An artist's impression of a gamma-ray burst, a powerful jet of energy lasting from less than a second to several minutes. The most powerful events in the universe, they are thought to be mostly associated with the explosion of stars that collapse into black holes. A. Roquette/ESO hide caption

itoggle caption A. Roquette/ESO

The surface of Mercury is seen in a picture taken by NASA's Messenger spacecraft as it approached the planet during a fly-by on January 14, 2008. The image was taken about 11,000 miles from the planet and shows a region 300 miles across. NASA/Getty Images hide caption

itoggle caption NASA/Getty Images