The first global map of the asteroid Vesta generated from images acquired with the Framing Camera. In July and August 2011, Dawn orbited the third largest object in the asteroid belt initially in a 'mapping orbit' at an altitude of about 2400 kilometres above its surface. In doing so, the camera acquired hundreds of images with a spatial resolution of about 250 metres per pixel. What is referred to as ‘simple cylindrical projection’ was selected for the representation of this global map. In this map projection the south pole is not a point but is extended as a line the length of the equator. Thus, the south pole covers the entire lower edge of the image and all characteristics of this region are represented in distortion. The spatial resolution of the photo is 750 metres per pixel.
Three connected craters are found in Vesta's northern hemisphere; for this reason, scientists have named them 'snowman'. NASA's Dawn spacecraft obtained this image with its Framing Camera on 6 August 2011. The image resolution is about 260 metres per pixel.
Perspective view of a part of the edge of the south pole basin on Vesta
A visitor from Earth would encounter a dramatic landscape at Vesta's south pole: cliffs several kilometres high, deep trenches and craters which have formed the southern tip of this fascinating protoplanet in the asteroid belt, and a mountain massif up to 15 kilometres high. For the scientists working on the Dawn mission, it is not yet clear how this wild landscape was formed – collisions with other asteroids contributed, but also the internal processes that played a role during the asteroid’s early formation phases. This diagonal view was derived from a global digital elevation model of the asteroid created from stereo image data obtained with the German Framing Camera on board NASA's Dawn space probe at an altitude of 2420 kilometres above Vesta's surface. The images, which were acquired during Dawn's observation orbit, have a resolution of about 250 metres per pixel.
The Framing Camera on board the Dawn spacecraft will acquire images of the surface of Vesta and Ceres from various perspectives and under changing light conditions. These images will then provide the basis for creating 3D models of the asteroids.
Three-dimensional images allow the scientists to analyse geological formations in detail. The exact three-dimensional survey of numerous craters of an asteroid is also exciting. On the basis of the shape and height of their edges, the scientists can work out the force of the impact, the mass of the projectile and the composition of the impacted area.
Main task of the DLR scientists: assigning precise coordinates to the Dawn image data and preparing high precision maps of Vesta and Ceres
The Max Planck Institute for Solar System Research provides the systematically corrected raw image data from the Framing Camera needed for further processing. From this, the DLR Institute of Planetary Research then creates the three-dimensional image products for the mapping and scientific study of Vesta and Ceres. The stereo software used for this purpose is thoroughly tested. It has been used for years in the three-dimensional mapping of the Moon, Mars and Mercury. All image data from the two 'Dawn asteroids' will be collected and stored at DLR and will subsequently be transferred to NASA's Planetary Data System (PDS).
For the three-dimensional image data processing, the DLR researchers use what is known as stereo photogrammetric methods. Photogrammetry uses information from at least two images of the same region of the asteroid, but taken from different perspectives. This stereo data is processed in such a way that elevation information can be derived for each pixel, which then allows the generation of a highly detailed three-dimensional terrain model. The information produced from these perspectives of the surface, which change with each orbit of the spacecraft, is taken into account and continuously improved during stereo processing. Here, the accuracy of the 3D model being created also depends on the high-precision calibration of the Framing Camera, which was carried out before the launch with great care. In the finished model, details with elevation differences of less then 10 metres can be shown.
The stereo software was subjected to a thorough test before its real-life application on Vesta. This was based on simulations of photos of the asteroid prepared by the NASA Jet Propulsion Laboratory from earlier blurry images of Vesta acquired by the Hubble Space Telescope. From these, DLR researchers were able to develop a corresponding virtual 3D model of the asteroid before Dawn's arrival at Vesta.
