By Jonathan Amos BBC Science Correspondent From BBC

The Dawn probe will spend just over a year mapping and sensing the dwarf planet

The US space agency’s Dawn probe has gone into orbit around Ceres, the largest object in the Solar System between Mars and Jupiter.

A signal from the satellite confirming its status was received by ground stations at 13:36 GMT.

Ceres is the first of the dwarf planets to be visited by a spacecraft.

Scientists hope to glean information from the object that can tell them about the Solar System’s beginnings, four and a half billion years ago.

Dawn has taken 7.5 years to reach its destination. Its arrival has seen it pass behind the dwarf to its “dark side”.

Over the next month, controllers will re-shape the orbit to get it ready to begin the prime science phase in late April.

Over time, the intention is to progressively lower the orbit until the probe is just a few hundred km above the surface. By that stage, it will be returning very high resolution pictures.

“We feel exhilarated,” said Chris Russell, the mission’s principal investigator from the University of California, Los Angeles (UCLA).

“We have much to do over the next year and a half, but we are now on station with ample reserves, and a robust plan to obtain our science objectives.”

Similar narrative
The satellite has turned up at Ceres having previously visited asteroid Vesta.

Both objects reside in the belt of rocky debris that circles the Sun beyond Mars.

Of the two, Ceres is the bigger at 950km across; Vesta has a diameter of 525km.

The pair should tell a similar story, says Dr Carol Raymond, the mission’s deputy principal investigator at Nasa’s Jet Propulsion Laboratory.

“Both Ceres and Vesta, we believe, are proto-planets. They were on their way to forming larger planetary embryos and they were the type of object that merged to form the terrestrial planets,” she told the BBC’s Inside Science programme.

“But these two stopped before they reached that evolutionary stage, and so they are essentially these intact ‘time capsules’ from the very beginning of our Solar System; and that’s really the motivation for why Dawn is going there to explore them in detail.”

Dwarf status
Researchers think Ceres’ interior is dominated by a rocky core topped by ice that is then insulated by rocky lag deposits at the surface.

A big question the mission hopes to answer is whether there is a liquid ocean of water at depth. Some models suggest there could well be.

The evidence will probably be found in Ceres’ craters which have a muted look to them. That is, the soft interior of Ceres has undoubtedly had the effect of relaxing the craters’ original hard outline.

One big talking point has dominated the approach to the object: the origin and nature of two very bright spots seen inside a 92km-wide crater in the Northern Hemisphere.

The speculation is that Ceres has been struck by something, exposing deeply buried ices.

These will have vaporised on the airless world, perhaps leaving behind highly reflective salts.

The Dawn mission is expected to work at the dwarf planet for at least 14 months.

Dr Raymond commented: “The spacecraft will run out of hydrazine [fuel] at some point; it will lose the ability to maintain its attitude and therefore lose the ability to point its solar arrays towards the Sun and point its antenna towards the Earth.

“It will then lose power and be a tumbling satellite of Ceres in perpetuity, because our orbit has proved to be stable over a hundred-year lifetime, as required, so we don’t crash into Ceres and contaminate its surface.”

While Dawn takes the honour of being the first spacecraft to visit a dwarf planet, the next opportunity comes very quickly.

Nasa’s New Horizons probe is due to make a close flyby of Pluto in July.

This far-more distant world was demoted from full planet to dwarf status at an international astronomy meeting in 2006.

IMAGES:
An artist’s impression of Dawn firing its ion engine on approach to Ceres
Bright spots
The bright spots inside a 92km-wide crater have been the big surprise of the encounter so far

For more on this story and video go to: http://www.bbc.com/news/science-environment-31754586

Related story:

We might just learn what the mysterious bright spots on the dwarf planet are—and much more

By Marissa Fessenden From SMITHSONIAN.COM

In 1801, a astronomer peering up at the sky from Sicily was busy assembling a great catalog of stars, when he noted something that did not fit. Observations over the next weeks confirmed his hypothesis—the star was moving. He wrote to fellow astronomers: “I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet.”

The astronomer lost sight of the star and became ill before it could be found again. But he did offer up a name, writes Michael Hoskin for the Observatory of Palermo—Ceres, for the patron goddess of Sicily.

Now we know that this maybe-better-than-a-comet light in the sky is a dwarf planet. It’s largely made of ice and rock and is the largest body in the gap between Mars and Jupiter. But many questions about Ceres’ characteristics and origins remain, some of which will hopefully be answered with the arrival of NASA’s Dawn spacecraft on March 6.

One of the first questions Dawn can answer could be: What are those strange bright spots shining from inside the dwarf planet’s crater? An image captured on February 19 shows two spots that appear to be reflecting sunlight, writes Ian Sample for the Guardian. It’s possible that these are patches of ice exposed by collisions with small objects in the asteroid belt. Still, the brightness surprised researchers.

“We knew from Hubble observations that there was variation in the colouration and reflectivity of the surface. But when we got to Ceres we saw bright spots, and they are really, really bright,” Chris Russell, lead scientist on the Dawn mission at the University of California, Los Angeles, told the Guardian.

At this point, scientists are just speculating, as Dawn draws closer to its destination. The spots could be shiny minerals or ice cones pushed up by volcanic activity. Ceres might even be hiding liquid water under a frozen crust. And jets that emanate from the dwarf planet could be evidence of internal heating. Or they could just be sublimating ice from the surface. Another possibility: the liquid may have only existed in the past. But the possibility that Ceres might harbor life (currently or historically) has researchers excited.

Dawn, launched in 2007, is fresh off its successful 14 month orbit around Vesta, a massive asteroid that takes second place in the asteroid belt after Ceres. Comparing the two objects will help scientists get a clearer idea of the Solar System’s formation.

The image of the bright spots was taken from about 29,000 miles away from Ceres, but the mission plan has Dawn spiraling down to eventually reach a close orbit of 233 miles above the surface, according to NASA’s Jet Propulsion Laboratory. There the spacecraft will map the dwarf planet’s surface by measuring gravity, reading elemental signatures and snapping photos in stereo to create 3-D images.

So stay tuned for findings from the dwarf planet: NASA will hold a briefing on the mission on Monday, and NASA TV and Ustream will carry live coverage of the event. Then we’ll get to know the nearest dwarf planet over the next year.
For more on this story go to: http://www.smithsonianmag.com/smart-news/dawn-spacecraft-should-soon-answer-what-ceres-bright-spots-are-180954427/#1ivu03V5FKUiOqJy.99