Scientists with the National Aeronautics and Space Administration (NASA) realized a nearly 40-year-old dream in July 1999, when Deep Space 1 (DS-1), the first spacecraft to employ an ion engine as its main propulsion system, made a successful fly-by of an asteroid. Instead of a chemical reaction, the main engine of DS-1 created thrust by stripping electrons from the element xenon to create ions (electrically charged atoms) and ejecting them in a high-speed beam from the back of the vehicle. NASA built its first ion engine in 1960, but the expense of launching spacecraft into orbit made the technology too risky to use on an actual mission until DS-1. Although the ion engine aboard DS-1 produced only a tiny amount of thrust (about equal to the force of a single piece of paper resting on a table), the engine could run for very long periods using relatively little fuel, enabling it to eventually go much faster than a chemical rocket.

Although NASA focused much of its research and development effort in 2000 on reducing the cost of launching payloads into orbit around Earth, the most exciting area of space research was the progress being made in space propulsion technologies other than chemical rockets, which were still the workhorses of space. Although many of these systems are still experimental-and some are only on the drawing board-they may be the technologies that will power future missions to explore the universe.

The ion engine aboard the DS-1 is an example of a type of propulsion system known as Solar Electric Propulsion (SEP). This system uses solar panels to generate the electrical power needed to operate a highly efficient ion engine. However, because sunlight decreases in intensity as an SEP vehicle gets farther away from the sun, the technology was limited to robotic missions exploring only about to the distance of Jupiter. Scientists expected, however, that by 2005 they would be able to build SEP vehicles with much more efficient engines.