Here's the official announcement:
"The spacecraft was approximately 38,000 miles (61,000) kilometers from Ceres when it was captured by the dwarf planet’s gravity at about 4:39 a.m. PST (7:39 a.m. EST) Friday.Mission controllers at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California received a signal from the spacecraft at 5:36 a.m. PST (8:36 a.m. EST) that Dawn was healthy and thrusting with its ion engine, the indicator Dawn had entered orbit as planned."
After spending 14 months on Vesta, the largest asteroid in the belt between Mars and Jupiter, Dawn is now settled in for a long-term stay with Ceres. By comparing the asteroid and the dwarf, NASA scientists hope they'll learn about the formation of all the objects in the asteroid belt, the evolution of the planet Ceres and the history of our entire solar system.
Friday morning's orbital entry was pretty tame as far as space missions go. You may remember the intense mission control footage streamed during the landing of the Rosetta orbiter's Philae probe last fall: Engineers and scientists were quite literally on the edge of their seats waiting to find out whether the mission was a success. In an interview with The Washington Post on Thursday, Dawn's chief engineer Marc Rayman assured us that Dawn 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, he 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."
Dawn, which was designed and built by Orbital ATK in Dulles, Va., in partnership with NASA JPL, has a truly unique propulsion system. It's the first mission to rely on ion propulsion after the technology 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.
To Rayman and his colleagues, the actual entry isn't so exciting. They were incredibly confident, which is understandable given 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.
Rayman said he was most excited to hear about evidence of liquid water. Scientists know that there's probably ice, and that liquid water was most likely on the planet at some time. But there may be hidden lakes or oceans beneath the surface.
Joe Makowski, who serves as Dawn's program manager at Orbital ATK, explained that Ceres's water is what makes it so important to study.
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"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."
The scientists are also (understandably) excited about those bright spots, the two shining 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.
Unfortunately, we're not going to hear anything much for over a month. Dawn is going to be on the dark side of the planet for a while, so it won't be taking the increasingly-clear photos of Ceres to which we've become so accustomed. It takes its next set of photos on April 10 and will start its first intensive science observations on April 23.