Dawn has now started taking pictures of Ceres much closer than ever before, but only those from greater altitude (Survey orbit) have been released so far. 52 pictures from Survey orbit are now on the web at http://dawn.jpl.nasa.gov/multimedia/images/ceres.html
A new Dawn blog was posted today, at http://dawnblog.jpl.nasa.gov/
Dawn arrived in its third mapping orbit on Aug. 13, which was slightly ahead of schedule. So dallied a bit before began this third mapping phase on schedule on Aug. 17. When Dawn explored Vesta from comparable orbits, it pointed its scientific instruments at the illuminated ground whenever it was on the dayside. Every time its orbit took it over the nightside, it turned to point its main antenna at Earth to radio its findings to NASA’s Deep Space Network. That is not the plan at Ceres because of the failure of two of Dawn’s four reaction wheels. (same fault that Hubble and Stardust/Genesis had). A result is that it will take 11 days for Dawn to return a complete view of all of Ceres. Each “mapping cycle” takes 11 days. Dawn will conduct 6 mapping cycles recording visible and IR spectra, each one looking at a slightly different angle. This will effectively yield stereo views.
In addition to the stereo pictures and the many spectra (which reveal the nature of the minerals as well as the surface temperature), Dawn will use the color filters in its camera to record the sights in visible and infrared wavelengths. The probe also will continue to acquire spectra both of neutrons and of gamma rays. Precise measurements of Dawn’s radio signal will reveal more details of the dwarf planet’s gravitational field and hence the distribution of mass within. As at Vesta, Dawn’s polar orbits are oriented so that the craft always keeps the sun in view, never entering Ceres’ shadow, even when it is nighttime on the ground below.
Controllers had allowed the time in the spacecraft’s main computer to drift off, as there was no need to keep it particularly accurate. But recently the clock was off by so much that they decided to correct it, so before the mapping began, they adjusted it by a whopping 0.983 seconds. Dawn is 905 miles (1,456 kilometers) from Ceres. It is also 2.06 AU (191 million miles, or 308 million kilometers) from Earth, or 775 times as far as the moon and 2.03 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 34 minutes to make the round trip.