Astronomy Picture of the Day

Explanation: Two days ago, the MESSENGER spacecraft became only the second in human history to swoop past Mercury. The last spacecraft to visit the Sun's closest planet was Mariner 10 over 35 years ago. Mariner 10 was not able to photograph Mercury's entire surface, and the images it did send back raised many questions. Therefore, much about planet Mercury remains unknown. This week's MESSENGER flyby was only the first of three. Over the next few years MESSENGER will swing past Mercury twice more and finally orbit in 2011, but MESSENGER is currently moving too fast to enter orbit around Mercury now. This image was taken by MESSENGER two days ago upon approach to Mercury. Many detailed images of Mercury are expected to be sent back over the next few days. The data acquired by MESSENGER will help better understand how Mercury's surface was formed, and why it is so dense.