August 24th marks the 25th anniversary of one of NASA's greatest achievements: the first flyby of Neptune. At, 2.8 billion miles from the sun, it’s the farthest planet ever to pose for a close-up.
You can thank the Voyager 2 spacecraft for the close encounter. Launched in 1977, Voyager 2 sped past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. Voyager discovered that a large dark spot marked the face of the giant blue planet. It also quadrupled the number of Neptune's known moons from two to eight. New discoveries have since raised that number to 14.
The largest and most intriguing moon is Triton. Voyager discovered that, like Earth, Triton has an atmosphere, mostly of nitrogen. The spacecraft also found geysers erupting material that rises five miles above Triton's surface.
Today, 25 years later, Neptune remains the farthest planet to be visited by a spacecraft. But its record will soon be shattered. In July 2015, NASA’s New Horizons mission zips close to Pluto, which will make that distant world the farthest planet yet visited—for those intrepid individuals who still consider Pluto a planet.
—written by Ken Croswell, voiced by Steve Mirsky