High radiation levels beyond Earth orbit pose the biggest challenge to human exploration of deep-space destinations, experts say.

With current spacecraft technology, astronauts can cruise through deep space for a maximum of one year or so before accumulating a dangerously high radiation dose, researchers say. As a result, many intriguing solar system targets remain off-limits to human exploration at the moment.

"There is an equivalent of a Mach 1 — a sound barrier — that exists, in terms of galactic cosmic radiation," Alvin Drew, manager of NASA's Deep Space Habitat Project, said Wednesday (Dec. 19) during a presentation with the agency's Future In-Space Operations working group.

"Until we solve that, we are still in the age of wooden ships and canvas sail for going out in space," added Drew, an astronaut who has flown on two space shuttle missions. "Until we get to a point where we are looking at steam engines and ships of iron, we may be very limited in how far we can go." [Future Visions of Human Spaceflight]

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