Cape Canaveral, Fla. ? An overheated Russian oxygen generator caused a momentary fire scare aboard the international space station Monday.

Flight controllers briefly declared an emergency and told the station’s three crewmembers to trip a smoke alarm after the astronauts smelled fumes from a melted rubber gasket. The crew never donned gas masks but instead put on surgical masks, goggles and gloves as a precaution against the possible release of chemicals from the oxygen unit.

A small amount of potassium hydroxide, a mildly toxic substance that can irritate the skin and eyes, is suspected of having leaked from a vent on the generator. The astronauts cleaned up the clear liquid and sealed it in an airtight bag.

Mission managers later said the crew was never in any danger.

“There was never any smoke,” said Jeff Williams, a NASA astronaut who serves as the station’s flight engineer. “There was a smell and it was perhaps wrongly assumed to be a fire initially.”

The biggest remaining question concerns whether and when the Russian Elektron oxygen generation unit will be back in service. Work on the device will continue today.

This photo of the international space station was taken shortly after the space shuttle Atlantis undocked on Sunday. New solar panels installed by the Atlantis crew are at left. An oxygen generator on the international space station overheated and spilled a toxic irritant on Monday, forcing the three-man crew to don masks and gloves in the first emergency ever declared aboard the 8-year-old orbiting outpost.

The units have a history of problems on this station as well as a prior Russian outpost. The Elektron apparently overheated Monday morning about a half-hour after it was activated.

The station is stocked with plenty of oxygen candles, which are backup canisters of lithium perchlorate that produce breathable air when ignited. More than enough of the candles are available to support the station’s three current residents along with a trio of other astronauts who are on the way there.

A Russian Soyuz rocket carrying NASA astronaut Michael Lopez-Alegria, Russian cosmonaut Mikhail Turin and American space tourist Anousheh Ansari lifted off from the Baikonur Cosmodrome in Kazakhstan at 11:09 p.m. CDT Sunday. Turin and Lopez-Alegria will replace Williams and cosmonaut Pavel Vinogradov, who have lived on the station since March 30.

The new crew’s Soyuz capsule is scheduled to dock with the outpost at 12:24 a.m. CDT Wednesday. Ansari, the first woman to pay for a flight to orbit, will return to Earth with Williams and Vinogradov on Sept. 28.