Asteroids are finally being recognized as more than just a clutter of broken and battered rocks. There is evidence that Ceres may have liquid water beneath its crust, and Vesta may be volcanic. Spacecraft have flown by several asteroids since Galileo flew past the first asteroid, Gaspra, on October 29, 1991, and two have been orbited: Eros and Ikotawa. A highly sophisticated orbiter will at long last explore two of the largest asteroids in the Main Belt between the orbits of Mars and Jupiter in a way that has never been done before. The spacecraft Dawn will explore Vesta and Ceres and promises to make significant contributions to the knowledge of asteroids.
Dawn was launched on September 27, 2007 and flew by Mars on February 17, 2009 for a gravity assist into the asteroid belt. It will arrive at Vesta this summer on July 21 and will go into orbit as close as 60 miles above its surface during a year-long stay. Dawn bristles with instruments, which includes a camera, two spectrometers, an altimeter, and a magnetometer. It will thoroughly map Vesta and determine the mineral content of its surface. It will measure the elevations of all surface features and reveal details as small as a few feet across. Dawn will be able to detect an atmosphere and magnetic field if present, and determine how solid Vesta is internally.
Vesta was discovered by Heinrich Olbers on March 29, 1807 and is the second largest asteroid at approximately 330 miles in diameter along its widest axis. It orbits about 220 million miles from the Sun and takes about 3.6 years to orbit. Vesta rotates in about 5.3 hours. It is rather oblong, being squashed at the poles, but is deformed further at the south pole by a huge impact crater about 285 miles across. This crater is the most obvious feature seen by the Hubble Space Telescope and is up to seven miles deep with a rim that rises about seven miles above the surrounding terrain. A central peak towers up to eleven miles from the crater floor. It is estimated that about 1% of Vesta was excavated by the impact and may be responsible for a type of meteorite found on Earth, known as eucrites, which are identified as being from Vesta.
Vesta is unique among asteroids because it has completely separated (differentiated) into a metallic core with a molten mantle and rocky crust. The rising molten material from within Vesta appears to have breached the crust and covered portions of the surface with a magma ocean that quickly froze into dark lava plains or mare. No other asteroid has this characteristic and the meteorites that match Vesta’s composition are clearly of volcanic origin. This is Vesta’s biggest mystery because it is thought to be too small to generate enough heat to have volcanic activity. It is hoped that the giant impact crater will have excavated Vesta enough to reveal clues as to its volcanic history.
Dawn is using new rocket propulsion technology that will make exploring the asteroid belt easier by not only allowing it to fly from one asteroid to another, but also to orbit each one to make a detailed analysis. Dawn’s primary objective is to determine why Ceres and Vesta evolved so differently. It is equipped with huge solar panels that power an ion engine that shoots charged ions of xenon gas to accelerate it up to 68,000 miles per hour. This will allow Dawn to maneuver easily enabling it to go into orbit around an asteroid and then to break free of the orbit again.
Dawn will depart Vesta on July 20, 2012 rich with discoveries and fly out to Ceres where it will arrive on February 1, 2015. It will then orbit Ceres within 60 miles of its surface and carry out observations and measurements similar to what it did at Vesta. Its primary mission will end in July 2015 and will probably remain at Ceres, however there may be plans for it to depart and fly past another rather large asteroid, Pallas. Regardless, Dawn will mark the dawn of a whole new era of asteroid exploration when it arrives at Vesta in a few months.