NASA recently announced that its Dawn spacecraft has started to approach asteroid Vesta, for an orbital encounter with Vesta on 16 July. (An asteroid is a small rocky object in orbit around the Sun.) Vesta (530 km across) is small by planetary standards, but it is one of the largest known asteroids. It orbits the Sun between Mars and Jupiter, in a zone populated by millions of asteroids, termed the asteroid belt. Astronomers regard Vesta a protoplanet, a cosmic object that almost formed into a planet.
At the beginning of this final approach to Vesta, the Dawn spacecraft is 1.21 million km from Vesta, or about three times the distance between Earth and the Moon. Through the approach, Dawn will be mainly operating a special, highly efficient type of engine, known as the ion engine. It uses electricity to produce thrust, by ionizing and accelerating xenon gas. Dawn’s ion thrusters provide less thrust than conventional rocket engines, but are expected to provide propulsion for years during the mission, and allow better capability to change velocity.
"We feel a little like Columbus approaching the shores of the New World," said Christopher Russell, Dawn principal investigator. "The Dawn team can't wait to start mapping this Terra Incognita."
The ion engine will enable Dawn to match the orbit of Vesta, and spiral swiftly into orbit around the asteroid. When Dawn comes to within approximately 16,000 km from Vesta, the asteroid's gravity will capture the spacecraft in orbit.
Scientists will study Dawn’s images for possible moons around Vesta. None of the images from ground-based and Earth-orbiting telescopes have recorded any moons, but Dawn will acquire much more detailed images, to determine whether small objects have remained undiscovered.
Dawn's interplanetary voyage, a 4.8-billion-km spaceflight, began on 27 September 2007, with its launch, from Cape Canaveral Air Force Station in Florida. It will orbit Vesta for one year, then will begin a new long cruise phase, to arrive at its second destination, an even larger asteroid belt body, the dwarf planet Ceres (about 950 km across), in 2015.
Vesta and Ceres would help scientists unravel the secrets of our Solar System's early evolution, since the members of the asteroid belt are believed to be primitive objects, whose composition remained nearly unchanged, through the history of the Solar System. The mission will compare and contrast Vesta and Ceres. Dawn's science instruments will examine surface composition, topography and structure of the two objects. In addition, Dawn will measure the gravity from Vesta and Ceres, to probe their internal structures.
For more information about Dawn, visit:
http://www.nasa.gov/dawn and
http://dawn.jpl.nasa.gov
References
NASA
www.nasa.gov/
Wikipedia