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Ceres is the biggest unexplored rock in the inner Solar System. We're about to visit it.

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The dwarf planet Ceres, as photographed by the Dawn spacecraft. (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI)

This grainy, slowly rotating orb might not look like much.

But this series of images, recently captured by NASA's Dawn spacecraft, are something pretty amazing: they're some of the best photos we've ever taken of the dwarf planet Ceres, which inhabits the asteroid belt, and orbits the Sun about 257 million miles away from Earth.

Dawn was launched in 2007 in order to visit Vesta (a large asteroid), then Ceres. It successfully orbited Vesta in 2011, and after a journey of more than three billion miles, in March, it's scheduled to reach Ceres — the largest unexplored rock between the Sun and Pluto.

Wait. There's a dwarf planet named Ceres?

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Ceres and Vesta are both part of the asteroid belt. (Lunar and Planetary Institute)

Yes. The asteroid belt is a dense ring of orbiting rocks in between the orbits of Mars and Jupiter. It's made up mostly of relatively small asteroids, along with a few larger ones.

Vesta and Ceres are the two most massive objects in the asteroid belt — the latter is about 38 percent of the surface area of the continental US. Dawn was designed to explore them for the first time.

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Vesta and Ceres, shown next to Mars, Mercury, and the Moon for scale. (NASA/JPL-Caltech/UCLA)

Ceres is officially considered a dwarf planet — a technical designation for objects that are smaller than planets but bigger than most asteroids and comets. (Pluto was famously demoted to a dwarf planet in 2006.) Yet in many ways, Ceres and Vesta are similar destinations. When Dawn was launched, they were the two largest rocks in the inner solar system that we had yet to explore.

Vesta is heavily cratered, and rocky, while Ceres is much smoother, and icy. Both of them, however, are rocky bodies that resemble the ones that coalesced to form Earth and the other terrestrial planets in the early days of the solar system — so by studying them, scientists hope, we can learn more about the formation of these planets billions of years ago.

Dawn has learned a lot about asteroids so far

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Vesta, as imaged by the Dawn spacecraft. (NASA/JPL-Caltech/UCAL/MPS/DLR/IDA)

In 2011, Dawn entered Vesta's orbit and stayed there for 14 months. During that time, it took unprecedentedly detailed photos of the asteroid, spotting surface features that could be evidence of liquid water.

The probe also collected geologic data that allowed scientists to map its surface. These observations led scientists to infer that Vesta is different than any other known asteroid: it has differentiated geologic layers, including an iron core. It's believed that in the early days of the Solar System, all other asteroids like this ended up crashing into each other and coalescing to form the inner planets, but somehow, Vesta did not.

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Dawn's trajectory. (NASA/JPL-Caltech)

Dawn left Vesta's orbit in 2012, and began heading towards Ceres. If the mission is successful, it will be the first to ever orbit two different extraterrestrial objects, which it achieved by using an ion thruster system — an advanced form of propulsion that uses charged particles, rather than conventional propellant, allowing it to change trajectory while consuming much less fuel.

This is "The Year of the Dwarf Planet"

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An illustration of Dawn imaging Vesta. (NASA/JPL-Caltech)

Dawn is now approaching Ceres, and is projected to begin permanently orbiting it in March 2015. Some scientists believe the icy planet could contain a subsurface ocean. Data collected by Dawn in the coming years should tell us more.

When Dawn took the new images of Ceres on January 13, it was still about 238,000 miles away — so they're slightly less sharp than photos of Ceres taken by the Hubble space telescope years ago. But as the craft gets closer, it'll provide increasingly detailed images of the dwarf planet. By late January, NASA says, it'll be taking the highest resolution photos of Ceres we've ever seen — about 100 times more detailed than the ones so far.

Last week, the New Horizons space probe — which is quickly approaching the most famous dwarf planet, Pluto — officially began the "encounter phase" of its mission, and it's scheduled to become the first spacecraft to flyby Pluto in July. These two missions are why some experts are calling 2015 "The Year of the Dwarf Planet" — together, they'll teach us more about these objects than we've ever learned before.