In recent years, we have established that Mars has plenty of water to support human activities on the planet. That's a huge boost to those who argue for extending human civilization to the Red Planet. Actually getting people there, however, is still a problem. With current propulsion technology, flying people to Mars would take several months. That would expose the crew to a dose of radiation that could have extremely serious long term medical implications. The radiation threat, indeed, might be the single biggest obstacle to interplanetary spaceflight by humans.
One way to deal with the radiation issue would be to cut down on mission flight time. That's where the work of physicist and former astronaut Franklin Chang-Diaz comes in. He has developed what he calls the VASMIR, a propulsion system that could cut a one way flight between Earth and Mars to 40 days. Conceivably, that could mean a full mission to Mars could be completed in less than a year, which would make Mars exploration less daunting on many levels.
The catch? For VASMIR to have the power required to reach that 40 day level the ship would need to carry a nuclear reactor. NASA has always shied from full bloom nuclear power because the American people have been uncomfortable with it. However, if we are serious about flying deep space missions, nuclear fission, and eventually onboard nuclear fusion, will have to be in the technology mix.