Speaking at an Aerojet Rocketdyne plant, NASA administrator Charles Bolden said the program is looking into advanced propulsion technologies that can cut the current eight-month journey to Mars "in half." Technologies such as solar-electric propulsion are definitely in the cards, but NASA may look towards more unconventional solutions such as nuclear rockets as well.

Over the past few years, there's been a lot of attention on getting astronauts to Mars, mostly fuelled by crazy projects like Mars One, the success of the Curiosity rover, and heavyweights like Elon Musk saying he wants to colonise the planet.

The main problem with getting humans to Mars is that, with our current liquid-fuelled rocket engines, it takes a very long time to get there; about eight months or so. If we can cut the journey in half, we significantly reduce the amount of food and water needed—which in turn cuts down the weight of the spacecraft, which in turn reduces the amount of fuel needed, which in turn feeds a very positive feedback loop. Less time in outer space means astronauts will be bombarded by less radiation too.

Finding a propulsion technology that's better than liquid fuel, though, has proven difficult. NASA has been looking at a variety of different technologies for decades. An In-Space Propulsion roadmap (PDF) from 2010 lists no less than 41 different propulsion methods. One of the most promising propulsion techniques, at least in the short term, is solar-electric propulsion—gathering solar energy with photovoltaic cells, which then powers some kind of electric engine like a Hall effect ion thruster.

Aerojet Rocketdyne recently won a NASA contract to develop Hall effect ion thrusters. The main benefit of solar-electric propulsion (SEP) technologies such as ion thrusters is that the energy source (the Sun) lasts for a very long time, while liquid-fuelled rockets have a very finite duration. SEPs aren't quite ready to send humans to Mars, though. "The limiting power of this type of propulsion has been the power to drive it," Bolden said, according to Space.com's account of Bolden's visit to the Aerojet facility. "Aerojet Rocketdyne has partnered with different entities around the country in looking [at] how to get more energy density onto a solar cell. The more power we can get, the larger we can make the engine and its capability."

According to Space.com, Bolden also mentioned the possibility of using thermal nuclear rockets: rockets that use a nuclear reactor to heat gas, which then expands through the nozzle to create thrust. NASA did a lot of work on nuclear rockets with the NERVA program in the '50s and '60s, but it was eventually cancelled in 1972.

Bolden wants NASA to put more money into in-space propulsion technologies, noting that they could be "game changers." Bolden also stressed that he doesn't want NASA's rocket partners to fixate on moving cargo more quickly through space. "I want industry to focus on getting people to move really fast. I think we can do far better than we are doing today, but we've got to show our commitment by putting some money into it."