The pace at which space is being exploited and explored is at present slowed by the cost of launching payloads. Typically, a payload will be placed by a rocket into low-Earth orbit (around 400 km), and then be boosted higher by rocket thrusters. But just transporting a satellite from the lower altitude to its eventual destination can run to several thousand dollars per kilogram of payload. To cut expenses, space experts are reconsidering the technology used to place payloads in their final orbits. A distinctly different type of propulsion may provide a cheap, lightweight, and reliable alternative to conventional rocket thrusters. Called an electrodynamic tether, it is a current-carrying wire that harnesses the force exerted by Earth s magnetic field. NASAs US $7 million Propulsive Small Expendable Deployer System (ProSEDS) experiment will show that an 11-kg, 5-km-long, 1.2-mm-diameter aluminum wire can rapidly remove a rocket s upper stage from orbit. Should this and subsequent experiments succeed, they will pave the way to replacing rocket thrusters with tethers in certain space applications.