Ceres
Ceres, dwarf planet
orbiting the Sun in the asteroid belt between the orbits of Mars and Jupiter at
an average distance of 415 million km (259 million mi). Ceres revolves around
the Sun once every 4.6 years and rotates on its axis in just over nine hours.
Ceres is by far the largest body in the asteroid belt, with an average diameter
of about 950 km (about 590 mi) and a mass over four times that of the next
largest asteroidal body. Long ranked as an asteroid, Ceres accounts for almost a
third of the mass of the entire asteroid belt.
In 2006 the International Astronomical Union (IAU)
reclassified Ceres as a dwarf planet because it has a rounded shape from effects
of its own gravity, a feature not confirmed yet for other asteroids. It is not
classified as a true planet because it has not cleared the neighborhood of its
orbit of other objects—it is surrounded by smaller rocky bodies in the asteroid
belt. (The IAU reclassified Pluto and Eris as dwarf planets on similar
grounds—both objects are associated with the Kuiper Belt, a region containing
thousands of small icy bodies.)
Studies based on observations by the Hubble Space
Telescope indicate that Ceres may have a differentiated internal structure, with
a separate mantle layer containing water-ice surrounding a rocky core. Its
surface is dark and is likely covered with a thin layer of material containing
hydrocarbons similar to carbonaceous chondrite meteorites.
The National Aeronautics and Space Administration (NASA)
is currently planning a spacecraft mission to explore Ceres and the asteroid
Vesta. Called Dawn, the unmanned probe is scheduled for launch in 2007 and is
expected to orbit Vesta in 2011 then move on to orbit Ceres in 2015. The two
bodies are thought to represent different evolutionary stages in the formation
of rocky planets from primitive materials in the early solar system.
Italian astronomer Giuseppe Piazzi found Ceres in 1801
while cataloging stars. Piazzi observed the same area of the sky on consecutive
nights and noticed that one of the stars appeared to be moving. He named the new
object Ceres after the Roman goddess of agriculture. Ceres was the first member
of the asteroid belt to be discovered.
See also Solar System.
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