Voyager 2 generated this computer image of Neptune as seen from its moon, Triton. Triton is Neptune's largest satellite and is the only moon in the solar system to orbit opposite of its planet. Experts believe that Triton may have been a large comet that orbited the sun but got caught in Neptune's gravitational pull. Triton boasts the coldest known temperatures in the solar system, at minus 390 degrees F (that's minus 235 degrees C). NASA has discovered evidence of ammonia and water volcanoes on Triton.