On 20 April 2014 the MESSENGER spacecraft completed 3,000 orbits of the planet Mercury and is about to get closer to the planet than ever before at an altitude of 199 km / 124 miles.
From the MESSENGER website:
“We are cutting through Mercury’s magnetic field in a different geometry, and that has shed new light on the energetic electron population,” said MESSENGER Project Scientist Ralph McNutt, of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md. “In addition, we are now spending more time closer to the planet in general — and that has, in turn, increased the opportunities for all of the remote sensing instruments to make higher-resolution observations of the planet.”
MESSENGER has been completing three orbits of Mercury every day since April 2012, when two orbit-correction maneuvers reduced its orbital period about Mercury from 12 hours to 8 hours. The shorter orbit has allowed the science team to explore new questions about Mercury’s composition, geological evolution, and environment that were raised by discoveries made during the first year of orbital operations.
APL’s Carolyn Ernst, the deputy instrument scientist for the Mercury Laser Altimeter (MLA), said the change from a 12- to an 8-hour orbit provided her team with 50% more altimetry tracks. “MLA coverage takes a long time to build up, and because of the small footprint of the laser, a lot of coverage is needed to obtain good spatial resolution. The more data we acquire, the better we resolve the topography of the planet,” she said. “The 8-hour orbit has also allowed us to make more MLA reflectivity measurements, which have provided critical clues for characterizing Mercury’s radar-bright deposits at high northern latitudes.”