Ceres is the biggest unexplored rock in the inner Solar System. Today, we will visit it.

Ceres, as photographed by Dawn on February 19, from about 29,000 miles away. (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

This is Ceres: a 590-mile wide ball of rock and ice that lies in the asteroid belt, orbiting the Sun about 257 million miles away from Earth. It's not technically an asteroid, but a dwarf planet, and it's the largest object in the asteroid belt — about a quarter as wide as the Moon.

Until now, Ceres was the biggest unexplored rock between the Sun and Pluto. We'd never seen it up close. But on Friday at around 7:39 am ET, NASA's Dawn spacecraft entered its orbit after a seven-year journey.

A video of Ceres captured by Dawn from about 29,000 miles away. (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Dawn was launched in 2007 and spent a year studying Vesta (a large asteroid) in 2011. After a journey of more than 3 billion miles, it's now closing in on Ceres and noticing all sorts of interesting things — such as the presence of two weird shiny dots on Ceres' surface.

Dawn finally entered Ceres' orbit on Friday, March 6. At 1 pm ET, the Slooh Observatory will broadcast a live view of Ceres as seen from telescopes on Earth. Here's what you need to know about the mission.

Wait. There's a dwarf planet named Ceres?

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

Yep. 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.

Ceres is the largest one — it has a surface area that's about 38 percent of the size of the continental US. The second-largest one is Vesta, which Dawn visited a few years ago.

(CWitte)

Ceres was first discovered in 1801 — the first asteroid belt object to be spotted — and for decades, astronomers actually considered it a planet. It's now officially considered a dwarf planet — a technical designation for objects, like Pluto, that are smaller than planets but bigger than most asteroids and comets.

Ceres' orbit. (Orionist)

Ceres is the only object in the asteroid belt big enough that the force of its gravity has given it a rounded shape (as occurs on all planets). Compared to Earth, it's really cold: the surface is −36 °F at its warmest. It's lightly cratered, compared to other asteroids.

What's exciting about Ceres?

Based on measurements of its mass, scientists believe Ceres may have a small rocky core surrounded by an icy mantle — one that might contain as much fresh water as is present on all of Earth.

A hypothetical rendering of what Ceres' interior might look like. (NASA, ESA, and A. Feild)

Last year, scientists operating a space telescope detected two jets of water vapor shooting out of Ceres' surface. The most likely explanation is that they result from surface ice being vaporized as it's exposed to solar radiation.

But they could theoretically be evidence of ice volcanoes erupting, or even of a subsurface ocean trapped under the ice. This is the reason why some scientists believe that Ceres is just as exciting as a place like Jupiter's moon Europa, which is believed to have a subsurface ocean that could potentially support life.

An illustration of what Ceres' ice volcanoes might look like. (Y. Gominet and B. Carry/CNRS/IMCCE-Observatoire de Paris)

Still, Ceres probably doesn't have life — and we don't have an explanation for what might heat its interior enough to produce a liquid ocean, like we do for Europa.

The bottom line is that we still know vanishingly little about the dwarf planet. Scientists speculate that it could even have its own tiny moons, or a thin atmosphere made up of water vapor. We've never seen it up close, and will learn a lot over the next year.

What's this about shiny dots?

As Dawn has closed in on Ceres, its photos have revealed a pair of strange, shiny dots inside a crater on its surface.

Ceres, from 29,000 miles away. (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Right now, there are two main theories for what the dots might be:

1) Ice. The more mundane explanation is that these shiny spots are just patches of ice, reflecting a bit of sunlight. Slate's Phil Plait suggests that their location in a crater may not be a coincidence: an impact could have exposed subsurface ice, as we've previously seen, for instance, on Mars.

2) Ice volcanoes. Chris Russell, the Dawn mission's principal investigator, suggests the dots could have a "volcano-like" origin. This could explain why they're so close together: they could be two vents from the same underlying ice volcano. We've previously seen ice volcanoes on Neptune's moon Triton and Saturn's moon Enceladus, among other places. The problem with this is we wouldn't expect Ceres to have tectonic activity (because it's too small).

3) Ceres is actually the third Death Star. Okay, no one actually believes this.

What is Dawn going to do now?

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

After leaving the asteroid Vesta in 2012, Dawn began heading toward 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.

After it enters Ceres' orbit on March 6, Dawn will spend about a year collecting all sorts of data — mapping the dwarf planet's surface, measuring the abundance of different elements on it, and looking for ice and water vapor. This information will help us better understand Ceres, and figure out whether it does have a layer of ice, or even a liquid ocean underneath.

Ultimately, the goal of all this is to better understand how Ceres originally formed. We know that it coalesced during the early days of the Solar System, during the same time that other bits of rock (left over from the protoplanetary disk that surrounded the early Sun) were coming together to form the Earth and other planets. So, on a very basic level, learning about the formation of Ceres will teach us about the formation of the Earth and the rest of the solar system, billions of years ago.

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