the development of a solar/ion engine may offer a new form of propulsion between the stars. Such slow-moving engines would generate little thrust, but they can maintain that thrust for years at a time. These engines concentrate solar energy from the sun, heat up a gas like cesium, and then hurl the gas out the exhaust, giving a mild thrust that can be maintained almost indefinitely. Vehicles powered by such engines might be ideal for creating an interplanetary “interstate highway system” connecting the planets.
