Dawn Serving a Delicious Feast for Scientists

SCIENCE CORNER

An image of Haulani Crater that Dawn acquired during its 3rd mapping orbit at an altitude of 915 miles (1,470 kilometers). The crater is 21 miles (34 kilometers) in diameter (Photo – NASA/JPL-Caltech/UCLA/MPS/DLR/IDA).

Everyone who hungers for new knowledge about the cosmos or for bold adventures far from Earth can share in the sumptuous feast Dawn has been serving.

By Dr. Marc D. Rayman

One of the major objectives of the mission was to photograph 80 percent of Ceres’ vast landscape with a resolution of 660 feet per pixel. That would provide 150 times the clarity of the powerful Hubble Space Telescope. Dawn has now photographed 99.8 percent with a resolution of 120 feet per pixel.

This example of Dawn’s extraordinary productivity may appear to be the limit of what it could achieve. After all, the spaceship is orbiting at an altitude of only 240 miles (385 kilometers), closer to the ground than the International Space Station is to Earth, and it will never go lower for more pictures. But it is already doing more.

Since April 11, instead of photographing the scenery directly beneath it, Dawn has been aiming its camera to the left and forward as it orbits and Ceres rotates. By May 25, it will have mapped most of the globe from that angle. Then it will start all over once more, looking instead to the right and forward from May 27 through July 10. The different perspectives on the terrain make stereo views, which scientists can combine to bring out the full three dimensionality of the alien world. Dawn already accomplished this in its third mapping orbit from four times its current altitude, but now that it is seeing the sights from so much lower, the new topographical map will be even more accurate.

Dawn captured this view of Oxo Crater on Jan. 16 from an altitude of 240 miles (385 kilometers). This is the only location (so far) on Ceres where Dawn has clearly detected water (Photo – NASA/JPL-Caltech/UCLA/MPS/DLR/IDA).

Dawn is also earning extra credit on its assignment to measure the energy of gamma rays and neutrons, as the gamma ray and neutron detector (GRaND) can reveal the atomic composition down to about a yard (meter) underground, and last month we saw initial findings about the distribution of hydrogen. However, Ceres’ nuclear glow is very faint. Scientists already have three times as much GRaND data from this low altitude as they had required, and both spectrometers in the instrument will continue to collect data. In effect, Dawn is achieving a longer exposure, making its nuclear picture of Ceres brighter and sharper.

Using the radio signal to track the probe’s movements allows scientists to chart the gravity field and thereby learn about the interior of Ceres, revealing regions of higher and lower density. Once again, Dawn performed even better than expected and achieved the mission’s planned accuracy in the third mapping orbit. Because the strength of the dwarf planet’s gravitational tug depends on the distance, even finer measurements of how it varies from location to location are possible in this final orbit.

Thanks to the continued smooth operation of the mission, scientists now have a gravitational map fully twice as accurate as they had anticipated. With additional measurements, they may be able to squeeze out a little more detail, perhaps improving it by another 20 percent before reaching the method’s limit.

Dr. Marc D. Rayman is the Dawn Mission Director and Chief Engineer at JPL. Marc greatly enjoys sharing the thrill of interplanetary adventures with the public.