Sending humans much farther than Mars will require scientific breakthroughs we cannot anticipate, but it is still worth researching right now, argues Ron Litchford, who oversees NASA’s propulsion portfolio as a technologist in the Space Technology Mission Directorate. “It’s not just dreaming,” he says. “Having a vision gives you some guidance and helps inform how to invest today.
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While the technical challenges of a human Mars mission are real, there is nevertheless a clarity to what it will require. For example, we do not know yet what type of propulsion we will actually use, but that is partly because we can probably do it with a number of technologies the space agency is already working on. “Between chemical, nuclear thermal, solar electric, and maybe even nuclear electric propulsion, we’ll find a way,” Litchford says.
After Mars, things get trickier. A human mission to the Red Planet needs a propulsion system capable of transporting a crew to a speed in excess of 10 kilometers per second, or roughly 4 kilometers per second more than a stable low-Earth orbit. Putting humans into Jupiter’s vicinity would require speeds more along the lines of 30 kilometers per second. For Neptune, it’s 200 kilometers a second.
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We simply cannot and likely never will be able to reach the outer planets with any of our current propulsion technologies—at least, not in an amount of time that would make sense for a human mission. Four or five years is probably the maximum we can keep astronauts in transit barring some breakthrough in how we shield them from exposure to cosmic radiation.
One path forward is to initially leave humans behind. NASA has already sent numerous missions to the outer solar system, and as remote robotic missions become more sophisticated, they could light the way to more distant human missions.
Philip Lubin, a physics professor at the University of California at Santa Barbara, is focused on what it will take to explore beyond the interstellar medium that exists between solar systems and to reach nearby stars, the closest of which are about 4 light-years away.
“If you want to achieve interstellar capability, then, in addition to the distance, you have to set a few other parameters,” Lubin says. “What timescale can you live with? You could use Voyager and figure it will take 100,000 years to get to the nearest star, if you aim in the right direction. Is that good enough? Most people would say no. So, how fast do you want to go? How much mass do you want to carry?”
By starting with tiny spacecraft and propelling them with directed energy—powerful laser beams, for instance—Lubin and his colleagues believe they already know how to achieve speeds of about a quarter of the speed of light and reach a nearby star in a human lifetime and possibly in as little as 20 years......🦘🦘
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