MESSENGER’s mission profile specifies an unusually elliptical orbit, made necessary by the harsh conditions so near the Sun. It is equipped with an efficient solar shield to protect it from the worst of the Sun’s heat and radiation and is designed to always remain between the Sun and the probe. Mercury itself is so hot, though, that if MESSENGER were to stay near enough to the planet’s surface to use it’s more sensitive instruments it would rapidly overheat. The solution is a long swooping elliptical orbit, similar to that of a comet. The orbiter swoops down for a close look, scans the planet’s surface rapidly, then retreats for a long cool-down period. The specific parameters have been chosen to synchronise with Mercury’s rotation so that, with each swoop, MESSENGER can study a different part of the planet. Over time, the entire planet will be surveyed.
For more information, visit the official mission page here: http://www.nasa.gov/mission_pages/messenger/main/index.html