With their primary oxygen source broken and another running out, the two astronauts aboard the International Space Station (ISS) will ignite their first oxygen-generating candles Friday to test a reserve air supply, a NASA spokesperson said.
ISS Expedition 11 commander Sergei Krikalev and flight engineer John Phillips will burn two potassium perchlorate canisters, or "candles," before switching over to the reserve oxygen supply system full-time next week.
"They wouldn't need to use two candles tomorrow, but they're going to test them," NASA spokesperson Kylie Clem told SPACE.com Thursday, adding that the two astronauts are in no danger of running out of oxygen before the June arrival of a fresh supply ship.
On May 19, Krikalev and Phillips repressurized the ISS interior with oxygen from tanks aboard the docked Russian cargo ship Progress 17, but those supplies should be depleted over the weekend, NASA officials said.
The space station's primary source of oxygen, the Russian-built Elektron generator, failed for good on May 13. A finicky device nearing the end of its service life, the Elektron generator uses electrolysis to strip oxygen and hydrogen from water. The oxygen is used to sustain astronauts while the hydrogen is dumped overboard.
Several of the last ISS crews worked with the Elektron to prolong its operation, including boosting power to the unit to keep the oxygen flowing.
In addition to the Elektron, Progress 17 oxygen, and potassium perchlorate canisters, a 100-day air supply is also available inside two high-pressure tanks attached to the U.S.-built Quest airlock, NASA officials said.
While NASA has stated that the Expedition 11 crew has 84 oxygen canisters onboard - a 42-day supply - Clem said there are actually more, but station engineers factored in the possibility of low-functioning or defective canisters in their final estimate.
"One candle is one day's worth of oxygen for one crewmember," Clem said, adding that past ISS crews have burned the oxygen-generating canisters.
In 2004, Expedition 8 commander Michael Foale and flight engineer Alexander Kaleri burned the canisters for oxygen while working to pin down an air leak that was traced to a flexible hose.
During today's test, the Expedition 11 crew will burn two potassium perchloratecanisters, one after the other, inside a device that fires an igniter to begin the process. Carbon fiber filters absorb non-oxygen impurities and a fan distributes the oxygen into the ISS, Clem said.
A single canister can take between five and 20 minutes to burn completely, NASA officials added.
A fresh load of oxygen and other supplies are set to launch aboard the Progress 18 spacecraft on June 16 and dock at the ISS two days later. Replacement parts for the broken Elektron will not reach the space station until August, when an additional Progress cargo ship will make another unmanned delivery.
Krikalev and Phillips are now one month into a six-month stay aboard the ISS. They launched toward the space station aboard their Soyuz TMA-6 spacecraft on April 14 in a space shot staged from Baikonur Cosmodrome in Kazakhstan.
The two astronauts are the fifth two-person crew to the orbital laboratory and hope to receive NASA's first space shuttle crew to fly since the Columbia disaster in July. A second shuttle mission is slated to dock at the ISS in September, delivering astronaut Thomas Reiter of the European Space Agency, before Krikalev and Phillips return to Earth on Oct. 7.
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