Between Mars and Jupiter lies a little-known dwarf planet named Ceres, one of the last unexplored bodies in the solar system. Its mysteries include the presence of ice and maybe liquid water, and two shining reflective spots in one of its craters.
On Friday, NASA's Dawn spacecraft entered orbit around Ceres to solve some of these mysteries. It's the first mission to visit a dwarf planet, and also the first time a spacecraft has orbited two different alien bodies during its mission. Dawn recently spent 14 months at Vesta, the largest asteroid in the belt between Mars and Jupiter, and is now settled in for a long-term stay with Ceres, its final stop.
By comparing the asteroid and the dwarf planet, NASA scientists hope to learn about the formation of all the objects in the asteroid belt, the evolution of Ceres and the history of the entire solar system.
Joe Makowski, Dawn's program manager at Orbital ATK, explained that Ceres' water is what makes it so important to study.
"The first body that we visited, Vesta, was what we expected - very dry, basically rocky and heavily cratered, and probably pretty much intact in that fashion for billions of years," he said. "Ceres is in the same asteroid belt, but it's very different. It's very large - almost twice the diameter of Vesta - and it contains a lot of water. Visiting both will enable us to understand why one ended up so dry and one has so much water, and in turn how they developed and why they're so different."
Dawn is going to be on the dark side of the planet for about a month. It will take its next set of photos on April 10 and will start its first intensive science observations on April 23.
Friday morning's orbital entry was pretty tame, as space missions go. It was not like the landing of the Rosetta orbiter's Philae probe on a comet last fall, when engineers and scientists were quite literally on the edge of their seats waiting to find out whether the mission was a success.
Marc Rayman, Dawn's chief engineer, said the spacecraft wouldn't elicit much nail-biting from its creators.
"This is going to be a typical day on Dawn for people. When a conventional spacecraft enters an orbit, it has to execute this big whiplash-inducing, bone-rattling maneuver to drop in," Rayman explained. But the ion propulsion system and precise maneuverability that made Dawn's two-visit mission possible means that it has more than one shot.
"If for some reason there's a glitch" on Friday, Rayman said, "a cosmic ray or just bad luck, and it doesn't go into orbit, that's okay. We'll just restore it to normal operations and go into orbit some other day."
NASA said that the spacecraft was captured by Ceres gravity at about 7:39 a.m. EST, when Dawn was approximately 38,000 miles out.
Dawn, which was designed and built by Orbital ATK of Dulles, Virginia, in partnership with NASA's Jet Propulsion Laboratory, has a unique propulsion system. It's mission is the first to rely on ion propulsion, a technology that was successfully tested on the Deep Space 1 mission in 1998. Launched in 2007, Dawn has used just 10 percent of the fuel that a spacecraft with a traditional combustion engine would use.
"It's like having a car that gets 300 miles per gallon," Rayman said.
Traditional engines work by heating or compressing fuel. When the heated fuel is pushed out of a rocket's nozzle, it pushes the rocket in the other direction. Ion engines do the same pushing, but without heating or compressing fuel: The engine charges the gas xenon, giving its particles the electrical charge that turns them into ions. Two metal grids with voltage between them work to shoot the ions out of the thrusters at extremely high speed, which pushes with great thrust to send the spacecraft in the other direction.
Rayman and his colleagues were confident because of Dawn's forgiving nature when it comes to mishaps.
"Dawn had already been operating its ion engine for more than five years, so this is just like any one of about 1,800 days. It's just flying along, emitting its blue-green beam of ions. It's very much a routine day for us in every sense of the word," Rayman said.
Except for the thrill of finally reaching their destination.
"It's exciting that we're on the verge of actually exploring this alien world," he said.
Scientists are excited about the evidence of liquid water and also about those bright spots, the two reflective patches that have shown up in the middle of one of Ceres's craters.
"I don't think it's possible to look at those without thinking of shining beacons calling out to us as travelers on the cosmic seas," Rayman said.