NASA’s Dawn spacecraft officially began its approach to the dwarf planet Ceres this week. Ceres was the first asteroid discovered and at 590 miles (950 km) across, the largest of the Main Belt asteroids that cycle between the orbits of Mars and Jupiter. No probe’s ever visited Ceres, and our best photos taken by the Hubble Space Telescope show only a mottled, blurry disk.
Dawn is currently 400,000 miles (640,000 km) from Ceres, approaching it at around 450 miles per hour (725 km/hr). It’s the only spacecraft ever to orbit two solar system targets. You probably recall its first mission (2011-2012) to the asteroid Vesta, also in the Main Belt. There we learned that Vesta mimicks a planet in miniature, having “differentiated” through radioactive heating into core, mantle and crust.
Millions of miles and more than a year separated that success from its next eagerly awaited target now just 9 weeks away.
“Ceres is almost a complete mystery to us,” said Christopher Russell, principal investigator for the Dawn mission, based at the University of California, Los Angeles. “Ceres, unlike Vesta, has no meteorites linked to it to help reveal its secrets. All we can predict with confidence is that we will be surprised.”
Yes, meteorites. We have tons of samples of Vesta in pieces fallen to Earth as eucrite, Howardite and diogenite meteorites, a subset of a larger clan of meteorites called achondrites. Achondrites (Ay-KON-drites) are rocks that have been melted and processed on asteroids and resemble volcanic rocks on Earth.
The two planetary bodies are thought to have gone down different evolutionary roads. Ceres likely formed later than Vesta and possesses a cooler interior. The fact that Vesta appears to have very little water tells us it formed earlier, when radioactive material was more abundant. Heat from radioactivity would have cooked out the water. Ceres, in contrast, has a thick ice mantle and may even have an ocean beneath its icy crust.
Great explainer on Dawn’s ion propulsion engine
NASA employed ion propulsion to travel the many millions of miles of the dual mission cheaply and efficiently. Instead of conventional rocket fuel, an electric charge is applied to atoms of xenon, a gas used in photographic flashes (strobes). Next, an electrically-charged plate at the back of the engine accelerates the electrified atoms out of the thruster. As they leave, 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. Check the video for more details.
Later next month, we’ll start to receive better images of Ceres than Hubble’s been able to provide. We’ll update the mission’s progress at that time.