On March 6, the Dawn spacecraft was captured by the gravity of Ceres which is, depending on your preferences, either the largest asteroid of the solar system, or one of many dwarf planets. On its approach, Dawn’s cameras took these images of the two hemispheres from a distance of 28,000 miles.
These are by far our best views ever of this object, but no new ones have been released since then. What’s going on? Is NASA hiding evidence of alien life? Although I have to rank this speculation alongside the moon-hoax conspiracy theories in its utter lack of plausibility, I wondered myself about the lack of new data. So I did a little digging, and the explanation is both more prosaic and more interesting.
Here is a diagram of the path taken by the spacecraft on its approach, looking down on Ceres from above its north pole. The sun is out of the image to the left.
The white circles are at one-day intervals, and the closer together they are, the more slowly the spacecraft is traveling. Today, on March 18, Dawn is near the apex of this path, moving only at 37 miles per hour relative to Ceres, getting ready to fall back toward its target picking up speed as it does so. You can see from this that it is now farther from the dwarf planet than it was on its approach.
Dawn has a remarkable rocket engine on board, an ion propulsion system that can operate for months at a time.
Its thrust is almost nothing (equivalent to the force of a sheet of paper on your hand), but operating over long periods of time allows it to move the spacecraft around the solar system, albeit quite slowly. If the spacecraft’s engine were shut down just as it entered orbit, it would loop around Ceres in a long elliptical path. Instead, it continues thrusting to put Dawn into a close circular orbit with an altitude of 8400 miles. And that orbit is a polar one, which allows it to image the entire surface. This shows the same orbital approach, this time from the side. North is up and the sun is again out of the image to the left.