The possible first-ever evacuation of the International Space Station, if a Russian spacecraft is not launched in November, would risk the loss of the orbiting lab, a NASA official has warned.
"There is a greater risk of losing the ISS when it's unmanned than if it were manned," Michael Suffredini, the ISS program manager for the US space agency, said on a conference call with reporters.
"The risk increase is not insignificant," he added.
Russia on Monday delayed its next manned Soyuz spacecraft mission to the ISS by at least a month after an unmanned cargo vessel using a similar rocket crashed into Siberia instead of reaching orbit on August 24.
The station crew normally consists of six -- currently three Russians, two Americans and one Japanese -- working six-month rotations.
Neither NASA nor the Russian space agency will allow the astronauts aboard the space station to remain beyond a mandated six-month limit because of the risk posed by exposure to radiation.
And mid-November is considered the last chance to bring the Soyuz space capsule safely back to the steppes of Kazakhstan because of the lengthening night.
Crew safety and the "very big investment" that the Russian and US governments have made in the ISS would guide future decisions, Suffredini said during the call on Monday.
"We prefer not to operate in that condition without crew on board for an extended period of time," he said.
"But assuming the systems keep operating we can command the station from the ground and operate it on orbit indefinitely," he added.
Suffredini said some of the scientific experiments on board, however, would have to be suspended because they require the presence of astronauts.
Others, including the Alpha Magnetic Spectrometer, a sophisticated particle physics detector that searches for antimatter and dark matter, and which measures cosmic rays, could be operated from Earth.
Suffredini said he was optimistic the Russians would be able to determine why the upper stage of the rocket launching the supply capsule -- which is the same used for the manned Soyuz -- malfunctioned, and would be able to send up another crew of astronauts to relieve those on board the space station.
The ISS, which orbits 350 kilometers (220 miles) above Earth, is a platform for scientific experiments bringing together space agencies from Russia, the United States, Europe, Japan, and Canada.
Launched in 1998, it was initially expected to remain in space for 15 years until an agreement was reached to keep it operating through 2020.
An evacuation of the ISS was planned after the Columbia shuttle disaster killed seven astronauts in 2003, but NASA later decided to keep staff on board the station at all times.
Russian officials expect the next manned launch of a Soyuz craft to take place in late October or early November -- it had initially been scheduled for September 22.
A crew comprising Russians Andrei Borisenko and Alexander Samokutyaev and NASA astronaut Ron Garan went up to the ISS in March to honour the 50th anniversary of the first voyage of space pioneer Yuri Gagarin.
But this month's failed launch was a spectacular blow for Russia after it had become the sole nation capable of taking humans to the ISS following the July retirement of the US space shuttle program.
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