Imagine a soccer ball held 3.5″ (9 cm) from your eyes. That is the scale of how close the spacecraft Dawn is to the surface of Ceres in polar orbit of 5.4 hrs. Actual altitude is about 240 mi (385 km), as close as the International Space Station is to Earth. Dawn will remain in this low altitude mapping orbit (LAMO) forever. It does not have enough fuel to move to another orbit or location in space. The primary mission will end in mid-2016. The mission might be extended if science, resources, and money warrant.
The orbit diagram below shows the path of Dawn the final three months of 2015. In October, the spacecraft was 915 mi (1470 km) from Ceres traveling 400 mph (645 km/h) shown as an outer green ring. This orbit was called the high altitude mapping orbit (HAMO). Upon completion of scientific duties, the ion propulsion engine was turned on. That weak thrust caused Dawn to spiral inward toward Ceres. The blue lines in the diagram show the spiral path.
After nearly 7 weeks of ion engine thrusting, Dawn reached the LAMO shown as the inner green ring. Because it fell deeper into the gravity field of Ceres, it gained speed. Dawn is now going 610 mph (980 km/h). Images are returned daily and are available to browse at this fine link. Bookmark and return often.
This image from LAMO gives a sense of the detail and perspective possible on the surface of Ceres.
GRaND View Mapping
Because Dawn is in polar orbit the entire surface is visible to the instruments on board. There are three primary types of instrumentation. As noted above, images will allow a detailed visual map to be made of the surface features. In addition, scientists want to know the elements and chemical makeup of the surface. Mapping those from orbit is accomplished using the gamma ray and neutron detector (GRaND). The 21 sensors in GRaND measure the energy of gamma photons and neutrons emitted by matter at the surface. The signals are very weak. The LAMO allows for the strongest signals and most detailed mapping possible.
Cosmic rays from space strike the surface of Ceres. The nucleii of elements are excited by the absorption of the cosmic ray energies. The nucleii in turn emit gamma photons and neutrons to rid themselves of the extra energy. By analyzing the spectra of the gamma and neutron energies, GRaND will allow scientists to learn the composition of the various substances on the surface.
Gravity Field Mapping
Scientists are eager to probe the interior of Ceres over the next months. As Dawn passes over the surface at close range, it will feel slightly speeded up and slowed down by the presence of mass concentrations beneath the surface. The variations in the speed of Dawn will be measured from Earth using the communications antennae. The speed variations will show up as doppler shifts in the frequency of the telemetry signal. A global map of gravity strength, and therefore density of underlying material, will emerge. An example of such a map of Earth is from the GRACE mission. Red bumps indicate high density and stronger than normal gravitational pull. Blue is less density and weaker pull. Images credit: NASA | JPL