Neptune's atmosphere is primarily made up of helium, methane and hydrogen, making it unsuitable for sustaining human life. Because Neptune is a gas giant, it has no surface, which means there is no solid ground to land on. Neptune is also dark, extremely cold and is subjected to supersonic winds.
As of 2014, there has only been a single mission to Neptune, which was flown by the unmanned Voyager 2 spacecraft. Although there has been no manned missions to Neptune, astronomers speculate that a human may be able to at least survive a fly-by of Neptune in a secure spacecraft.