Dreaming of a journey to the outer solar system? A group of physicists at Johns Hopkins have the same dream, and have published a report on what's required to send humans to Neptune. Hint: Lots of cash and magnetoplasmadynamic thrusters.
Led by Ralph McNutt, the group speculates about how the propulsion system would work, as well as how to get there. Below, you can see the scale of the potential spacecraft. Ars Technica explains:
Their travel plan to Neptune, for example, was designed with a 5-year total mission duration to limit the radiation exposure. That would take us to Neptune in less than two years, which means going significantly faster than anything we have ever launched. For example, the New Horizons probe currently en route to Pluto-the fastest man-made object in space-has reached the distance of 16.74 AUs from the sun as of this writing. That took about 4.5 years, and it's only a little more than halfway to Neptune's orbit at 30 AU.
To design the mission, the scientists also gave considerations to the size of the living space in which the astronauts would spend 5 years of their lives. They have compared the sizes of past large space habitats such as the Skylab, the Russian Mir, and the International Space Station to determine that approximately 200 cubic meters per person of habitable volume makes for a healthy living space. If the habitable space is spherical, a crew of 10 would need a 7.8 meter radius, or about twice the size of Discovery 1 in 2001: A Space Odyssey.
Not surprisingly, this dream voyage is not cheap. Based on the costs of historic large technological endeavors, the scientists project that a mission to Neptune would cost roughly 4 trillion dollars. To put this number in perspective, in 2009, the GDP of the US was about 14 trillion dollars, and the US federal budget was about 3.1 trillion dollars, of which NASA received only $17.6 billion. Due to the extreme financial cost associated with the plan, the authors conclude that, for the mission to materialize at all, it would have to be a large-scale international collaborative effort.
You can read McNutt et. al.'s paper here (it's a PDF), and find out more about the voyage our descendants might someday take to the outer system.