Mercury in retrograde © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Today we"re traveling to outer space to catch a glimpse of the Caloris Basin on the planet Mercury. This small planet—about the size of Earth"s moon—is riddled with craters, but none as spectacular as the Caloris Basin. One of the largest impact craters in the solar system, Caloris is about 950 miles across and ringed by mile-high mountains.