Nasa administrator Charles Bolden has claimed that nuclear thermal propulsion is the "most effective" way of sending humans to Mars.
"We are on a journey to Mars and most people believe that, in the end, nuclear thermal propulsion will be the most effective form of propulsion to get there," he told Congress.
Bolden also praised Alabama's Marshall Space Flight Centre, a nuclear thermal propulsion centre that has been developing the technology for years. "Marshall is the place that people go when they want to discuss in-space and leaving-the-Earth propulsion issues," he said.
Although nuclear-powered engines are smaller, they're much more powerful than conventional rockets. Fluid, usually hydrogen, is heated to incredibly high temperatures to create plasma, which is then channelled through a nozzle to create thrust. But because temperatures can reach 2000°C, Nasa has struggled to find materials that can withstand the journey.
Earlier this month Russia announced that it would be testing its own nuclear engine for deep space travel in 2018. Experts working on the Russian project believe the new technology could take humans to mars in just six weeks. By comparison, today's engines take up to 18 months to reach Mars.
Getting humans to Mars is a major focus for Nasa over the coming decades. The space agency has been growing vegetables in Martian soil, tested a number of different rocket systems and has recently launched InSight, a mission to explore the planet that could provide the strongest evidence yet for life on Mars.