Dwarf Planet Ceres


Previously, I told you the fascinating story of Ceres’s discovery and complicated identity crisis; now I’m ready to tell you about the dwarf planet specifically.


Let’s take a quick trip out beyond our Moon, past Mars (if you find yourself at Jupiter, you’ve gone too far), and into the realm that is commonly known as the asteroid belt. Now, contrary to popular depictions, the asteroid belt isn’t crammed full of asteroids. As a kid, I remember seeing illustrations of the asteroid belt that made it look as densely packed as Saturn’s rings. That depiction is a gross exaggeration. In fact, while there are billions of bodies orbiting out in the asteroid belt–it is believed that there are somewhere between 1 and 2 million asteroids with a diameter of 1 kilometer or more–the area is still mostly just empty space. If you were to board a rocket that would fly through the asteroid belt, the chances of actually smacking into anything are extremely slim. Of the asteroids in the belt that have a diameter of 10 km or more, a collision is only likely to occur about once every 10 million years. So anyway, this is the home of one such body, the dwarf planet Ceres.

Compared to the other bodies in the asteroid belt, Ceres is huge. Ceres has a diameter of 950 kilometers (590 miles). This is a little smaller than the width of Texas or Montana. Ceres comprises between a quarter and a third of the mass of the entire asteroid belt. King among the asteroid belt, Ceres falls short when facing up against the other planets in our solar system. Compared to Earth and the Moon, Ceres has the mass of .00015 that of the Earth and .0128 that of the Moon. (For some perspective, it would take almost 80 Cereses to equal the mass of just the Moon.)

Size comparison of Earth, the Moon, and Ceres.

Size comparison of Earth, the Moon, and Ceres.


Ceres orbits the Sun at an average distance of 415 million kilometers (257 million miles), in a nearly circular orbit. At this distance from the Sun, and at the speed that Ceres is traveling, one year on Ceres is equivalent to 4.6 years on Earth.

Ceres is believed to consist of a thin, dusty crust situated above a fairly thick layer of water-ice. At the center of the dwarf planet is a thick rocky core.

Cutaway image showing Ceres's layers.

Cutaway image showing Ceres’s layers.
“Ceres Cutaway” by NASA, ESA, and A. Feild (STScI)


Ceres, of course, has less mass than the Earth, and thus you would weigh less standing on a scale on Ceres than you would on Earth.  If you weigh 150 pounds on Earth, then you weigh a mere 4.2 pounds on Ceres!

Ceres is one of the latest planets to be explored by high-tech modern spacecraft. NASA’s Dawn spacecraft is currently orbiting the dwarf planet at a just recently arrived distance of only 2,700 miles above its surface. For about a month, Dawn will orbit and study Ceres from this location. The spacecraft will complete an orbit every three days, constantly kicking images and other important data back to Earth. For some perspective, the resolution Dawn can obtain while imaging Ceres is somewhat comparable to what it would be like for you to observe a soccer ball from 10 feet away. Subsequent to the 2,700 mapping orbit, Dawn will venture even closer to the dwarf planet providing better and better views of Ceres. By the end of 2015, Dawn will be concluding its mission at an altitude of only 230 miles. Dawn’s cameras at this distance will be able to produce images with a resolution 850 times greater than that of what Hubble would be able to produce. Now, that soccer ball is a mere 3.3 inches away! At this distance, Dawn will be in a fairly stable orbit around the dwarf planet and is expected to become its satellite into perpetuity.

I’ll have more to share about Dawn soon.

For now, let’s all celebrate the fact that we’re still exploring–exploring not just planets and asteroids and moons, but exploring actual worlds. Let’s celebrate the fact that we’re learning new things about this particular world on a daily basis and that this will continue for many months to come. And, let’s celebrate the fact that with all that we know today, it’s a tiny amount compared to what we still get to learn in the future. Having a lot to learn, I think, is much more exciting than already knowing it all.

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