After 14 months at Vesta, the Dawn spacecraft swung around for its second mission target: Ceres. After a rogue cosmic ray jumbling the best laid plans in September, the spacecraft is in its approach phase to slip into orbit around Ceres on March 6, 2015 just a half-day behind schedule.

Top image: Artist's concept of Dawn approaching Ceres. Credit: NASA/JPL-Caltech

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After leaving Earth, Dawn flew past Mars, spent 14 months at Vest, and is en route to Ceres. Image credit: JPL/NASA

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The Dawn spacecraft has a split objective: visiting the biggest and second biggest objects in the main asteroid belt. The debate on what to call Ceres and Vesta is ongoing — are they dwarf planet, asteroids, comets, protoplanets, or some confusing combination depending on the context?! — but their rank as the biggest objects in the asteroid belt is undisputed.

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Dawn launched on September 27th, 2007. Image credit: KSC/NASA

Measuring in at an average diameter of 950 kilometers (590 miles), Ceres is thea monster, with Vesta a relative pint-sized 525 kilometer (326 mile) average diameter. Ceres is also the heftiest chunk of rock and ice in the ring, accumulating roughly 30% of the mass of the entire asteroid belt in one lumpy spheroid. Vesta is again a distant second, at 8% of the mass, leaving the remaining 60% of the asteroid belt distributed amongst literally millions of rocks.

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Vesta! Image credit: NASA/JPL

Arriving at Vesta in 2011, the spacecraft spent just over a year photographing the rock before moving on. We had some idea of what we would find on the distant world from meteorites recovered here on Earth, but the extensive observations helped scientists create this detailed geological map of a world in miniature.

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Now the spacecraft is closing fast on its next destination, travelling at 725 kilometers per hour (450 mph) and closing the distance to Ceres from over a million kilometers at the start of this month to just 640,000 kilometers (400,000 miles). That's only 1.6 times the average distance between the Earth and the moon with a spacecraft travelling at airliner speeds!

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Where is Dawn? Check the latest location here! Image credit: NASA/JPL

This unprecedented mission to go into orbit around two distinct targets is made possible by the spacecraft's ion drive: an electrical charge ionizes xenon gas, with a charged metal grid accelerating the ions out the thruster. Between building up speed and changing orbits, the thruster has been responsible for generating a combined equivalent of accelerating the probe by 10.2 kilometers per second! (This isn't Dawn's actual velocity: geek out here for an extensive discussion on the topic). You can learn more about how the ion drive functions here or here.

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The drive has been performing beautifully, with one notable mini-disaster. In September 2014, a rogue cosmic ray struck an electrical component, stopping the thruster as the operations team spent four days coaxing it back into normal operation. The hiccup happened during a critical juncture when the orbit was particularly sensitive to any deviations of thrust, and was just enough to disrupt the flight plan. The team was left with two choices: do nothing and make the rendezvous with Ceres much later than planed, or adjust the spacecraft's approach path for a totally different approach while sticking to (pretty much) the original schedule.

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Dawn will lead Ceres in orbit for the revised approach, spiralling in from ahead. Image credit: NASA/JPL

The alternate route of approaching from the lead means that the Dawn will allow itself to be captured by the dwarf planet then work thrust and Cere's gravitation pull together to drag the spacecraft into a tighter orbit, arriving just a half-day later than originally planned. While the approach path is significantly different in direction and complexity, by April 23rd the original and revised plans will merge, placing Dawn exactly where it would have been without the interference of a high-energy particle triggering an engine stutter.

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Along with being set up to break records by entering orbit around a second destination, Dawn has already set a record by wracking up the longest accumulated thrust time of any spacecraft. Running for over five years and an ever-increasing percentage of its total time in space, Dawn's ion drive surpassed the previous record-holder, Deep Space 1, years ago. With the engine's success, a new ion drive prototype is already being tested for potential deployment in an asteroid retrieval mission.

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From its current viewpoint, Ceres is as big in Dawn's night sky as Venus is for us humans on Earth, photographed as just 9 pixels across. By the end of January, the photographs we'll be seeing will be the best views ever of the not-so-tiny dwarf planet. And we have almost no idea of what to expect.

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Ceres hasn't pelted our planet with stray bits of rock for us to extrapolate its composition, but we do have some educated guesses at least about how Ceres might differ from Vesta. We think Vesta is older, formed when radioactive material was more abundant and keeping its interior warmer even today.

The current best view of Ceres as captured by the Hubble Space Telescope. Image credit: NASA/ESA, J. Parker/P. Thomas/L. McFadden

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Ceres is likely younger with a cooler interior, possibly with a thick ice mantle or even a deep liquid ocean within its icy crust.

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Dawn's first view of Ceres captured on July 20, 2010. Image credit: NASA/JPL-Caltech/MPS/DLR/IDA

Depending on how chemical contaminates impact melting temperatures, that crust could be very, very thin, maybe even a wispy meter of dust and ice protecting a mantle up to a hundred kilometers thick. While it's undoubtably too chilly now for active convection, Ceres may have had a warm and gooey interior in the past, a geologic past written on the surface in distinctive scars and features visible to prying eyes.

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Artist's concept of water vapour from Ceres, with data inset. Image credit: ESA/ATG medialab/Küppers et al.

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All that mention of ice is for good reason — the current guess is that Ceres may be up to 30% water by mass! We know the dwarf planet isn't static: a thin plume of water was spotted escaping from Ceres. It wasn't much, and it may not be an all-season event, so whatever Dawn sees when it slips into orbit is bound to be a surprise!

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With the last photo taken in early December, Dawn is gearing up to take its next photograph of Ceres on January 13th. You can browse raw data from the Dawn spacecraft here, and keep up with their mission blog here.