What happens when a fire starts in space?

Until Tuesday, NASA didn't really know — and for a good reason. Scientists and engineers spend years of their lives and millions of dollars building the highly complex, gravity-defying pieces of machinery we send into space. You'd be hard-pressed to find one willing to let you start a fire on their ship, just to see what might happen.

"We tried for years, trying to find a vehicle and a circumstance where this would work," NASA's David Urban said in a video. "Initially they had a 'not on my spacecraft' reaction."

It took some negotiating, but Urban and his colleagues worked out a solution they'd all be happy with: conducting the experiment on a trash-filled Cygnus resupply vehicle making its return trip from the International Space Station. The Cygnus was going to burn up anyway on its reentry into Earth's atmosphere. Why not get a little extra science out of the deal?

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So at 4:55 p.m., Tuesday, inside an insulated container on a ship miles above the Earth, NASA began the largest intentional fire ever set in space.

The capsule that contained the experiment held a video camera and other sensors, as well as an exhaust system to keep air moving through to feed the flames. With a radio signal from scientists at NASA's Glenn Research Center in Cleveland, a hot wire touched a towel-sized card of woven cotton and fiberglass. Within seconds, there was fire.

Gary Ruff, project manager of the experiment dubbed "Saffire I" (a portmanteau of "safe" and "fire"), watched closely as data streamed down to his desk at NASA Glenn. By the morning after the fire, he could tell that it took eight minutes for the material to burn — a lot longer than it would have taken on Earth. He also knew that the Cygnus's smoke alarm had gone off — good news for spacecraft fire safety.

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Saffire I was the first of three experiments aimed at understanding how fire behaves in microgravity.

"We want to know how big can a fire get, how rapidly does it grow, how rapidly do the conditions inside the vehicle change?" Ruff said. That knowledge will help NASA refine its computational models for space fires, choose better materials for building spacecraft and develop procedures for fighting fire.

These concerns are becoming more and more pressing, Ruff added, as NASA gets closer and closer to its first long-distance manned mission in space: a trip to Mars. The years-long trip offers plenty of opportunity for something to go wrong, and astronauts need to be prepared to fight any fires, literal and otherwise.

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Smaller experiments conducted at space stations — and a few larger, unintentional ones — have shown that fire can be unpredictable and dangerously tenacious in the absence of gravity. The worst accident happened in 1990, when an oxygen generator on board the Russian Mir space station malfunctioned and was set ablaze.

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"The fire spewed with angry intensity, sparks – resembling an entire box of sparklers ignited simultaneously – extended a foot or so beyond the flame’s furthest edge," Jerry Linenger, an American astronaut on board the space station at the time, wrote in his book "Off the Planet." "Beyond the sparks, I saw what appeared to be melting wax splattering on the bulkhead opposite the blaze. But it was not melting max. It was molten metal. The fire was so hot that it was melting metal."

The fire eventually consumed the entire oxygen canister and burned itself out, but it raised the temperature inside the space station to 100 degrees and filled the air with smoke. Linenger recalled wanting to open a window, then realizing that wasn't an option. It was only by quickly donning oxygen masks that the crew was able avoid suffering from smoke inhalation.

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The Cygnus will remain in orbit while the rest of data from the experiment — including video of the burn — is downloaded to NASA computers on Earth. When the data transfer is complete, sometime next week, the ship will begin its descent back to Earth. It, the Saffire capsule and the 4,000 or so pounds of trash it's carrying from the ISS will burn up for good somewhere over the Pacific Ocean.

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