NASA has successfully maneuvered a craft called Messenger into orbit around Mercury, no small feat because of the gravitational pull of the sun.
Mercury is not only difficult to get to, but it’s has some of the most extremes in the solar system. Temperatures there swing wildly by 1,100 degrees. While it gets up to 800 degrees on the planet closest to the sun, it also is so cold and dark in some craters that the temperatures don’t get above 300 degrees below zero. Radar even shows that there is likely frozen ice in those craters, something Messenger will try to confirm.