The ion rocket is another means of providing thrust, more gradually but also more efficiently than a conventional rocket.
The material ejected here is xenon, a heavy inert gas, compressed inside strong containers (mercury vapor was also considered). As in the nuclear rocket, the energy which propels the xenon jet comes from a different source; here it is electrical energy from solar cells.
The reason an ion rocket is much more efficient than ordinary rockets is the way its jet is produced. Rather than confining a hot gas in a chamber and ejecting it through a nozzle--a process limited by the temperature which the nozzle can stand--an ion rocket first strips negative electrons from the xenon atoms, leaving them as "ions", atoms with a net (positive) electric charge. The ions can now be accelerated by electrical forces, to velocities much higher than those obtained from a hot gas, but without the need for a high temperature. Inside every video tube is an "electron gun" which similarly accelerates the narrow beam of electrons that paints the video picture on the screen.
Incidentally, the emerging jet of ions must be combined with a stream of negative electrons from a separate electron gun. Without this addition, only positive ions would be emitted, and the satellite would quickly become negatively charged by the stripped electrons left behind. The negative charge would then pull back the ions and undo all the work of the ion gun.
Of all the exotic means of propulsion in space, this one is probably closest to practical use. The XIPS ("zips") ion engine developed by the Hughes corporation was tested in the laboratory and then aboard a Russian spacecraft, launched 6 October 1997; it is the one shown on top of this page. An experimental satellite "Deep Space 1", equipped with an ion engine was designed to test the concept and to be the first in NASA's "New Millenium" series.
As with solar sails, solar ion engines are mainly practical in the inner solar system, where ample sunlight is available. For more distant missions it is in principle possible to drive an ion engine by a small onboard nuclear reactor. Such reactors have already flown in space, although they are currently out of favor, in part because a reactor-equipped Russian spacecraft re-entered the atmosphere and crashed into a frozen lake in Canada. For exploring the outer solar system, however, some sort of nuclear power seems essential.
Deep Space 1"Deep Space 1" was successfully launched 24 October 1998, but the initial 17-hour test of its ion engine on November 10 ended prematurely after 4.5 minutes, which was tentatively blamed on a small short circuit of the accelerating grids, caused by a tiny piece of loose metal. Operators kept trying to restart the engine, hoping to evaporate the offending piece, and apparently that did the job, because some weeks later the engine restarted. The ion rocket has operated satisfactorily after that.
On 28 July the spacecraft flew by asteroid Braille, passing at a distance of about 26 km and performing observations. It continued to Comet 19P/Borrelly, observing its properties and its interaction with the solar wind. For more, see here and here.
SMART 1(Added 16 November 2004) A European satellite demonstrating ion propulsion has reached an orbit around the Moon. SMART-1 ("Small Missions for Advanced Research and Technology -1") was launched aboard an Ariane-5 rocket on September 27, 2003, an 809-lb spacecraft with large solar panels and 181 pounds of xenon fuel, of which 131 have been used so far. After a conventional launch to Earth orbit it started its ion engine to gradually increase its orbital radius, in the process circling Earth 331 times. It will continue to use its engine to lower its orbit around the Moon, to a polar orbit passing 185 miles above the south pole and 1850 above the north pole.
SMART-1 carries scientific instruments to take images and observe x-rays and infra-red emission from the Moon. For more details, see an article here.
Next Stop: #34 Orbits in Space
Author and Curator: Dr. David P. Stern