... is going to fly by the planet Mercury later today (at 19:04:39 Greenwich time). The MESSENGER spacecraft is making the first of three gravity-assist passes by the planet, modifying its orbit around the sun before making a final manouevre to achieve orbit of Mercury in 2011. In the course of these flying visits, the probe will be mapping the planet and making other studies to prepare for the eventual orbital mission.
Mercury has only been visited by one other mission --- three flying passes by the Mariner 10 probe over thirty years ago --- and placing a modern probe in orbit will dramatically increase our knowledge of the solar system's smallest planet.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. (Image taken on 12th January, 1.2 million kilometres from the planet: details are available at the MESSENGER web site.)