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NASA's Space Place

The Largest Asteroid Doesn’t Quite Cut It As a Planet

Alex H. Kasprak
National Aeronautics and Space Administration

From a distance, Ceres might look a bit like any other planet you might see. It’s spherical, after all, and that’s one of the biggest requirements of planethood. But put it next to any other planet and you would quickly see that it is seriously tiny. It’s not even 1/5th the size of Mercury, which is the tiniest planet in the Solar System. This small size is one of the reasons why Ceres is classified as a dwarf planet.

Its location, floating amongst thousands of bits of other space rock in the Asteroid Belt, is another clue that it hasn’t reached full planethood. For a space object to be honored with the title ‘planet,’ it has to have enough gravity to clear away all the junk around it. But, alas, poor Ceres can’t muster that kind of gravitational strength.

Still, amongst the other asteroids, Ceres is a giant. Because of this, it is considered both a dwarf planet and the largest asteroid—the only dwarf planet/asteroid in our solar system. It is also the closest dwarf planet to Earth. The others, including famed Pluto, are all near or beyond the orbit of Neptune.

But it’s really hard to see. It’s tiny, after all, and it’s also not especially bright. We haven’t really had a chance to take a close look. That is about to change!

After over seven years of travel, a trusty spacecraft named Dawn is fast approaching Ceres. It has already begun to take pictures. As it gets closer and closer, those pictures will become clearer and clearer. It has already revealed a cratered surface. By March, it will enter into orbit around Ceres, giving us all kinds of new information about this mysterious little world.

When it does that, it will become the first spacecraft ever to visit a dwarf planet!

Learn more about asteroids and other tiny worlds with a factsheet from NASA’s Space Place! http://spaceplace.nasa.gov/posters/en/#solarsystem.

An image of Ceres taken by the Dawn spacecraft on January 13th, 2015. The images will get clearer as Dawn gets closer. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.


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