Mercury is the closest planet to the sun. It is an airless ball of rock less than half the size of Earth. It is difficult to see because it never strays far from the sun. The sun does rise and set on Mercury, but very slowly. One "day" on Mercury lasts 176 Earth days. We can’t see many markings on Mercury even through a large telescope. Mercury looks a lot like our moon. It is only 40 percent bigger than our moon. Mercury’s surface is covered with craters up to 250 miles across. In the daytime Mercury is hot enough to melt tin and led (750 degrees). But by night it gets to a very cold –275 degrees. Mercury is a hostel planet and no one is likely to visit it in a long time, if ever.