Venture to the last protoplanet

July 22, 2011

Text: Thorsten Dambeck

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This image of the asteroid Vesta was taken on July 15 shortly after the Dawn spacecraft swung into orbit around the... [more]

This image of the asteroid Vesta was taken on July 15 shortly after the Dawn spacecraft swung into orbit around the asteroid. [less]

This image of the asteroid Vesta was taken on July 15 shortly after the Dawn spacecraft swung into orbit around the asteroid.

© NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

© NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

When astronomers point the Hubble telescope towards the Vesta planetoid, they play their last trump card. With its eagle eye, the space telescope is known to provide the clearest possible images of celestial bodies. However, Vesta has beaten even Hubble: the photos it took in 2007 are reminiscent of the view of Mars which is obtained with a run-of-the-mill telescope - a few blurred light and dark patches and a slightly non-spherical form; it is difficult to recognise any further detail on the small body, although Vesta is the third-largest planetoid. Ceres, number one in terms of size, has nearly twice the diameter - around 1,000 kilometres. Astronomers have known about the two asteroids for two centuries; however, only now is their exploration really taking off: Vesta and Ceres are on the flight schedule of Dawn, one of NASA’s probes. It entered into an orbit around Vesta on Saturday after a journey taking just under four years.

On board are two cameras - so-called framing cameras - which were constructed and developed by a team led by scientists from the Max Planck Institute for Solar System Research (MPS) near Göttingen, Germany. Both instruments have an identical construction and the feat they have to pull off is to convert an expanse of terra incognita in the solar system into charted territory. Dawn’s mission is also directed at the chemical-mineralogical composition of the surface rocks of both celestial bodies. The ultimate aim is to answer one of astronomy’s most important questions: how did the planets of the solar system come into being?

For a long time it was assumed that asteroids were debris from a burst planet beyond Mars’ orbit, as several hundred thousand members of Vesta’s family orbit the sun, most of them in the so-called main belt between Mars and Jupiter. However, the hypothesis of the shattered planet has long been passé and, at any rate, it would have been tiny: all the planetoids of the main belt taken together do not even amount to the mass of our moon. Planetologists now tend to hold the view that the planetoids are a kind of planetary building material that was left over when the worlds of our solar system were built.