In early March, NASA's Dawn spacecraft will enter the orbit of the dwarf planet Ceres. "We're going to see a whole new world," says Marc Rayman, Dawn's chief engineer and mission director. Eight years since its launch, and four years after visiting the second-largest object in the asteroid belt, Vesta, Dawn will be the first spacecraft to orbit two alien protoplanets in a single mission.

That achievement is made possible by its ion engines, which are about one foot in diameter. Inside each engine, xenon atoms are bombarded with electrons to form ions. Metal grids at the back of each engine are charged to about 1,000 volts and shoot out the ions at up to 90,000 miles per hour. The thrust is minuscule. (To approximate the pressure the engines exert on Dawn, take a sheet of paper and place on it on your palm. That's it!) But in a zero-gravity, frictionless environment, the effect of this thrust gradually builds up. At its fastest, Dawn can move at about 24,000 miles per hour. And because the ions move at such a high velocity, the engines need less propellant, making them 10 times more efficient than conventional chemical fuels.

Ion propulsion "is taking over the industry," says John Brophy, who developed Dawn's engine.