NASA's Dawn spacecraft slipped into orbit around the dwarf planet Ceres Friday, setting the stage for unprecedented close-range observations of the largest body in the asteroid belt between Mars and Jupiter, an enigmatic world featuring heavily cratered terrain, smooth plains and mysterious bright spots.
The solar-powered spacecraft, launched in 2007, was captured by Ceres' gravity at about 7:39 a.m. EST (GMT-5). An hour later, radio signals from Dawn confirmed the spacecraft was in good health as it moved into orbit and that's its low-thrust ion propulsion system was firing as expected.
"Since its discovery in 1801, Ceres was known as a planet, then an asteroid and later a dwarf planet," Marc Rayman, Dawn chief engineer and mission director, said in a NASA statement. "Now, after a journey of 3.1 billion miles and 7.5 years, Dawn calls Ceres home."
Launched in 2007, Dawn flew past Mars for a velocity boosting gravity assist in 2009 and entered orbit around the asteroid Vesta in July 2011. After 14 months of unprecedented exploration, the spacecraft's ion propulsion system fired up to boost the probe out of orbit and onto icy Ceres, a 900-million-mile voyage that took two-and-a-half years to complete.
At the moment of capture, Dawn was about 38,000 miles from Ceres. Over the next several weeks, the spacecraft will slowly spiral inward, reaching an altitude of about 8,400 miles by April 23 to begin close-range science operations. By the end of the year, it should be orbiting at an altitude of just 235 miles, a bit lower than the International Space Station orbits Earth.
Unlike more typical planetary missions that use chemical rockets requiring make-or-break maneuvers to achieve orbit or descend to the surface, Dawn's arrival at Ceres was a much more sedate affair, thanks to its low-thrust ion engine and Newton's law of gravity.
"If you had been in space watching the event ... it would not have looked much different from the 1,885 days of ion thrust that had preceded it," Rayman said in a blog posting. "The spacecraft was perched atop its blue-green pillar of xenon ions, patiently changing its course, as it does for so much of quiet cruise."
The ion propulsion system uses electricity generated by two huge solar panels to accelerate xenon ions, imparting a gentle push that, over many months, can boost the spacecraft to enormous velocities.
"In this phase of the mission, the engine expends only a quarter of a pound (of xenon fuel) per day, or the equivalent of about 2.5 fluid ounces," Rayman wrote. "So although the thrust is very efficient, it is also very gentle. If you hold a single sheet of paper in your hand, it will push on your hand harder than the ion engine pushes on the spacecraft at maximum thrust."
At the current throttle setting, he said, it would take Dawn "almost 11 days to accelerate from zero to 60 mph."
"That may not evoke the concept of a drag racer," he added. "But in the zero-gravity, frictionless conditions of spaceflight, the effect of this whisper-like thrust can build up. Instead of thrusting for 11 days, if we thrust for a month, or a year, or as Dawn already has, for more than five years, we can achieve fantastically high velocity. Ion propulsion delivers acceleration with patience."
As a result, Dawn's arrival at Ceres was a decidedly low-key affair. There was no question the spacecraft would be captured by the dwarf planet's gravity, thanks to the months of thrusting it took to get to this point.
"So the flight team was not tense. They had no need to observe it or make a spectacle out of it," Rayman wrote. "The drama is not in whether the mission will succeed or fail, in whether a single glitch could cause a catastrophic loss, in whether even a tiny mistake could spell doom. Rather, the drama is in the opportunity to unveil the wonderful secrets of a fascinating relict from the dawn of the solar system more than 4.5 billion years ago."
Dawn, the centerpiece of a $473 million mission, is the first spacecraft to orbit more than one body, the first to visit a dwarf planet -- Ceres -- and the first operational science probe to rely on ion propulsion.
Discovered in 1801, Ceres was the first -- and largest -- body ever found in what is now known as the asteroid belt, a heavily cratered, roughly spherical world measuring 606 by 565 miles.
"One of the first things you notice is how round Ceres is," Carol Raymond, Dawn deputy principal investigator, said Monday. "And Ceres' roundness is one of its planetary characteristics. We also know that Ceres is much lighter than the rocky planets and so we know it retained a lot of water and light volatile elements that were present in the solar nebula when Ceres was formed.
In contrast, bodies like the moon and Vesta suffered melting from major impacts that caused the water and other light elements to boil away, "leaving them dry and rocky," Raymond said.
"One of the prime motivations of the Dawn mission is to examine these building blocks of the planets, Vesta and Ceres, which are two intact proto-planets from the very dawn of the solar system," she said. "They're literally fossils that we can investigate to really understand the processes that were going on at that time."
One major mystery is what might be the source of two brilliant spots of light seen in a crater on Ceres during Dawn's approach.
"Suffice it to say, these spots were extremely surprising to the team, and they have been puzzling to everybody who's seen them," Raymond said Monday. "The team is really, really exited about this feature because it is unique in the solar system."
The bright spots presumably are reflected sunlight, possibly from ice or salt deposits that may have been uncovered by a recent impact. Other, less likely, scenarios include the action of ultra-low temperature "cryovolcanism." But for now, no one knows. And given their unusual brightness, interest is high.
"We will be revealing its true nature as we get closer and closer to the surface," Raymond said. "So the mystery will be solved, but it is one that's got us on the edge of our seats."