<P> From Earth, Neptune goes through apparent retrograde motion every 367 days, resulting in a looping motion against the background stars during each opposition . These loops carried it close to the 1846 discovery coordinates in April and July 2010 and again in October and November 2011 . </P> <P> Observation of Neptune in the radio - frequency band shows that it is a source of both continuous emission and irregular bursts . Both sources are thought to originate from its rotating magnetic field . In the infrared part of the spectrum, Neptune's storms appear bright against the cooler background, allowing the size and shape of these features to be readily tracked . </P> <P> Voyager 2 is the only spacecraft that has visited Neptune . The spacecraft's closest approach to the planet occurred on 25 August 1989 . Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for Voyager 1's encounter with Saturn and its moon Titan . The images relayed back to Earth from Voyager 2 became the basis of a 1989 PBS all - night program, Neptune All Night . </P> <P> During the encounter, signals from the spacecraft required 246 minutes to reach Earth . Hence, for the most part, Voyager 2's mission relied on preloaded commands for the Neptune encounter . The spacecraft performed a near - encounter with the moon Nereid before it came within 4,400 km of Neptune's atmosphere on 25 August, then passed close to the planet's largest moon Triton later the same day . </P>

What is the composition of the atmosphere on neptune