Asteroids can no longer be considered a rubble pile of boring, shattered, cratered worlds. They have turned out to be as different from each other as the planets are. Galileo flew past the first asteroid, Gaspra, on October 29, 1991 on its way to Jupiter; spacecraft have flown by many others since. Three have been orbited: Eros, Itokawa, and Vesta. The Japanese spacecraft, Hayabusa, actually landed on Itokawa and successfully returned a few grains of its surface back to Earth. A highly sophisticated spacecraft, Dawn, will in a few months continue in the wake of its successful Vesta mission to go into orbit around the largest asteroid, Ceres.
Dawn was launched on September 27, 2007 and flew by Mars on February 17, 2009 for a gravity assist into the asteroid belt. It went into orbit around Vesta on July 16, 2011 until September 5, 2012 when it departed for Ceres. It orbited as close to Vesta as 60 miles and revealed a battered world rich in diversified minerals and geology. Vesta was found to have two huge craters, one nearly on top of the other, near the south pole that is around seven miles deep with a central peak eleven miles high. The impacts have gouged out a huge part of Vesta's southern hemisphere giving it a misshapen appearance. Rows of ridges girdle the equatorial region and there are craters and landslides everywhere.
Giuseppe Piazzi discovered the first asteroid, Ceres, on January 1, 1801. It is the largest asteroid at approximately 605 miles in diameter, round, and classified as a dwarf planet. It orbits about 250 million miles from the Sun and takes about 4.6 years to orbit. Ceres contains about one-quarter of the asteroid belt's total mass. Although it shines as bright as magnitude +7.0, only the largest surface features can be fuzzily resolved with the Hubble Space Telescope. The most interesting is a bright white spot that may be an icy impact crater. There is also a dark circular area of unknown origin. It rotates swiftly in about nine hours and may have a rocky core overlain with an icy mantle. The mantle may be as thick as 70 miles and may contain more fresh water than Earth (keep in mind that most of Earth's water is not considered fresh, as watery as it may seem). A thin atmosphere is suspected, which may contribute to an expected frosty surface, and unusually balmy temperatures as warm as 0 degrees F. It will be Dawn's job to confirm these proposed theories and observations.
Dawn uses new rocket propulsion technology that makes exploring the asteroid belt easier by not only allowing it to fly from one asteroid to another, but also to orbit each one at various altitudes. It bristles with instruments, which include a camera, two spectrometers, an altimeter, and a magnetometer. Dawn's primary objective is to determine why Ceres and Vesta evolved so differently. It has obtained one piece of the puzzle with its successful mission to Vesta now behind. Dawn is equipped with giant solar panels that power an ion engine that shoots out charged ions of xenon gas to accelerate it up to 62,000 miles per hour. This allows Dawn to easily maneuver into orbit around an asteroid, break free, and then travel to another asteroid and orbit it.
Dawn will arrive at Ceres on March 6, 2015 and will orbit it as close as 60 miles. It will carry out observations and measurements identical to what it did at Vesta. Its primary mission will end later in 2015; however, it will continue to explore Ceres for up to a year or more, as long as it remains functional. Beyond Ceres there are no plans for it to fly to another asteroid, although Pallas has often been considered a target. The aging spacecraft has had problems therefore Dawn will remain at Ceres and eventually crash into its surface in the far distant future. Dawn clearly has marked the dawn of a whole new era of asteroid exploration.