March 2, 2015

Dawn orbiter nears the finishing line

It is only a few more days until the Dawn spacecraft enters orbit around Ceres on 6 March 2015, marking humankind's first visit to a dwarf planet. What Ceres has disclosed to scientists so far has raised more questions than it has provided answers: "The localised, extremely bright patches we see on the camera images are unusual, and so far we have been unable to find an explanation – a possible reason could be exposed ice that reflects a large proportion of the light falling on its surface," says Ralf Jaumann from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Institute of Planetary Research. The wide variation in crater forms and their preservation states have also surprised the scientists. For instance, the dwarf planet has a crater with a diameter of 300 kilometres that is nevertheless shallower than expected for an impact depression. It also has impact basins that have high walls and a mountain at their centre. "I have to say that Ceres is far more exciting than I had expected," says Jaumann.

An unknown world located over 500 million kilometres from Earth

The NASA Dawn spacecraft orbited the asteroid Vesta from July 2011 to September 2012 and then continued its journey towards its primary mission destination, the dwarf planet Ceres. Since then, Dawn has travelled over 228 million kilometres, moved further into the Asteroid Belt between Jupiter and Mars and is now venturing onwards at a distance of over 500 million kilometres from Earth. As the spacecraft draws closer to Ceres, the curiosity and excitement of the researchers is growing. This is the first time that an orbiter has investigated two celestial bodies during one mission; asteroid Vesta was a 'dry', rocky asteroid, rugged and covered with craters, mountains and canyons. In contrast, dwarf planet Ceres, a 'wet', icy asteroid, will not only have a dirty, dusty crust of ice, it may even possess an ocean of water beneath its surface.

Yet however different these inhabitants of the asteroid belt may be, they each preserve traces of the early developmental stages of Earth: "Both asteroids are fossils from the birth of the Solar System, and can shed light on its origin," says Dawn Deputy Principal Investigator Carol Raymond from NASA's Jet Propulsion Laboratory (JPL). Although Vesta, with a diameter of 530 kilometres, and Ceres, with a diameter of almost 1000 kilometres, are the only known intact asteroids, the gravitational field of Jupiter prevented them from becoming planets. Jaumann is convinced that: "Many of the things that we will learn during the Dawn mission will help us acquire a better understanding of how the Solar System came into being 4.5 billion years ago."

As promising as it is mysterious

The researchers discovered an amazing and scientifically interesting new world with asteroid Vesta. As the orbiter approaches dwarf planet Ceres, it is revealing itself to be equally promising and mysterious. Discussions and speculation centre on the extremely bright patches visible in one of Ceres’ craters: "They are quite puzzling for interested observers and planetary researchers alike," says Jaumann. In the images, acquired at a distance of 46,000 kilometres, the patches cover no more than one pixel, which makes them less than four kilometres across. This resolution is not high enough for any kind of detailed explanation. "It may be cryovolcanic activity or exposed ice. We will just have to wait until we have images with higher resolution." Analysis conducted using the infrared spectrometer on the Herschel Space Observatory has already determined that there is water vapour around the dwarf planet. It is possible that the emission of water vapour and the bright patches are related. However, a complete explanation will not be possible until the spacecraft has entered its orbit around Ceres and is able to image the dwarf planet from a smaller distance.

The craters themselves, their shape and distribution, suggest that Ceres will be a rewarding object to study. Researchers had expected a fairly flat surface – particularly at the equator – as the processes occurring in the thin layer of ice would cause a flattening and lowering of any crater features over time. However, the scientists have discovered not just one particularly large crater, which barely resembles an impact crater, but a whole series of noteworthy impact basins with mountains located at their centres. Indeed, the entire surface of the dwarf planet is extremely varied and exhibits an extraordinary diversity of structures. "We will not be able to deduce the thickness of the ice crust we suspect is located beneath the dust-covered surface until we have analysed these crater shapes," explains Jaumann.

Timetable until mission end

The Dawn spacecraft will be captured by Ceres' gravitational field at a distance of 41,000 kilometres on 6 March 2015. The next images of the dwarf planet will not follow until 10 April. Dawn will then investigate Ceres for 20 days in April and May from an orbit with an altitude of 13,500 kilometres. The orbiter will then draw closer to Ceres in June, first to an altitude of 4400 kilometres and then to 1470 kilometres. The closest approach will be in December 2015, when Dawn will orbit Ceres at an altitude of just 375 kilometres. The planetary researchers will acquire a constant stream of data for 14 months – until the end of June 2016, which they will use to investigate their target. Once its propellant is exhausted, Dawn will be unable to point its instruments, but will nevertheless remain as an artificial satellite in orbit around the dwarf planet for a long time to come.

The third dimension

"There are many mysteries about Ceres that we will not be able to resolve until we have access to the third dimension", says Jaumann. From May 2015, Dawn will begin its systematic mapping of the dwarf planet from a variety of directions. The DLR Institute of Planetary Research will use this data to produce a three-dimensional elevation model of the celestial body. For this, the DLR Institute has planned exactly how many images need to be acquired at which locations in order to compute the first precise topography of a dwarf planet.

The Dawn mission

NASA's Jet Propulsion Laboratory (JPL) in Pasadena manages the Dawn mission; JPL is a division of the California Institute of Technology. The University of California, Los Angeles, is responsible for the overall Dawn mission science. The camera system on the spacecraft was developed and built under the leadership of the Max Planck Institute for Solar System Research in Göttingen, Germany, in collaboration with the DLR Institute of Planetary Research in Berlin and the Institute of Computer and Communication Network Engineering in Braunschweig. The Framing Camera project is funded by the Max Planck Society, DLR, and NASA/JPL.

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Manuela Braun

Editor HR
German Aerospace Center (DLR)
Central HR Marketing
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Prof. Dr. Ralf Jaumann

Freie Universität Berlin
Institute of Geological Sciences
Planetary Sciences and Remote Sensing
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