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NASA's Dawn probe has started "setting its sights" for its forthcoming rendez-vous with Vesta (the second largest asteroid) set for 16th July 2011, when both objects will be 188 million km from the Earth. Launched in 2007, Dawn has already traveled 2 billion km along a trajectory that will first bring it alongside Vesta and then allow it to enter orbit around the asteroid.
Although the path necessary for a close approach can be calculated with sufficient precision before launch, the corrections necessary to place the probe into orbit around Vesta can only be determined in the weeks and days before arrival. Therefore, it is essential that the target be photographed against the background star field in order to determine the precise location of the probe.
Dawn began this delicate phase of its journey on 3rd May, obtaining its first image of Vesta (above) where the asteroid appears overexposed so as to show its location against the background stars.
When the image was taken, Dawn was at a distance of 1.21 million km from the asteroid, a distance that will reduce to about 2,700 km at the beginning of August.
At that point Dawn will begin taking a series of multi-angle images that mission scientists will use to make a topographical map of the asteroid. Eventually, the probe will enter into an orbit 200 km above the surface (Vesta is 530 km in diameter) obtaining high resolution images of the surface and making various other measurements of its structure.
The stay at Vesta will last for one year, after which Dawn will be sent towards Ceres, the largest asteroid, with a diameter of 950 km (technically a "dwarf planet" rather than an asteroid), where it will arrive in 2015, after another 3 billion km journey.
From the analysis of the data from the two asteroids, and by comparing their chemical and physical properties, as well as their gravitational pull on the probe, we will learn about their internal structure and their origin. We will thus develop a much better picture of the origin of the solar system, given that the asteroids are thought to be the remnants of its formation.
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