Messenger at Mercury.
Credit: NASA/Johns Hopkins University
Applied Physics Laboratory/Carnegie
Institution of Washington
MESSENGER will have an unusually elliptical orbit in which it will spend most of its time far from Mercury, regularly swooping in close in the same way that a comet orbits the Sun. This is necessary because of the harsh conditions so close to the Sun. MESSENGER is equipped with a massive heat shield which is permanently held between the spacecraft and the Sun, but Mercury itself is so hot that the spacecraft would rapidly overheat if it was parked in a close orbit. Unfortunately, many of the instruments need to operate from a short range, so the elliptical orbit was designed as a compromise. MESSENGER swoops in close to take some readings, and then retreats to cool down. The orbit has been precisely synchronized so that, as Mercury rotates, every part of its surface will eventually be scanned by MESSENGER.
MESSENGER's orbit about Mercury is highly elliptical; it's lowest point above the
surface is 200 kilometers (124 miles) while the highest point is more than
15,000 kilometers (9,300 miles)
So for now, we all wait eagerly for the data to start flooding back. We will soon have detailed maps of Mercury's magnetic field and its interactions with the Sun, we will be better able to identify features that were spotted in earlier flyby's, and gain a better understanding of Mercury, the smallest planet.