On April 12, 1961, the cosmonaut Yuri Gagarin became the first human to enter space. Upon seeing our planet from his spacecraft he is reported to have said, "I see Earth! It is so beautiful."
More than 57 years on from that momentous flight, space travel continues to generate feelings of excitement, wonder and awe.
Today, the idea of sending humans to Mars is not such a far-fetched proposition. Elon Musk's SpaceX, for example, says it has an "aspirational goal" to send a cargo mission to the Red Planet in 2022. A second mission, carrying both cargo and crew, is being targeted for 2024.
For its part NASA, together with the Department of Energy's National Nuclear Security Administration, has been working on a nuclear reactor power system that it says could "enable long-duration crewed missions to the Moon, Mars and destinations beyond."
A small, lightweight fission system dubbed the "Kilopower," it can provide as much as 10 kilowatts of electrical power. This, NASA says, is "enough to run several average households," continuously, for "at least 10 years."
Lee Mason is NASA's principal technologist for power and energy storage. He explained to CNBC's Sustainable Energy why it was so important to develop new sources of power for spaceflight.
"Most of our current spacecraft are powered by solar arrays and batteries which depend, obviously, on sunlight," he said.
"But we want to go to missions in which there is no sunlight available – permanently shadowed craters on the Moon, the northern latitudes of Mars where sunlight is very limited," he added. "It's in those applications that we see Kilopower really fitting well."
In May 2018, NASA said that it had successfully demonstrated the system, with its Kilopower Reactor Using Stirling Technology (KRUSTY) experiment ending with a 28-hour full power test.
That test, according to NASA, simulated a mission and included a reactor start-up, "ramp" to full power, steady operation and shut down.
"The test demonstrated that the reactor could do exactly what it needs to do on a mission, as far as operating at the power levels that we need it to, staying stable during all the operations and then being able to supply all the power that's needed for specific missions that come up in the future," Mark Gibson, Kilopower's lead engineer, said.