The MESSENGER probe makes its first flyby of Mercury on Monday. The first probe to visit Mercury since Mariner 10 in 1974 and 1975, MESSENGER will make two more passes of the innermost planet before settling into orbit in 2011. What makes this probe particularly interesting is that it’ll map the half of Mercury that Mariner 10 missed — 33 years later, and we still don’t know what half the planet looks like. Here’s a good article from the Planetary Society.