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9. Dawn Spacecraft Approaches Dwarf Planet Ceres
John Austin
images of dwarf planet ceres

Images of Ceres obtained by NASA's Dawn spacecraft on 12 February 2015, from a distance of 83,000 km. The two views are of opposite hemispheres as the dwarf planet rotated. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

The Dawn spacecraft has now sent back the best images yet of a dwarf planet and is due to arrive at Ceres on 6 March 2015. In the image, craters and white patches are clearer than in previous images and these provide issues to investigate. The image resolution is about 8 km, giving the sharpest images of a dwarf planet to date. Note the essentially spherical shape of Ceres, which distinguishes it from a large asteroid such as Vesta.

The Solar System

The solar system consists of the sun, the 8 well-known planets with their attendant moons, dwarf planets, asteroids and comets. In the early solar system, material would have existed throughout the solar system but became gravitationally settled to form the planets and other bodies. Dwarf planets are distinguished from planets mainly in that their orbits still contain other debris such as asteroids and comets. They are also typically much smaller than planets (and indeed Earth's moon) but still largely spherical in shape. As recently as 2006, Pluto was reclassified as a dwarf planet because its orbit still contains material that has not been collected up. Whereas the planets are relatively stable, dwarf planets, asteroids and comets have the potential for further change by sweeping up the remaining material along their orbits. Comets can be destroyed by the sun by losing material as their orbits pass closest to the sun.

Orbit of the dwarf planets Pluto and Eris compared with the outer planets. Credit: NASA JPL.

Only 5 dwarf planets have so far been discovered: Ceres, Pluto, Makemake, Haumea and Eris. Most dwarf planets (the Plutoids) orbit the Kuiper belt outside the orbit of Neptune, while Ceres lies within the main asteroid belt between Mars and Jupiter. The Kuiper belt itself was discovered only as recently as 1992. A 6th object, 2012 VP113 thought to be a dwarf planet, of diameter about 450 km, was discovered in 2012 but its closest distance to the sun is 80 AU (1 AU is the distance from the Earth to the sun, about 150 million km). In comparison, Pluto is the largest object in the Kuiper belt and its distance from the sun varies from 30 to 49 AU. There are likely to be be dozens of dwarf planets yet to be discovered. They have escaped discovery simply because they are small objects and too dark to be detected by our telescopes.

In addition, almost 700,000 asteroids and 3000 comets are known. These numbers continue to increase as new discoveries are made. Asteroids differ from Dwarf planets in having an irregular shape. At this stage in their evolution they do not have sufficient mass for gravity to have reshaped them into the more spherical shape of dwarf planets and planets.

Comets differ from asteroids in that their orbits are very eccentric and take them close to the sun. While passing close to the sun, its radiation and the solar wind (the stream of particles from the sun) evaporate material from the comet forming the characteristic tail.

Dawn's mission

Dawn spacecraft location on 18 February, showing the distance to Ceres as 0.00034 AU = 51,000 km. Credit: NASA JPL.

Synthesis of the best views of the asteroid Vesta, over the period July 2011 to september 2012. Credit NASA/JPL-Caltech/UCAL/MPS/DLR/IDA -

By NASA's high standards the Dawn mission must already count as a huge success. In 2011 and 2012 the Dawn spacecraft encountered the asteroid Vesta and returned a tremendous amount of high quality data. Vesta is the largest known asteroid, so by comparing the geology and physical characteristics of the asteroid with that of the dwarf planet Ceres, scientists can investigate how the early solar system formed, and in particular how the planets ultimately formed in the early solar system. Thanks to the Dawn spacecraft we now have excellent data on the asteroid Vesta. Its diameter varies between 446 km and 573 km with a mean of 525 km, whreas Ceres has a diameter of about 950 km. Vesta has a density of 3456 kg/m3, which is smaller than the Earth's mean density of 5513 kg/m3. However small asteroids and comets typically have a much lower density. For example, the comet 67P/Churyumov-Gerasimenko, which was recently visited by the European Space Agency Rosetta probe, was found to have a density of only 470 kg/m3 and is only about 4 km across. Although asteroids and comets do vary in porosity (or fraction of empty space between the granules), as their sizes increase so we would expect gravity to compress their material and reduce the porosity. Hence, the largest asteroids and dwarf planets should have higher densities than the comets and small asteroids which may only be tens of kilometres across.

Rendezvous with Ceres

The Dawn spacecraft is now approaching the dwarf planet Ceres at about 5000 km per day, and as shown above has already begun to send back high quality images ahead of its arrival on 6 March. Thereafter it will go into orbit in order to carry out extensive scientific analyses. If the studies of Vesta are a guide, then there is much more to come and in due course an improved understanding of how the planets coalesced from the solar system debris.

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Article initially prepared 18 February 2015.

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Website revised by John Austin, 18/2/2015. © Enigma Scientific Publishing, 2015.