NASA’s Dawn spacecraft orbited Vesta for 14 months in 2011 and 2012, and then departed for Ceres, where it’s orbited since March 2015. This week, Dawn’s been taking stereo views of the dwarf planet’s terrain from just 233 miles (375 kilometers) up or about 20 miles lower than the altitude of the International Space Station.
Dawn’s dual-asteroid mission will officially end this summer, but principal investigator Chris Russell of the University of California at Los Angeles and his team are asking NASA for an extension. Spacecraft normally crash-land on their targets at mission end, but that won’t happen with Dawn because it’s not been sterilized. Scientists don’t want to take the chance of introducing some kind of microbe onto the otherwise pristine dwarf planet. Instead, it makes more sense to use Dawn’s thrusters to move it away from Ceres and send it on a mission to a third asteroid.
The probe can’t go too far because its understandably low on fuel, but Russell figures why not make the best of what’s left by visiting another asteroid nearby. For now, he’s keeping the choice a secret, but if NASA approves the extension, he’ll divulge the target. Unlike Dawn, the European Space Agency’s Rosetta orbiter has been sterilized and will be crashed into comet 67P/C-G at the conclusion of its mission later this year.
Instead of conventional rocket fuel, Dawn uses the element xenon as propellant. Xenon shows up in everyday life in the new, rather terribly bright xenon car headlights and in photographic flashes (strobes). An electrically-charged plate at the back of the engine accelerates electrified xenon atoms out of the thruster. As the atoms depart out the back end of the craft, they push back against the engine, nudging the spacecraft forward.
Ion engines thrust with a light touch that begins slowly but adds up soon enough in the frictionless environment of space to great speeds. Will the funds be found to continue on to a third asteroid? Cross your fingers NASA will provide the funding.