ARRIVING AT A NEW WORLD: NASA’ s Dawn probe captured this view of dwarf planet Ceres’ northern regions on 14th and 15th April, 2015, shortly after its arrival, from a distance of about 22,500 kilometres( 14,000 miles). Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA
and versatile ion propulsion system were discussed in Issue # 10 of‘ RocketSTEM’(‘ Journey to the Beginning of the Solar System’, February 2015), as were the results of the first phase of its mission at asteroid 4 Vesta. Since then, it has continued its journey on to dwarf planet Ceres, which it has been orbiting since March 2015. Here we provide a summary of what Dawn has revealed so far about this small but significant world, described by Dr. Rayman before the encounter as‘ an intriguing and mysterious orb that has beckoned for more than two centuries’ since its discovery by Giuseppe Piazzi on 1st January 1801.
Why was Ceres a significant target?
Ceres, like Vesta, was chosen for study by the Dawn mission because it is thought to hold keys to unlocking the secrets of the Solar System’ s early history. This has, indeed, proved to be the case for Dawn’ s first target, Vesta, which was confirmed by the spacecraft to be one of the last remaining rocky proto-planets, a remnant building-block of the kind that formed the terrestrial planets, and the only known asteroid with an Earth-like differentiated internal structure of core, mantle, and crust.
As for Ceres, much of its attraction as a target for scientists was the fact that it is an‘ oddball’ of the asteroid belt. Thought at its discovery in 1801 to be a comet, then considered a planet, it was finally re-designated an asteroid in the 1850s when increasing numbers of small bodies were discovered orbiting the Sun between Mars and Jupiter. However, with an average diameter of 945km( 587 miles) it is much larger than any other object in this region of the solar system. It is massive enough for its gravity to have pulled it into a roughly spherical shape, which led to it being reclassified once more in 2006, this time as a‘ dwarf planet’. Finally, Earth- or near-Earth-based observations of Ceres hinted at the presence of ice and clay minerals at or near its surface, and even traces of water-vapour in its proximity, indicators perhaps of a different origin to that of many asteroids and of possible geological activity at its surface.
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