WASHINGTON (AFP) — The US space agency is set to launch on Thursday the space probe Dawn on an eight-year mission to unlock the secrets of the solar system and how it was born.
After several delays in recent months, NASA said Sunday it is finally ready to launch the unmanned craft on its mission to Ceres and Vesta, the two largest asteroids orbiting the sun, in the asteroid belt between Mars and Jupiter.
The deep-space explorer, measuring 1.64 meters (yards) long and 1.27 meters (yards) wide, aims to provide a better understanding of the building blocks that formed the terrestrial planets and how the two "protoplanets" followed different evolutionary paths.
The National Aeronautics and Space Administration set the liftoff window for Thursday between 1120 GMT and 1149 GMT, or 7:20 am to 7:49 local time at Cape Canaveral, Florida.
The first of Dawn's three main objectives is to probe the first moments of the solar system's creation 4.6 billion years go by gathering information about the two giant asteroids between Mars and Jupiter.
Secondly, it aims to find out what kinds of elements form terrestrial planets such as Earth, Mars, Mercury, Venus and Ceres.
And finally, its mission plans to explain why and how Vesta and Ceres followed a different evolutionary and formative path, particularly the role that water may have played in their development.
Ceres, discovered in 1801, has a spherical shape and has a diameter of about 960 kilometers (596 miles). Scientists believe it may have a layer of thick ice under its crust, covering a rocky core.
Ceres was classified in 2006 as a "dwarf planet," according to a new definition by astronomers to describe asteroids in the solar system.
The decision by the International Astronomical Union was the result of a debate about the status of Pluto, which is now classified as a dwarf planet along with Ceres and another celestial body, Eris.
Vesta, discovered in 1807, is smaller than Ceres but the third largest asteroid in the solar system. With a diameter of 520 kilometers (323 miles), Vesta has a rocky surface without a trace of water and a hot interior.
Scientists are especially interested in the enormous crater on the south pole of Vesta, 460 kilometers (285 miles) wide and 13 kilometers (eight miles) deep, which is believed to be the result of a major collision.
Astronomers estimate that five percent of all meteorites found on Earth are the result of this gigantic jolt.
Dawn is scheduled to enter orbit around Vesta in October 2011, proceed to Ceres in May 2012 and then begin orbiting Ceres in February 2015 -- traveling a total distance of 5.1 billion kilometers (3.1 billion miles).
The spacecraft is equipped with innovative ion propulsion engines, which consume relatively little fuel and only accelerate at a gradual pace. The engines use an electrical charge to accelerate ions from xenon fuel.
Dawn is also carrying a high-definition camera and two spectrometers.
After having cancelled the Dawn project previously, NASA revived the mission in 2006 after an investment of 449 million dollars.
It is the ninth mission out of 10 planned in NASA's Discovery program that employs unmanned vehicles to explore space, often focusing on asteroids.
Weather concerns postponed Dawn's liftoff on Wednesday.
"Weather prevented technicians from completing the loading of fuel on the Delta rocket's second stage," NASA said, rescheduling it for Thursday.
The US space agency in July postponed Dawn's mission until September due to "limited launch opportunities."
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