|Date of discovery||January 1, 1801|
|Name of discoverer||Giuseppe Piazzi|
|Name origin||Roman goddess of agriculture|
|Order from primary||5|
|Semi-major axis||2.76678175 AU|
|Titius-Bode prediction||2.8 AU|
|Sidereal year||1680.973163 da|
|Avg. orbital speed||17.882 km/s|
|Inclination||10.586404° to the ecliptic|
|Sidereal day||9.075 h|
|Rotational speed||0.091584 m/s|
|Mass||9.47 * 1020 kg|
|Mean radius||476.2 km|
|Equatorial radius||487.3 km|
|Polar radius||454.7 km|
|Surface gravity||0.2787 m/s²|
|Escape speed||0.5152 km/s|
|Surface area||2,849,631 km²|
|Mean temperature||167 K|
|Maximum temperature||239 K|
|Number of moons||0|
|Composition||Rock and water|
Ceres is named for the Roman goddess of agriculture, from which name the word cereal derives.
Piazzi was searching for the "missing planet" that, according to Bode's law, should exist between Mars and Jupiter. Bode's law predicted an object having a semi-major axis of 2.8 AU, remarkably close to the actual semi-major axis of Ceres.
When Piazzi first observed Ceres, he thought he was looking at a comet. But comets normally move much faster than Ceres does, a fact Piazzi appreciated. He observed Ceres 24 times and reported his discoveries to Bode and other astronomers.
Shortly after the publication of Piazzi's findings, Ceres was lost in the glare of the sun. Karl Friedrich Gauss, then 24 years old, predicted Ceres' path and suggested where to look to reacquire Ceres. On December 31, 1801, the astronomer Baron von Zach found Ceres very near where Gauss said he might.
For many years Ceres was listed as a planet, even after the asteroids Pallas, Juno, and Vesta were discovered. With the discovery of yet more objects in what would later be known as the Asteroid Belt, Ceres was reclassified as an asteroid, the largest of all such bodies. But Ceres, unlike any other object in the asteroid belt, has a hydrostatic-equilibrium shape, characteristic of an object sufficiently massive that its self-gravity forces such a shape despite the rigid-body forces that normally hold a solid object's shape.
In 2006, the discovery of Eris provoked a fresh look at all the bodies of the solar system, and in particular what constituted a planet and what didn't. As a result of the debate, Eris and Pluto, an object even smaller than Eris, were classed as dwarf planets--a new classification with definite criteria. Ceres meets these criteria, and thus Ceres is considered a dwarf planet--the smallest of three known bodies of that class.
Recent observations made by the Hubble Space Telescope suggest that Ceres might be very rich in water ice. Specifically, astronomers have concluded that Ceres has a layered interior, with a rocky inner core, a thick layer of water ice, and a dusty outer crust. McFadden specifically says that Ceres is an "embryonic planet," one that (according to evolutionary theory) tried to form a planet but could not because Jupiter, being so close, so perturbed the local gravitational field that Ceres could not grow any larger than it is. Findings supporting the water-ice theory include:
- Most objects having hydrostatic-equilibrium or round shape have differentiated, or layered, interiors.
- Spectrography of the surface shows evidence of water-bearing minerals.
Ceres has no known satellites.
Observation and Exploration
Until today, the only observation of Ceres has been by telescope. Astronomers have inferred a remarkable number of the physical properties of Ceres from this observation, but wish to learn much more.
On September 27, 2007, the Dawn mission began officially with the launch of Dawn, the first rocket probe to visit the asteroid belt. The Dawn craft carries an ion engine that it will use on the second part of its journey after it makes rendezvous with Mars. Dawn's two targets are the asteroid Vesta and the dwarf planet Ceres.
- Blue, Jennifer, ed. "Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers." United States Geological Survey, March 31, 2008. Accessed June 2, 2008.
- "Orbital elements for 1 Ceres." JPL Small-Body Database, JPL, NASA, May 14, 2008. Accessed June 2, 2008.
- Carey, Bjorn. "Largest Asteroid Might Contain More Fresh Water than Earth." Space.com, September 7, 2005. Accessed January 22, 2008.
- "Asteroid 1 Ceres." The Planetary Society. Accessed January 22, 2008.
- Parker, Joel William, Thomas, Peter C., McFadden, Lucy A., Mutchler, M., and Levay, Z. "Largest Asteroid May Be 'Mini-Planet' with Water Ice." The Hubble Site, September 7, 2005. Accessed January 22, 2008.
- "Dawn Mission Overview." JPL, NASA, March 31, 2008. Accessed June 2, 2008.
- Rayman, Marc D., Fraschetti, Thomas C., Raymond, Carol A., and Russell, Chrisopher T. "Dawn: A mission in development for exploration of main belt asteroids Vesta and Ceres." JPL, NASA, April 5, 2006. Accessed June 2, 2008.
- Hamilton, Calvin J. "Dwarf planet Ceres." Views of the Solar System, 2007. Accessed January 22, 2008.