Outbound from Mercury . ... Credit Copyright: Mariner 10 , NASA Explanation: After just passing Mercury, the robot spacecraft Mariner 10 looked back. ... Mercury , the closest planet to the Sun, is heavily cratered much like Earth's Moon . As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius in the half facing away from the Sun, to an unbearably hot 400 degrees Celsius in the half facing toward the Sun. ... NASA Official: Jay Norris. ...