The next generation of space probes is being powered by highly efficient electric plasma rockets. These rockets—powered by ionized xenon gas—produce very low thrust compared with their solid- or liquid-fueled cousins, but use so little propellant that they last much longer. Their constant acceleration results in speeds that ultimately surpass those of chemical rockets. This makes them ideal for deep-space missions. The Dawn spacecraft is on just such a mission: a journey first to the giant asteroid Vesta and then on to the dwarf planet Ceres.

Engines that can electrically produce and manipulate ionized gas are so efficient that they require only one tenth the fuel that a chemically propelled rocket would need to reach the asteroid belt. The thrust from a plasma engine is equivalent to the weight of a sheet of paper resting on your palm. But because the thrust and acceleration is constant, speeds of 12.5 miles (20 kilometers) per second and greater are eventually achieved.

Two types of electric rockets are currently in use: the ion thruster, which generates thrust using a beam of positive xenon ions, and the Hall thruster, which uses xenon ions and electrons for propulsion. On the drawing board is the magnetoplasmadynamic (MPD) thruster, which will reach even greater speeds than today's engines. They are planned for use by piloted spacecraft and for transport of heavy cargo.