How much would it cost to send a manned mission to Neptune before the end of the 21st century? About four trillion dollars. That’s the estimate of a study examining the question of manned missions to the outer planets (PDF), which used Neptune as a case study. The basic premise is to send six people, in living quarters roughly the size of the International Space Station, to and from Neptune within five years — it can’t be longer because of cosmic radiation, and then you’d have to add more shielding (to say nothing of additional consumables). Essentially, interplanetary travel much beyond Mars and the asteroids is pretty much a non-starter barring the development of ridiculously efficient methods of propulsion and preposterously lightweight radiation shielding. Via io9. (Image credit: Voyager 2 image of Neptune, NASA/JPL.)