Messenger reveals Mercury’s previously unseen face

Monday of this week saw NASA’s Mercury Messenger spacecraft make its first flyby of the planet. This is the first in a series of three passes required to dump orbital energy before the probe can enter Mercury orbit in 2011.

That’s the thing about spacecraft trajectories; you cannot simply go from A to B. Or at least not without a lot more fuel than can be carried onboard. Add to that the problem that B can have moved a rather long way since you left A. Travel to the outer planets and you need to gain energy; to move into the inner solar system requires that you get rid of energy due to the Earth moving around the sun at around 30 kilometres per second.

On Monday, the Messenger spacecraft came to within 200 km of Mercury’s surface, and the onboard instruments made many valuable scientific measurements during the flyby. I include here two visual light photographs taken with wide- and narrow-angle cameras. Click on the images below for higher resolution popup windows.

Mercury Messenger image from January 2008, of the side of the planet hidden to the 1970s probe Mariner 10 Narrow-angle image from Mercury Messenger, taken just under an hour before closest approach in January 2008

Neither of these images is from the point of closest approach. The wide-angle photo on the left was taken with the spacecraft 27,000 km from the planet, about 80 minutes after closest approach. When NASA’s first inner solar system probe Mariner 10 flew past Mercury three times in 1974 and 1975, the same hemisphere was sunlit during each encounter, and the spacecraft was able to image less than half the planet. This latest image from Mercury Messenger shows the side of the planet previously hidden from view.

The narrow-angle image on the right was acquired when Messenger was about 18,000 km from the planet, 55 minutes before closest approach. In the image can be seen smooth plains at the centre, a number of impact craters, and the 200-km wide Sholem Aleichem crater at the bottom right. This feature was named after the renowned Yiddish writer and humourist who died in 1916. The shadowed area on the right of the image is the terminator, or day-night boundary.

Check out the Mercury Messenger website at Johns Hopkins University in Baltimore for more details of the mission, and an archive of similar images and other data. Planetary scientists and space systems engineers at Johns Hopkins are responsible for the science payload on the Messenger spacecraft. BBC News also has some explanatory text worth a read.