On April 18, 2014, NASA's Lunar Atmosphere and Dust Environment Explorer, also known as LADEE, ended its mission by crashing into the moon. LADEE had spent seven months orbiting the moon and studying its exosphere. The exosphere is a thin layer of gas that's kind of like an atmosphere, but the molecules are more spread out and are definitely not breathable. LADEE conducted a lunar dust experiment in which it collected and analyzed the dust particles floating around in the exosphere. This experiment was intended to help NASA solve the mystery behind the faint glow Apollo astronauts reported seeing on the lunar horizon. LADEE discovered neon in the exosphere, but it wasn't enough to account for the glow, and the mystery remains unsolved. When LADEE ran out of fuel, NASA intentionally crashed it into into the lunar surface. It was traveling 3,600 miles per hour during the crash and created a crater almost 10 feet wide.