The largest and most mysterious resident of the debris belt between Mars and Jupiter is an icy world called Ceres, and it’s on the threshold of being explored up close for the first time by NASA’s Dawn mission, which is scheduled to enter Ceres’s orbit on March 6.
Discovered in 1801 by the Italian astronomer Giuseppe Piazzi, Ceres was initially thought to be a full-fledged planet. That began to change when one of Piazzi’s rivals, the astronomer William Herschel, noted that Ceres only appeared as a point of light in his telescope rather than a resolved disk, like the other known planets. To Herschel that meant Ceres was probably too small to be considered a planet, and he coined the term “asteroid” to describe its starlike appearance. Ceres received a minor upgrade to “dwarf planet” in 2006, part of the same process that demoted Pluto to the same status.
Whatever you call may call it, Ceres is one of the most geologically interesting and strange objects in the solar system. Its shape, size and composition—round, roughly the size of Texas and at least 20 percent water ice—place it at the poorly understood transition point between rocky worlds like Earth and icy worlds like Jupiter’s Europa, Saturn’s Enceladus, and other large moons of the outer solar system. Other than blurry Hubble Space Telescope images from 2004, its surface had scarcely been glimpsed until Dawn’s approach. As the spacecraft’s ion engines slowly push it toward Ceres, the dwarf planet’s details are now coming into focus, revealing tantalizing new details with practically every new image.
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