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The Dawn spacecraft has been exploring the two largest objects in the asteroid belt since 2007, and here's what we've learned so far!

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Sources:
https://dawn.jpl.nasa.gov/
https://www.nasa.gov/centers/glenn/about/fs21grc.html
http://iopscience.iop.org/article/10.3847/2041-8213/aa66cd/meta
http://science.sciencemag.org/content/355/6326/719
http://www.planetary.org/blogs/emily-lakdawalla/2014/10091306-what-did-dawn-learn-at-vesta.html
https://www.nasa.gov/home/hqnews/1997/97-191.txt
https://dawn.jpl.nasa.gov/mission/timeline_trajectory.html
http://science.sciencemag.org/content/355/6326/706.1
https://dawn.jpl.nasa.gov/mission/journal_11_28_16
https://dawn.jpl.nasa.gov/multimedia/pdfs/Dawn_Vesta_Ceres_Lithograph.pdf

Images:
https://commons.wikimedia.org/wiki/File:Dawn_Flight_Configuration_2.jpg
https://commons.wikimedia.org/wiki/File:Dawn_ignition.jpg
https://commons.wikimedia.org/wiki/File:Ceres-Vesta-Pluto-and-Moon-size-fr.png
https://commons.wikimedia.org/wiki/File:Vesta_in_natural_color.jpg
https://www.jpl.nasa.gov/blog/2014/3/so-close-yet-so-far-away-dawns-trajectory-explained
https://commons.wikimedia.org/wiki/File:Vesta-HST-Color.jpg
https://en.wikipedia.org/wiki/File:Rheasilvia_and_older_basin,_Vesta.jpg
https://photojournal.jpl.nasa.gov/catalog/PIA15510
https://commons.wikimedia.org/wiki/File:MillbillillieMeteorite.jpg
https://commons.wikimedia.org/wiki/File:Tatahouine_Diogenite_72g.jpg
https://commons.wikimedia.org/wiki/File:Howardite_QUE94200.gif
https://commons.wikimedia.org/wiki/File:PIA20183-Ceres-MapOfBrightSpots-20151210.jpg
https://commons.wikimedia.org/wiki/File:PIA20358-Ceres-DwarfPlanet-HaulaniCrater-CloseupColor-20160419.jpg
https://www.nasa.gov/feature/jpl/dawn-discovers-evidence-for-organic-material-on-ceres
We spend a lot of time visiting other planets and moons, but other places in the solar system can also teach us plenty about the history of our neighborhood.

So in 2007, NASA launched the Dawn spacecraft to investigate the two largest objects in the asteroid belt between Mars and Jupiter: a giant asteroid called Vesta, and an icy dwarf planet named Ceres. Dawn was the first mission to orbit something in the asteroid belt, and the first to orbit two bodies besides Earth.

The mission was supposed to end a year ago, but NASA voted to keep it going, and today marks the one-year anniversary of Dawn’s extended mission. In the last decade, Dawn has taught us a lot more about Vesta and Ceres than we expected, and there are still more discoveries on the way. Vesta and Ceres were selected as targets because they’re so different from each other.

Vesta is dense and rocky like the planets closer to the sun, but Ceres is icy like the smaller worlds in the outer solar system. So visiting both of them was like getting double the science in one mission. First, Dawn traveled 2.8 billion kilometers to get to Vesta, where it arrived in 2011.

Before Dawn, we already knew a little about the asteroid thanks to the Hubble Space Telescope, like that it has a massive crater on its south pole. The crater is almost as wide as the asteroid itself. If Earth had a crater that big relative to its size, it would fill the entire Pacific Ocean basin.

The crater also has a huge mountain on the edge, which Dawn eventually confirmed is about 22 kilometers higher than the surrounding terrain. That makes it almost three times taller than Mount Everest and one of the tallest mountains in the entire solar system! But besides taking some sweet pictures of the crater, one of Dawn’s biggest accomplishments at Vesta was confirming a hypothesis we had: that some meteorites we’ve found on Earth came from Vesta.

Vesta is unusual for an asteroid because it’s a lot like a mini-planet. It has a liquid metal core, a mantle, and a crust made out of lava, which we were able to figure out using instruments like Hubble, and we haven’t really seen an asteroid like that anywhere else in our solar system. About 6% of the meteorites we’ve collected on Earth look a lot like they could be from those unique, different layers of Vesta, and we call them HED meteorites, or howardite, eucrite, and diogenite meteorites.

Based on their composition, eucrites could have come from the hardened lava on Vesta’s surface, diogenites could have come from inside the asteroid, and howardites look like a mix of the two, which could have happened when Vesta’s crater was formed. And Dawn pretty much confirmed that was right! Using its different spectrometers, Dawn confirmed the data we’d measured from farther away was correct, and Vesta is probably responsible for a whole class of meteorites all by itself.

Which is a pretty big accomplishment for one asteroid! After 14 months at Vesta, Dawn packed up its bags and started a 1.5 billion kilometer, almost three-year journey to the dwarf planet Ceres, where it arrived in 2015. And it’s been there ever since.

As we were approaching Ceres, one of the first things we noticed were these bright spots like big, shiny reflectors stuck all over the surface. These spots were a huge mystery for a while. Were they water ice?

Clay? Salt? Aliens?

After analyzing how the spots absorbed light, astronomers concluded they’re probably magnesium sulfate, or as we call it here on Earth, Epsom salt. So you can think of that the next time you’re preparing to drink an Epsom salt laxative. The salt deposits were probably created when asteroids smashed into Ceres’ surface, exposing the salt-filled ice that was hidden beneath.

Then, the exposed ice turned into a vapor, and the salt deposits were left behind. We suspected from previous research that Ceres had a lot of water ice, but Dawn helped confirm that during its other observations. And the ice seems like it’s everywhere, even if it’s hiding underneath the surface.

But this February, Dawn found something that we didn’t expect: organic molecules, which are the building blocks of life. Although it couldn’t detect exactly what the molecules were, the spacecraft observed tar-like compounds made of carbon and hydrogen on Ceres’ surface in an area of about 1000 square kilometers. Because the concentration of molecules is so high, it seems unlikely that the molecules came from an outside source like an asteroid impact, since the impact usually scatters molecules all over the place.

Instead, they probably formed from water and heat based chemical reactions on Ceres, which could mean it used to have a warm, watery environment. We might investigate that more in the future, but for now, Dawn’s main job in its extended mission is to give us a clearer picture of Ceres’ surface and composition. When cosmic rays from the Sun hit Ceres, they create radiation and neutrons that Dawn can use to identify the different molecules on and even underneath the surface.

Dawn did a lot of this during its mission, but since we’ve got some extra time, we’re hoping to get the clearest data we can. The mission will officially end when Dawn runs out of fuel, which could happen around the end of 2018. After that, it’ll probably just keep orbiting Ceres, like a permanent little travel buddy.

But we’ve already found a lot, and we’ll be sure to keep you updated on any exciting discoveries in the future! Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who help make this show possible. If you want to help us keep making episodes like this, you can go to patreon.com/scishow to learn more.