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Dawn Spacecraft Slips Quietly Into Orbit Around Dwarf Planet Ceres

Image: Ceres and Dawn

An artist's conception shows NASA's ion-powered Dawn probe approaching Ceres from its dark side. NASA / JPL-Caltech

NASA's Dawn spacecraft slipped into orbit around the dwarf planet Ceres on Friday, in a manner as cool and quiet as the soft blue glow of its ion engines.

Eight years and 3 billion miles after its launch, the boxy probe was captured by Ceres' gravitational pull at 7:39 a.m. ET (4:39 a.m. PT), said Marc Rayman, Dawn's chief engineer and mission director at NASA's Jet Propulsion Laboratory in Pasadena, California.

Confirmation that Dawn was healthy and on the right track came about an hour after the event, during JPL's routine communication session with the spacecraft. But because of the gentle, steady thrust of Dawn's ion propulsion system, there was never much question that the orbital mechanics would work out the way Rayman and his teammates expected.

"We feel exhilarated," UCLA astronomer Chris Russell, the Dawn mission's principal investigator, said in a celebratory NASA news release.

What Can We Learn From Visiting Ceres? 0:43

This wasn't your typical orbital insertion.

"Usually, there's a big, bone-rattling, whiplash-producing maneuver," Rayman told NBC News, "but Dawn flies most of the time on this pillar of blue-green xenon ions, just like a spacecraft from science fiction. ... It's a beautiful celestial pas de deux, these two dancers together. I think it's really a remarkable scene to imagine. It's so different from what we're accustomed to from five decades of previous space exploration."

This also isn't your typical target for an interplanetary mission. Ceres is a type of world that's never been visited before.

With a diameter of 590 miles, it's the biggest object in the main asteroid belt between Mars and Jupiter, orbiting about 250 million miles from the sun. When Ceres was discovered in 1801, it was considered one of the major planets — but as more asteroids were discovered, it came to be left off the list. In 2006, the International Astronomical Union classified Ceres as a dwarf planet, along with Pluto, because it's big enough to retain a round shape but doesn't necessarily stand out in a celestial crowd.

Dawn is the first spacecraft to go into orbit around a dwarf planet. Another NASA probe, New Horizons, is due to fly past Pluto in July. Will these close-ups change Ceres' planetary label again? Rayman doesn't much care about the nomenclature. "Whatever you call it, it's something very special," he said.

Ceres could have a huge reservoir of water ice beneath its cratered crust — and in the solar system's early days, it might have even been suitable for life. Studying the dwarf planet could provide new insights into how the solar system was formed. And then there's Ceres' biggest mystery: a pair of bright spots that shine like alien headlights when sunlight hits them just the right way.

"It's neat to have such an intriguing mystery," Rayman said. "You can't look at them without being mesmerized."

Dawn also has the distinction of being the first probe to go into orbit around two worlds beyond Earth. Earlier in the $473 million mission, Dawn spent 14 months mapping Vesta, the second most massive object in the asteroid belt. Then Dawn moved on, firing its ion thruster to make the three-year trek between the two asteroids.

The ion drive isn't suited for getting anywhere with a burst of power. The maximum propulsive push is roughly equal to the weight of a piece of paper pressing down on your palm. But the solar-powered ion engines build up speed steadily over time. Rayman said Dawn's total velocity change since its launch in 2007 amounts to 23,800 mph (38,300 kilometers per hour).

"You can compare that to the 17,500 miles per hour it takes to go from the surface of Earth to low Earth orbit," he said.

By now, Dawn has matched Ceres' orbital track so closely that it's traveling a mere 100 miles an hour relative to the dwarf planet. Because of that slight but steady push, Friday's entry into orbit was pretty much a typical day at the office.

"While it's important, and it's a historical milestone from the standpoint of humankind's exploration of the cosmos, from the spacecraft's standpoint, it's not different," Rayman said. Only one person was scheduled to be on duty at JPL's mission control when Dawn entered orbit. (And it wasn't Rayman.)

Dawn is now about 38,000 miles (61,000 kilometers) from Ceres, but it's approaching the dwarf planet from its dark side. For that reason, there'll be nothing to see (or take pictures of) until April 10, when the probe's camera is due to capture some images of Ceres' crescent for navigational purposes.

The mapping mission begins in earnest on April 23, when Dawn settles into an 8,400-mile (13,500-kilometer) pole-to-pole orbit. The spacecraft will alternate observing campaigns with orbital maneuvers, working its way down to an altitude of just 235 miles (378 kilometers) by November.

Image: Ceres
NASA's Dawn spacecraft took this picture of Ceres' slim crescent on March 1, just a few days before slipping into orbit around the previously unexplored mini-world. NASA / JPL-Caltech / UCLA / MPS / DLR / IDA

Dawn's primary mission is scheduled to end in June 2016, but it could keep watch on Ceres for months or even years longer, until the hydrazine fuel for its maneuvering thruster system runs out. Even when that fuel is gone, the spacecraft will remain in a stable orbit around Ceres for decades afterward. Dawn's eventual end, like Friday's orbital insertion, is expected to be virtually devoid of drama.

"The drama is not in whether this mission is going to succeed or fail," Rayman said. "It's not whether there's going to be some small glitch that spells doom for the mission. ... To me, the drama is in the opportunity to explore a mysterious alien world that has beckoned for more than two centuries — and we finally have a robotic ambassador from Earth, answering that invitation.

"That, to me, is where the drama is."

Keep track of Marc Rayman's mission updates on NASA's Dawn Blog. To celebrate Dawn's arrival, the Slooh community observatory will present live telescope imagery of Ceres from Australia during an online video show beginning at 1 p.m. ET Friday. Slooh astronomer Bob Berman and host Will Gater will be joined by Lucy McFadden, a co-investigator for the Dawn mission from NASA's Goddard Space Flight Center. Twitter users can ask the team questions by including the hashtag #SloohDawn in their tweets. The Slooh show is available via Slooh.com.