| MadSci Network: Astronomy |
The answer depends upon your definition of "possible."
If you mean possible in the sense, Are there any physical laws that prevent it? Then the answer is, Yes. At least some initial prototypes of the Orion concept were even constructed. Of course, even though physical laws don't prevent the Orion concept (or other similar ones) from working, there are still huge technological challenges to be overcome. For instance, at these speeds, one might have to worry about the impact of interstellar dust on the spacecraft.
If you mean possible in the sense, Will one be built in my lifetime? Then I think the answer is, at best, Maybe. NASA has a Prometheus One project to send an unmanned nuclear-powered spacecraft to the Jovian moons, but that's not targetted to fly until 2015. Given that launch dates often slip, it could easily be after 2020 until this mission flies. (Presuming that it continues to be supported!) A crewed spacecraft could easily not occur for decades after this, and this is still only for travel within the solar system. I'm not aware of any similar plans by ESA.
Try the links in the MadSci Library for more information on Astronomy.