Steamboat Springs The discovery of the minor planet Ceres by astronomer Giuseppe Piazzi in 1801 set off a series of discoveries of many other mini-worlds circling the sun in the huge gulf of space between the orbits of Mars and Jupiter. Originally classified as planets, the sheer numbers of these small objects demanded a new classification. Astronomers settled on the term “asteroid,” which means “star-like planet.”
Today, more than 40,000 of these asteroids have been observed, ranging in size from just less than 621 miles across for Ceres to small chunks only one-third of a mile across. In 2006, Ceres was reclassified as a dwarf planet, along with Pluto.
German astronomer Heinrich Olbers discovered the second largest asteroid in 1807 and allowed his colleague Karl Gauss to select the name for it — Vesta. Vesta, at 329 miles in diameter, has an unusually bright surface for a chunk of rock and is the only main-belt asteroid that can become bright enough to be seen with the naked eye. Astronomers have determined that Vesta is covered with a light-colored lava that was extruded onto its surface long ago.
Hubble Space Telescope images of the Colorado-sized asteroid Vesta reveal tantalizing hints of craters and lava flows and what appears to be a colossal impact crater near its south pole. The central mountain peak in this 300-mile diameter hole rises 11 miles high — twice as high as Mount Everest, Earth’s highest peak. About 200 space rocks called HED meteorites that have fallen to Earth are thought to be fragments thrown out from this violent impact on Vesta.
NASA’s Dawn spacecraft has been maneuvering toward the mysterious asteroid Vesta since its launch in September 2007, and it will arrive at Vesta and settle into orbit July 16. After scrutinizing Vesta’s surface and environment for 11 months, Dawn will fire up its innovative ion-propulsion engine and depart for its second target, the largest of the asteroids and dwarf planet designate, Ceres. Dawn will arrive at Ceres in February 2015 and will orbit and study this water-rich world for several months.
By exploring these two enigmatic mini-worlds in the asteroid belt, we hope Dawn will reveal more about the history of our own world and the dawn of our solar system, hence the spacecraft’s name. Watch the news for the first detailed glimpses of Vesta as July 16 approaches and, for more information, visit NASA’s Dawn Mission home page at http://dawn.jpl.nasa.gov.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. Check out Jimmy’s website at www.jwestlake.com.