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.
On course for Ceres since September 2012
After completing its investigations 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, Dawn circled three quarters of our central star before reaching the heaviest of all the objects in the asteroid belt in March 2015. It is now entering into orbit to analyse the celestial body. This makes Dawn the first spacecraft to travel in an orbit between two objects located beyond Earth in the Solar System.
Just as it did during its approach to Vesta, Dawn has used its gradually increasing proximity with Ceres to examine the asteroid's immediate environment since the start of 2015. The investigation of Ceres is divided into three phases. Over the course of its mission, Dawn will approach the surface of the celestial body until it reaches a distance of approximately 400 kilometres. 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.