<P> The exploration of Neptune has only begun with one spacecraft, Voyager 2 in 1989 . Currently there are no approved future missions to visit the Neptunian system . NASA, ESA and also independent academic groups have proposed future scientific missions to visit Neptune . Some mission plans are still active, while others have been abandoned or put on hold . </P> <P> Neptune has also been scientifically studied from afar with telescopes, primarily since the mid 1990s . This includes the Hubble Space Telescope but most importantly ground - based telescopes using adaptive optics . </P>

When was neptune last visited by a spacecraft