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Nasa is preparing the Dawn spacecraft for lift-off on its long-delayed mission to explore an asteroid belt between Mars and Jupiter for clues to the birth of the solar system.
If successful, it will be the first robotic probe to put itself into orbit around two different bodies,
thanks to an extremely efficient electric engine, powered by xenon gas.
Dawn's targets are two of the largest bodies in the asteroid belt, Vesta and Ceres, which scientists believe hold clues to the solar system's formation.
Blast-off of the unmanned rocket carrying Dawn is set for 11.20pm tonight from Cape Canaveral Air Force Station.
Dawn's last launch attempt in July was cancelled after a string of technical problems and weather delays cut into the satellite's launch opportunity.
Nasa decided to launch its higher-priority Mars probe and then a space shuttle mission in August before returning Dawn to the launch pad.
The spacecraft is perched atop an unmanned Delta 2 rocket, which was to boost Dawn into orbit around Earth so it can unfold its 19.7m solar wing panels and fire one of its three ion engines for a four-year trip to Vesta.
Dawn will slingshot itself around Mars in February 2009, but most of its speed will be built up slowly over time - very slowly.
The probe's engines work by pumping electrically charged ions of xenon through an electric field, which accelerates the particles and prepares them for an 142,400km/h escape into space.
The force of the gas causes the spacecraft to move in the opposite direction.
The motion, which is about equal to the pressure of a sheet of notebook paper on the human palm, is so gentle it would be useless on Earth.
But in space, where there is no counteracting gravitational force, momentum builds up over time.
It will take Dawn about four days to accelerate from zero to 96 km/h, but it will consume only 1.2 litres of fuel to do so.
After 12 days, the spacecraft will reach more than 288km/h and after a year Dawn should be zipping along at a respectable 8800km/h, having used only 57 litres of fuel.
"I call it acceleration with patience," said Dawn's lead engineer, Marc Rayman with Nasa's Jet Propulsion Laboratory in Pasadena, California.
Traditional chemical thrusters are about one-tenth as efficient as Dawn's ion engine and require far more fuel, which is why once a spacecraft reaches its intended target it settles into orbit and stays there.
Dawn's first target is Vesta, the fourth-largest asteroid in the region and the only object in the asteroid belt visible from Earth with the naked eye. It has an iron core and signs of lava flows on its surface.
Scientists believe it is very similar to Earth's moon.
Vesta also has a gaping crater, so deep it exposes the asteroid's mantle.
Dawn is expected to help answer questions about Vesta's composition and how it formed.
The spacecraft has three science instruments to study surface features and determine chemical composition. "From that we try to work back to how the whole thing was put together and what happened to it," said lead scientist Chris Russell, with the University of California at Los Angeles.
After six months of observations, Dawn will leave orbit and begin a three-year trek to the dwarf planet Ceres, the king of the asteroid belt.
Though relatively close to Vesta, icy Ceres formed under vastly different circumstances. It contains water-bearing minerals and possibly a weak atmosphere.
By comparing the two worlds, scientists hope to learn more about how the solar system formed.
- Reuters
