day make it possible to send dozens or hundreds of soup-can-sized spacecraft into orbit around Saturn or onto the surface of Mars.

Efficient propulsion will create more opportunities for solar system travel.

But getting to interstellar space will require more power. Nuclear-powered ion propulsion will be needed, said Brophy, who has worked almost exclusively on ion propulsion for 18 years.

If the success of DS1 is any indication, SMART-1 stands to be quite a successful test mission. DS1 exceeded expectations, and an identical test engine, still running in a vacuum chamber in a JPL lab, has processed three times the xenon it was design to handle.

The scientists at ESA expect SMART-1 will perform just as well, if not better. "You don't know until you fly it in space, and if something is wrong it will be too late," Estublier said. "But the Moon group has big expectations."

Editor's Note: Yesterday's Science Tuesday feature explained what SMART-1 will do for scientists eager to gather more data on the Moon and possible water there.