The ion thruster, which uses electrical power from solar panels to power a rocket firing an ion beam instead of one that burns solid or liquid fuel, is an old idea which has become increasingly influential in recent years thanks to technological advances. The prospects for ion-powered spacecraft, perhaps even (one day) manned spacecraft, are now bright.
Unlike conventional rocket engines, which burn liquid or solid fuels and are propelled by emitting the exhaust gases from the burning fuel, ion thrusters use electricity to generate a beam of ions which pushes the spacecraft forward. Ion-powered rockets, says NASA scientist Marc Rayman, are about "acceleration with patience." In contrast to the very short but incredibly powerful burns used to accelerate conventional rockets, ion thrusters supply a slow, steady push through a continuous burn that could last for months or even years. A detailed mechanical description of how an ion thruster works has been published online by NASA.
The advantages of ion thrusters were long appreciated theoretically - indeed, the idea was first published a century ago by Russian scientist Konstantin Tsiolkovsky - but, until now, conventional rockets were always cheaper and easier to build. Until the last decade or so, deep space probes with ion thrusters existed mainly in science fiction, where they are commonplace. The exception are a few types of small ion thrusters, like Hall thrusters, which Princeton University says were developed by the United States and the Soviet Union for navigational purposes on satellites orbiting Earth. Satellites must occasionally adjust their orbits, either to move into new positions or to resume their usual position after being gradually slowed by the drag from the trace particles in Earth's upper atmosphere. For this purpose, thrusters powered by solar panels are much more efficient than conventional rocket sources with their heavy fuel requirements.
For deep space travel, however, ion thrusters have really only been more effective than conventional rocket engines since new technological advances were made beginning in the 1990s. In 2000, NASA's Deep Space 1 project set a record by successfully using an ion thruster in space for 200 continuous days. Since then, a number of interplanetary space probes were launched using new ion thrusters, like NASA's Dawn probe, sent to the asteroid belt, and the European Space Agency's SMART-1 probe, which was deliberately crashed on the Moon's surface in 2006.
With the feasibility of ion thrusters for space probe missions now proven, NASA says, the ion thruster has become "the propulsion of choice" for next-generation missions. NASA planned to use a new ion thruster to power the Jupiter Icy Moon Orbiter, although that project has since been cancelled. It may still be used on the mission's successor, the Jupiter Icy Moon Explorer, which the European Space Agency hopes to launch in 2022. Because of budget cuts, most ambitious deep space probe missions are now in grave jeopardy, but there is a good chance that future missions will be powered by ion thrusters. In the meantime, NASA continues to develop experimental designs like HiPEP in the hopes that they will one day be deployed on an actual spacecraft.