The NASA spacecraft Dawn has spent more than seven years traveling across the solar system to intercept the asteroid Vesta and the dwarf planet Ceres. Now in orbit around Ceres, the probe has returned the first images and data from these distant objects. But inside Dawn itself is another first—the spacecraft is the first exploratory space mission to use an electrically-powered ion engine rather than conventional rockets. The ion engine will propel the next generation of spacecraft. Electric power is used to create charged particles of the fuel, usually the gas xenon, and accelerate them to extremely high velocities. The exhaust velocity of conventional rockets is limited by the chemical energy stored in the fuel’s molecular bonds, which limits the thrust to about 5 km/s. Ion engines are in principle limited only by the electrical power available on the spacecraft, but typically the exhaust speed of the charged particles range from 15 km/s to 35 km/s. What this means in practice is that electrically powered thrusters are much more fuel efficient than chemical ones, so an enormous amount of mass can be saved through the need for less fuel onboard. With the cost to launch a single kilogram of mass into Earth orbit of around $20,000, this can make spacecraft significantly cheaper. This can be of great benefit to commercial manufacturers of geostationary satellites, where electric propulsion can allow them to maneuver adding new capabilities to the satellite during its mission. However, for scientific missions such as interplanetary travel to the outer regions of the Solar System, electric propulsion is the only means to carry useful scientific payload quickly across the enormous distances involved. More via Quartz.
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