As I have mentioned before, there is a fusion reactor, roughly spherical, 5 feet in diameter being developed, funding by the U.S. Navy, that when and if the Navy stops being so squeemish and gives them the $200 million they need instead of a couple of million at a time, and the thing works as expected, it would have a great impact on the world economy, be able to get people to Mars before they go stir crazy or succumb to radiation, and generate ample power on the ground once they get there, making it a lot easier to stay a while. If it ends up not working after a fair test, okay--at least we tried. But what people have said would prevent it from working has already been proved wrong in tests of scaled-down prototypes. So it looks promising.
But an interesting thing about this Polywell system is that, even though it's thought of as a future reactor for civilian use, by its very nature it can be configured as a fusion rocket with little modification. The Navy's interest, of course, is in powering ships with fusion, which apparantly is not an urgent goal.
If that happens, we could be on Mars actually doing stuff a lot sooner than you think.
But with chemical rockets, don't hold your breath.
With a VASIMR sytem, it's a bit hard but doable. It would just take a lot of money and development of a speciallized fission reactor for that use, since we have none with that sort of configuration on the shelf. If there happened to be something like that already available, then VASIMR would look very attractive but still need quite a bit of funding just for a stunt Mars mission.
Another thing some people are looking at for Mars missions is positron rocketry. One company is working on that, there is an article about it on NASA's Web site, and it recently got a lot easier to produce positrons in quantity. However, storage bottles for that have not yet been developed, and virtually the only use for them would be in space travel.
