Pluto is about two-thirds the diameter of Earth's Moon and probably has a rocky core surrounded by a mantle of water ice. More exotic ices like methane and nitrogen frost coat its surface. Owing to its size and lower density, Pluto's mass is about one-sixth that of Earth's Moon. Pluto's 248-year-long elliptical orbit can take it as far as 49.3 astronomical units (AU) from the sun. Pluto has five known moons, the largest being Charon, about half the size of Pluto forming a fine binary system. NASA's space probe New Horizons flew by on July 14, 2015 as close as 12,500km. Icy mountains as tall as 3500 meters were detected on the equator region. Mission scientists have found Pluto to be 1,473 miles (2,370 kilometers) in diameter, somewhat larger than many prior estimates. Further, a tenuous atmosphere mostly nitrogen extends up to 1600 meters above the surface. Pluto and Charon are locked in a gravitational resonance where not only does Charon keep the same face to Pluto (just like Earth's moon faces Earth) but also, Pluto always sees the same face of Charon. Pluto's largest moon Charon has a chasm running 4-6 miles deep. The little outermost known moon Hydra has a surface primarily composed of water-ice.