You may not have heard of Ceres. This enormous asteroid was originally classified as a new planet when it was discovered by Giuseppe Piazzi and rediscovered by Carl Gauss in the very early 1800s. Ceres is a massive asteroid in the asteroid belt between Mars and Jupiter - the biggest asteroid here, in fact. Ceres is so large that its gravity has formed it into a somewhat spherical shape, making it able to be classified as a dwarf planet and making Ceres the only dwarf planet in the inner solar system.

While originally named a planet, Ceres was downgraded after the rest of the asteroid belt was discovered shortly after Ceres was. That being said, Ceres is by far the largest asteroid in the inner belt and carries the largest share of the belt's mass.

Dwarf Planet Ceres up close
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Being far closer to us than ultra-remote giants like Pluto, Eris, and Makemake, Ceres also happens to be the dwarf planet scientists know the most about.

After talking about Pluto and Eris I thought it would be interesting to write about some of the other dwarf planets, and what better place to start than the one that's closest to home? Let's dive into Ceres.

Where is Ceres?

As you know from elementary school, between the planets Mars and Jupiter lies an asteroid belt full of various rocks called asteroids. Some of these rocks are the size of boulders, some of them are the size of mountains, and some of these rocks are extremely enormous: Take Vesta, for example. Vesta is a very large lopsided asteroid 500 kilometers across, with enough land on the surface to cover four states of Kansas.

This is Vesta, not Ceres.
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But Ceres is the biggest asteroid of them all. Over 900 kilometers across, Ceres is essentially a micro planet, with its round shape. Around one third of all of the mass in the entire asteroid belt lies inside this dwarf planet. However, this should just be an indication of how little mass there actually is in the asteroid belt, since there are many moons of other planets larger and more massive than Ceres (Jupiter alone has four moons larger than Ceres, by a lot). Our own Moon is way larger than Ceres (over 3x wider).

Ceres compared to Earth and the Moon. As you can see, Ceres really isn't that big compared to the Moon. But its still a huge place, which just goes to show how large things in space really are.
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Numerous past asteroid impacts have scarred the surface of Ceres with many craters, not unlike our own Moon. With no atmosphere to smooth over these marks, they remain essentially indefinitely.

In between Mars and Jupiter, Ceres follows a slightly eccentric (elliptic) and inclined orbit. This makes it somewhat more difficult to reach via spacecraft than its distance would otherwise suggest. Years here are long: It takes almost 5 years for Ceres to travel around the sun once (of course this is nothing compared to other dwarf planets like Pluto).

Ceres' orbit. You can see how the orbit goes far below the ecliptic plane where the planets lie. Notice that's its called "1 Ceres" here - this is because it was the first asteroid discovered, hence the 1.
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Ceres has no natural satellites - no moons of Ceres exist. That being said, it does have something orbiting it right now, which we'll get to later.

What's on Ceres?

Ceres is similar in color to the moon, and honestly looks a lot like the moon in general: Crater-covered and gray. Here's another picture to show this before we continue:

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You wouldn't weigh very much if you were walking on Ceres - just 3% of your weight on Earth. But you, of course, could walk on Ceres - the surface is solid and there's enough gravitational force to keep you down. You just have to find a way to get there.

Ceres' dark gray surface is due to large amounts on carbon in the compounds that make up the surface. This places Ceres in a class of asteroids called Class-C Asteroids (although there is some debate on this). Going a bit off topic, there also exist very interesting asteroids essentially made of solid metal known as M-type asteroids, which are usually the subject of asteroid mining vaporware presentations (perhaps that was a bit harsh).

But that isn't all there is to the surface. Observations of Ceres have indicated that water ice essentially covers the surface, mixed in with the rocky dirt that lines the Cerean surface. This fact was discovered in a pretty interesting way: Neutrons produced by cosmic rays were detected coming off of the surface of Ceres. Since hydrogen nuclei are just protons which have very similar mass to neutrons, direct elastic collisions between neutrons and protons result in the proton taking away almost all of the kinetic energy of the neutron, leaving the neutron moving very slowly. Slow neutrons are easy to absorb, so less of them escape back into space, allowing us to detect hydrogen simply by picking up a lack of neutron radiation high above Ceres. I think that's pretty ingenious but perhaps its my radiation/nuclear bias showing.

Solid water ice may also be present in shaded craters on the surface of Ceres. Certain craters are situated in a way that they rarely ever receive direct sunlight, resulting in the centers of these craters becoming extremely cold relative to the rest of the planet. It is speculated that these "cold-traps" could be filled with ice over long periods of time by falling water vapor ejected from the surface (I've attached an article at the end that covers this ice business if you're interested!)

An example of a shadowed crater on Ceres that could contain ice deposits.
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Wait, where did these pictures come from: The Dawn Spacecraft

Remember that I mentioned Ceres is quite small. You can easily see the planet Venus from the surface of our home Earth, and it is very bright most of the time, but you can't see that Venus is an actual planet without a telescope (to see the disc). Venus is extremely large, being about the same size of Earth, and can be very close to Earth (less than 1 AU). Now think of Ceres, many times further away from us than Venus and over 10 times smaller. As such, it is quite difficult to get pictures of Ceres that show any detail from Earth (but it's much better than trying to image Pluto!!). Here's one of the best images of Ceres prior to very late 2014:

Ceres from the Hubble Space Telescope
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Now that picture isn't terrible (there's some clearly visible surface features), but it is somewhat frustrating: This dwarf planet is, relatively speaking, right there, and this is the best we can get. I suppose we shouldn't be too upset: See my post on the first mission to Pluto here for the downright terrible HST images of Pluto from Earth prior to New Horizons (not to trash the HST: these images are incredibly good for what it has to work with, namely an absurd distance between Pluto and us).

Enter the Dawn spacecraft. This big space probe was built to study the asteroid belt, and launched in 2007 aboard a Delta II rocket.

Dawn departs for the asteroid belt 11 years ago.
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Artist rendition of Dawn above dwarf planet Ceres. Notice that the rendered Ceres is based off of the blurry HST image, while the images of Ceres I showed you originally came from Dawn when it actually arrived and look quite different.
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Dawn first traveled to the mega-asteroid Vesta, which I already mentioned above. Using a gravity assist at Mars to gain speed, it took Dawn around 4 years to reach Vesta. This is because Dawn was meant to orbit Vesta, which means (in general) the spacecraft has to approach the target much more slowly than if it was attempting a flyby to make the amount of fuel you have to carry reasonable.

Dawn used an electric ion thruster to propel itself into the asteroid belt. Ion thrusters are quite cool, and are too complicated to get much into now, but they are essentially ultra-low thrust, ultra-high efficiency spacecraft propulsion. This means you can't use them to get off of Earth, but once you are in space you can use them for long periods of time to get up to high speeds without carrying much mass.

Artist rendition of Dawn's ion thruster. Yes, they actually glow blue when they turn on!
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By the way, the Vesta picture above also came from Dawn. Being even smaller than Ceres, you can imagine it is even harder to get a good picture of Vesta from Earth.

After about a year studying and orbiting Vesta, Dawn departed this large asteroid to travel to a bigger one: Ceres, our resident inner solar system dwarf planet. Thanks to the slow-burning ion thruster, it took two years to arrive, with Dawn beginning to orbit Ceres in early 2015.

Notice: I said 2015! That was two years ago. This along with New Horizons is absolute proof that we aren't even close to having explored everything in our solar system, and new discoveries are being made every year. And we get cool pictures of the deal. I personally remember checking NASA's website for incoming new Ceres images as everyone got to see this mini planet for the first time.

At Ceres, Dawn took all of the pictures you see here that don't look like they are part of an 8-bit video game. Dawn also provided massive amounts of data on Ceres. To be honest, without the discoveries from Dawn, this wouldn't be much of a post. So, if you liked the pictures and information described here, thank NASA and the Dawn team at JPL.

Dawn will continue its mission at Ceres until it runs out of monopropellant hydrazine fuel (used to power small thrusters that allow the spacecraft to rotate and maneuver itself). But Dawn isn't going anywhere: With no moons of Ceres or atmosphere, it will stay up there long after its mission ends, flying above Ceres for eternity (or close to it). So in a way, Ceres now has its own mini moon, courtesy of Earth.

The Bright Spot on Ceres

If you watched the images from Ceres come in when Dawn first arrived, you remember the bright spots. This weird spot (and others) showed up in the earliest images of Ceres, and possible explanations ranging from salt flats to water ice to aliens were rampant in the media. Here's what the enigmatic spot looked like as Dawn began its approach:

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After months of getting slightly better pictures, here's a close-up of the spot:

Spot #5. I really can't believe some people actually thought this was aliens...
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In the end, the bright spots ended up likely being giant salt deposits. The interesting part is that this means that salt is being transported to the surface, indicating some sort of geological activity going on underneath the surface. Perhaps one day we will send a lander to investigate.

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I hope you learned something new today about Ceres, our closest dwarf planet. This place really is quite a bit more interesting than a simple rock, despite me calling it that at the start.

If you have any questions, comments, or corrections, please let me know!

Another view of Ceres
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Thanks for reading!

Sources for Additional Reading
Water ice on Ceres (NASA)
Dawn spacecraft Wikipedia Entry
Ceres Wikipedia Entry
NASA Ceres Overview
Wikipedia Entry just for the Ceres bright spots
Asteroid Compositions (ESA)