December 14, 2015 -Dawn Preparing for New Observations
Dawn thrust with its ion engine on Dec. 11-13 to fine tune its orbit.
385 kilometers up. All the images you’ve seen so far are from more than two months of intensive observations of Ceres from an altitude of 1,470 kilometers and more.
The choice of orbital height works as follows: “Ceres’s bumpy gravity field does not allow perfectly smooth, regular orbits at low altitude. Moreover, the variations in the strength of the gravitational attraction transform the orbits. As the changing forces reshape the orbit, the ellipse gets more exaggerated, with the low points going lower and the high points going higher. The differences within one revolution grow. Thanks to the ingenious design of the orbital trajectory however, those same forces then will gradually attenuate the profile, causing it to become more round again. This pattern repeats. It is almost as if the orbit breathes slowly, its envelope expanding and contracting.”
When it finished fine tuning its orbit, it pointed its gamma ray and neutron detector (GRaND) at Ceres. GRaND measures the energies and numbers of these two components of nuclear radiation, from which scientists can determine the abundances of some elements on the dwarf planet, including the abundance of major rock-forming elements (O, Mg, Al, Si, Ca, Ti, and Fe), trace elements (Gd and Sm), long-lived radioactive elements (K, Th, and U), and light elements such as H, C, and N.
Question. How? Answer:
There are four mechanisms involved. Cosmic rays hitting the surface produce fast neutrons. These are slowed by some elements, which allow measurements of amounts of epithermal and thermal neutrons allowing measurements of hydrogen, carbon, samarium and gadolinium. The mechanism of inelastic scattering and the mechanism of thermal neutron capture followed by gamma ray emission allows measurement of elements H, O, Mg, Al, Si, Cl, Ca, Ti, Fe. And natural radioactivity allows measurements of K, Th, U.
Some of the gamma rays and neutrons emitted by Vesta and Ceres are produced by radioactive elements and others are created by the bombardment of the surface material by cosmic rays. As they emanate from the surface and travel into space, some will be intercepted by Dawn’s gamma ray and neutron detector (GRaND) which, despite its name, is very humble. (For the sake of having an interesting appellation, it’s fortunate that GRaND detects gamma rays and neutrons and not neutrons and gamma rays.) The gamma rays and neutrons reveal many of the important atomic constituents to a depth of one meter (three feet) or so on Vesta and Ceres, thereby adding to the detailed story Dawn will tell. As we know from the first log, Ceres may be rich in water. If it is, the signature of water may be contained in GRaND’s data.
Navigators are making precise measurements of the adjusted orbit to verify that it meets the needs for the intensive observation campaign that will begin on Dec. 18. In the meantime, the spacecraft will collect more radiation measurements as well as conduct some bonus photography and infrared spectroscopy on Dec. 16-17.
Later today (Dec 14) mission controllers will power on two of Dawn’s reaction wheels. (Two others failed in 2010 and 2012.) Engineers cannot confidently predict how long the two units will operate, but as long as they do, they will reduce the expenditure of hydrazine propellant, extending the spacecraft’s lifetime in this final phase of the mission. When either one fails, Dawn will return to using only hydrazine to control its orientation
