What’s the big deal about another photo of the moon? The big deal is it’s not the moon. It is Mercury. The Messenger spacecraft, launched in 2004, has reached Mercury and will make a number of close passes before entering orbit around the innermost planet. The first flyby occurred last week. The image here was taken on January 18, 2008 from a distance of about 27,000 kilometres.

My understanding is the flybys are required to slow the spacecraft because it’s going too fast to enter Mercury’s orbit. The flybys and the orbital insertion will leave Messenger a mere 200 kilometres above the surface. According to the mission project page, the second flyby will be later this year in October, the third in September 2009, and the spacecraft will finally settle into orbit in March 2011.

The spacecraft is moving at a good clip, too. The image was taken at a distance 27,000 kilometres, 80 minutes before the first close approach, which is just 200 kilometres above Mercury’s surface! I figure this indicates a speed of approximately 20,000 kilometres per hour.

Messenger is only the second spacecraft to fly inside Mercury’s orbit. The first was Mariner 10 in 1974. Mariner 10 managed to map less than half of Mercury’s surface. Messenger will map the whole planet with far better imaging equipment, from an orbit 1/3 closer than Mariner 10’s closest approach.

Mercury image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington