Since Mercury is swinging into view this month for one of the better evening apparitions of the year here is some information about this interesting planet.
    What we know about the planet closest to our sun comes from Earth-based observations, one fly-by mission, Mariner 10, and the recent first of three flybys of the MESSENGER spacecraft. The Mariner 10 spacecraft, launched in 1973 used a gravity assist from Venus to steer it into an orbit that took it around the sun and past Mercury three times between 1974 and 1975. The MESSENGER spacecraft has completed the first of three flybys that will help the spacecraft settle into an orbit around the planet by 2011. What we know about Mercury will be changing once that happens.
    Visit the MESSENGER web site for more information and images.
    The Mariner 10 mission included instruments designed to study Venus and Mercury's atmosphere, surface features, and physical characteristics. During the three fly-bys of Mercury the Mariner 10 spacecraft mapped approximately 50% of the planet surface (image on right) and discovered the presence of a very thin atmosphere and a magnetic field.

    Following the January 2008 flyby of Mercury by MESSENGER considerably more of the planet's surface has been imaged providing a look at unseen features. One particular striking crater, the Caloris Basin, about 1300 km in diameter, was only partially imaged by Mariner 10, but from the MESSENGER spacecraft we are now able to see this large crater in its entirety.
    With this new data the IAU, International Astronomical Union, has begun naming surface features on Mercury.Click on this thumbnail image to see a larger image showing the recently named features.