Breathing Easy on the Space Station

"Eventually, it would be great if we could use plants to (produce oxygen) for us," said Monsi Roman, chief microbiologist for the ECLSS project at MSFC. "The byproduct of plants doing this for us is food."

However, "the chemical-mechanical systems are much more compact, less labour intensive, and more reliable than a plant-based system," Perry noted. "A plant-based life support system design is presently at the basic research and demonstration stage of maturity and there are a myriad of challenges that must be overcome to make it viable."

Hydrogen that's leftover from splitting water will be vented into space, at least at first. NASA engineers have left room in the ECLSS hardware racks for a machine that combines the hydrogen with excess carbon dioxide from the air in a chemical reaction that produces water and methane. The water would help replace the water used to make oxygen, and the methane would be vented to space.

"We're looking to close the loop completely, where everything will be (re)used," Roman said. Various uses for the methane are being considered, including expelling it to help provide the thrust necessary to maintain the Space Station's orbit.

At present, "all of the venting that goes overboard is designed to be non-propulsive," Perry said.

NASA The ISS's first crew -- Bill Shepherd, Sergei Krikalev and Yuri Gidzenko -- aboard the Space Station.

The ISS will also have large tanks of compressed oxygen mounted on the outside of the airlock module. These tanks will be the primary supply of oxygen for the U.S. segment of the ISS until the main life support systems arrive with Node 3 in 2005. After that, the tanks will serve as a backup oxygen supply.

For example, while the crew were waiting for activation of a water electrolysis machine on the Zvezda Service Module, they breathed oxygen from "perchlorate candles," which produce O₂ via chemical reactions inside a metal canister.

"You've got a metallic canister with this material (perchlorate) packed inside it," Perry explained. "They shove this canister into a reactor and then pull an igniter pin. Once the reaction starts, it continues to burn until it's all used." Each canister releases enough oxygen for one person for one day.

"It's really the same technology that's used in commercial aircraft," he continued. "When the oxygen mask drops down, they say to yank on it, which actuates the igniter pin. That's why you have to give it a tug to begin the flow of oxygen."

Keeping the air "clean"

At present, carbon dioxide is removed from the air by a machine on the Zvezda Service Module based on a material called "zeolite," which acts as a molecular sieve, according to Jim Knox, a carbon dioxide control specialist at MSFC.