On an eight-year, 4.9 billion-kilometer (3-billion-mile) mission to answer basic questions about the formation of planets in our solar system, NASA's unmanned Dawn spacecraft will be the first to orbit two planetary bodies on a single voyage. Beginning in August 2011, Dawn will make the first of two rendezvous with the asteroid Vesta and dwarf planet Ceres (in 2015), two of the largest objects that lie within the asteroid belt between Mars and Jupiter. Scientists theorize that objects like Vesta and Ceres may be the "seeds" of planets that never formed. Dawn's scientific instruments will measure shape, surface topography, tectonic history, elemental and mineral composition, and search for water-bearing minerals. Dawn spacecraft itself will also be used to measure the masses and gravity fields of Vesta and Ceres.

A Delta II-Heavy rocket propelled the 1,217.7 kilograms (2,684.6 pounds) Dawn spacecraft into space from Cape Canaveral, Florida on September 27, 2007. It took three upper stages and nine solid-fueled booster rockets to send Dawn on its four-year flight to the asteroid belt via a February 2009 Mars gravity assist.

To speed it on its way, the Dawn spacecraft will get a gravity assist from the planet Mars in February 2009. The Dawn team may use the spacecraft's science instruments to observe the red planet. The Mars flyby will help propel the spacecraft farther out of the ecliptic, the plane containing the mean orbit of Earth around the sun. This is necessary because Dawn's next destination, the asteroid Vesta, has an orbit around the sun that is outside the ecliptic plane. Overall, the Mars flyby will change Dawn's velocity relative to the sun by 4,020 kilometers per hour (2,498 miles per hour). Dawn will explore Vesta from August 2011 through May 2012. It will then depart for a February 2015 arrival at Ceres. Dawn will end its primary mission after a six-month exploration of Ceres. At that time, the spacecraft will be in a "quarantine" orbit around Ceres at an altitude of about 700 kilometers (435 miles). This orbit ensures that the spacecraft will not impact Ceres for more than half a century.

Using the same set of scientific instruments to study each of the two bodies, scientists will be able to compare and contrast the objects, and hope to learn much about the formation and evolution of our solar system.

Theorized to be the "seeds" of planets that never evolved, Vesta and Ceres appear to have formed in different parts of the solar system more than 4.6 billion years ago, and have since evolved under significantly different circumstances. It's interior melted, and it's surface dry, Vesta's origins appear to have been hot and violent. Ceres, by contrast, seems to have been kept cool by water as it evolved. There is evidence of frost or vapor on its surface and, possibly, liquid water under the surface. As a result, Ceres remains in its primordial state, while Vesta has evolved and changed over millions of years.

The Dawn spacecraft is powered by two 8.3-meter-by-2.3-meter (27-foot-by-8-foot) solar panels. Power is stored via a 35-amp-hour rechargeable nickel hydrogen battery.

Dawn's innovative ion propulsion system will provide the spacecraft with enough thrust to reach Vesta and Ceres. The demanding mission profile would be impossible without the ion engines -- even a mission only to asteroid Vesta (and not on to Ceres) would require a much larger spacecraft and a dramatically larger launch vehicle. Each of Dawn's three 30-centimeter-diameter (12-inch) ion thrust units is movable in two axes to allow for migration of the spacecraft's center of mass during the mission. This also allows the attitude control system to use the ion thrusters to help control spacecraft attitude. The thrusters work by using an electrical charge to accelerate ions from xenon fuel to a speed 10 times that of chemical engines. The electrical level and xenon fuel feed can be adjusted to throttle each engine up or down. The Dawn spacecraft carries 425 kilograms (937 pounds) of xenon propellant.

The Dawn spacecraft carries three onboard scientific instruments: a Framing Camera, a Gamma Ray and Neutron Detector, and a Visible and Infrared Mapping Spectrometer. Dawn will also conduct a radio science experiment to measure the gravity fields of Vesta and Ceres.

The Framing Camera will take detailed optical images that will be used for both navigation and scientific research. In addition to detecting the visible light that humans see, the cameras will register near-infrared energy. Dawn carries two identical and physically separate cameras for redundancy, each with its own optics, electronics, and structure. Each camera is has eight gigabits of internal data storage, and an f/7.9 refractive optical system with a focal length of 150mm. Mainly to help study minerals on Vesta's surface, each camera is equipped with seven color filters. The Max Planck Institute for Solar System Research, Germany, was responsible for the cameras' design and fabrication, in cooperation with the Institute for Planetary Research of the German Aerospace Center and the Institute for Computer and Communication Network Engineering of the Technical University of Braunschweig.

Many scientists believe that Ceres may be rich in water. If true, the Gamma Ray and Neutron Detector will discover this while studying the elements that make up the surfaces of both Vesta and Ceres. As its name suggests, the Gamma Ray and Neutron Detector will use a total of 21 sensors to observe gamma rays and neutrons emanating from the surfaces of Vesta and Ceres. The instrument was built by the Los Alamos National Laboratory in New Mexico.

The Visible and Infrared Mapping Spectrometer will measure the surface mineralogy of both Vesta and Ceres. Each image records the light intensity at more than 400 wavelength ranges in every pixel. Scientists will compare the observations stored in its six-gigabit internal memory, with laboratory measurements of minerals to determine what minerals exist on the surface of each body. The Visible and Infrared Mapping Spectrometer is a modified version of an instrument that flew on both the European Space Agency's Rosetta and Venus Express missions. It also inherits technology from Cassini's visible and infrared mapping spectrometer. Provided by the Italian Space Agency, Dawn's Visible and Infrared Mapping Spectrometer was designed and built at Galileo Avionica in collaboration with Italy's National Institute for Astrophysics.

In addition, a radio science experiment will measure the gravity of Vesta and Ceres. By monitoring Dawn's signals with sensitive, Earth-based antennas, mission controllers can detect slight variations in the gravity fields of the two space objects. These variations will provide clues about how mass is organized within Vesta and Ceres.

The Dawn mission to asteroid Vesta and dwarf planet Ceres is managed by JPL for NASA's Science Mission Directorate, Washington, D.C. The University of California, Los Angeles is responsible for overall Dawn mission science. Other scientific partners include: Los Alamos National Laboratory, New Mexico; Max Planck Institute for Solar System Research, Katlenburg, Germany; DLR Institute for Planetary Research, Berlin, Germany; Italian National Institute for Astrophysics, Rome; and the Italian Space Agency. Orbital Sciences Corporation of Dulles, Virginia, designed and built the Dawn spacecraft.