Trav­el­ling to the dawn of the So­lar Sys­tem

The Dawn spacecraft set off on en route to Vesta and Ceres, the two most massive objects in the main asteroid belt, on 27 September 2007, propelled by its ion engines. Vesta was thoroughly studied in 2011 and 2012; Ceres is now one of the last big 'unknown' bodies in the inner Solar System – only Pallas, which has a diameter of over 500 kilometres, has not yet been visited by an orbiter. Ceres has an average diameter of 950 kilometres, which makes it the largest object in the asteroid belt, prompting its 2006 classification – together with Pluto – as a member of the group of dwarf planets.

The investigation of these two large bodies, which offer valuable information about the earliest period of the Solar System, is of fundamental importance to understanding the origin and evolution of Earth.

Dawn carries a German camera system, developed and constructed under the leadership of the Max Planck Institute for Solar System Research (MPS) in Göttingen, in cooperation with the DLR Institute of Planetary Research in Berlin-Adlershof and the University of Braunschweig Institute of Data Technology and Communication Networks. In addition to their designated purpose of mapping and researching the asteroids, the two identical 'Framing Cameras' are indispensable tools for navigating the orbiter. Dawn carries two other experiments besides the cameras – a spectrometer from the Italian space agency ASI, used to analyse the mineralogy on the asteroid surface, and a gamma ray/neutron spectrometer from Los Alamos National Laboratories (USA).

Dawn flew past Mars on its way to the asteroid belt in February 2009, using the planet's gravity to provide an increase in the spacecraft's velocity. The camera had already been operated during the approach to Vesta, the orbiter's first destination, scanning the environment for dust and – unfortunately without success – small moons. The actual investigation of Vesta's surface commenced on 15 July 2011 from a distance of 16,000 kilometres, when Dawn entered orbit around the asteroid. The spacecraft spent the next 18 months examining the second heaviest object in the asteroid belt, descending ever closer towards the surface before returning to a higher altitude orbit in preparation for its onward journey in the direction of Ceres.

A succession of three observation phases

During the first phase of observation, Dawn was in orbit 2420 kilometres above the asteroid's surface. In the 68 hours that it took the orbiter to circle Vesta, it acquired images of the entire surface and conducted spectral analyses. The individual images were used to compile a three-dimensional model of Vesta. The spectral analysis provided information on the mineralogical composition of the outermost layer of dust covering the surface of the asteroid.

In the second phase, Dawn orbited the asteroid at an altitude of 670 kilometres above the surface in 12 hours. Here, high-resolution images were acquired to analyse geological formations. Among other things, this phase involved a thorough investigation of a large double impact crater situated at Vesta's south pole.

Phase three took the orbiter down to just 180 kilometres above the surface. At this altitude, Dawn was able to orbit Vesta in four hours. This phase prioritised measurements using the gamma ray and neutron detector. The data acquired here permitted a detailed examination of the surface properties.

March 2015 – Arrival at Ceres

After concluding its exploration of Vesta in September 2012, Dawn set its sights on Ceres, situated a little further from the Sun. Travelling for three years along an outward spiral orbit, Dawn circled three quarters of our central star before reaching Ceres. The first image of the celestial body reached Earth in December 2014. However, from the great distance of 1.2 million kilometres, Ceres measured just nine pixels across and was visible as a bright spot in the recording. In March 2015, the spacecraft entered orbit around the dwarf planet Dawn is the first spacecraft to orbit two different Solar System bodies beyond Earth.

Just as it did during its approach to Vesta, Dawn has used its ever-increasing proximity to Ceres in order to examine the Ceres' surface. This investigation of the celestial body is divided into three phases. In March 2015, Dawn was located 41,000 kilometres from Ceres' surface, and is progressively moving closer – it will be a mere 375 kilometres from the surface in January 2016. By mid-May 2015, Dawn had acquired almost 2000 images and completely mapped the dwarf planet's surface for the first time. As these images of Ceres reach Earth, scientists are becoming more puzzled than ever. Bright spots are visible in the images, flat plains have been identified, a pyramid-like structure was discovered and unstable crater, landslides and fissures can be observed. The closer the spacecraft is to the surface, the more details can be seen in the pictures. Are the white spots patches of ice? Or salt? The scientists are gaining an insight into the complex geological processes at work on Ceres and are seeing traces of past activity.

Following what is known as the High Altitude Mapping Orbit (HAMO) at an altitude of 1470 kilometres, in October 2015 Dawn began a maneouvre to achieve Low Altitude Mapping Orbit (LAMO), which will be at 375 kilometres. There, Dawn will acquire images with a resolution of 140 metres per pixel, so much more detailed data is expected. In addition to images, the spacecraft will acquire spectral data.

At the end of the mission – scheduled for next year – the spacecraft is designed to circle the dwarf planet for at least 50 years, following a stable orbit in a form of 'quarantine'. The purpose of this is to prevent any microbes from Earth that may be adhering to the orbiter from contaminating the surface if the craft crashes onto the surface of Ceres. If traces of simple forms of life are to be discovered there in future, they should on no account originate from an earthly 'import'.

Dawn is the ninth mission in the NASA Discovery Program. This focuses on cost-efficient projects with a comparably small budget of approximately 500 million US dollars. 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 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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Falk Dambowsky

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