The crew members, who are healthy and not in any danger, will stay on the station for several additional months because the Russian Soyuz capsule that carried them into space was damaged by a micrometeoroid roughly 1 millimeter in diameter. Their vehicle was struck after they arrived, when the spacecraft was essentially parked at the station some 250 miles above Earth.
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"Space is not a safe place and not a safe environment," said Sergei Krikalev, executive director of Human Space Flight Programs for the Russian space agency Roscosmos. "That's why before every flight, before we actually start to fly out to the station, we have several teams working together developing emergency procedures. We were thinking about this scenario even before we started to fly."
Still, this will be the first time in 60 years of human spaceflight that a replacement spacecraft is launched to bring crew members home.
The damage was first detected and reported on Dec. 14. Cosmonauts Sergey Prokopyev and Dmitri Petelin were preparing for a spacewalk when ground teams noticed coolant leaking from the Soyuz spacecraft. Their spacewalk was postponed as NASA and Roscosmos investigated the leak.
After weeks of scrutiny, officials determined the missing coolant would make the ride home uncomfortably hot — crew members could experience temperatures in the high 90s or over 100 degrees Fahrenheit — and humid.
Russia is planning to launch an empty Soyuz in late February. The space station crew will move hardware, such as seat liners and spacesuits that are customized for each crew member, from the damaged spacecraft to the new spacecraft. The damaged spacecraft will then return to Earth, landing in Kazakhstan as planned, to bring home experiments and equipment.
Its return will also be an opportunity to collect data, should this scenario arise again in the future.
"We'll be taking some temperature measurements to measure how the vehicle does," said Joel Montalbano, NASA's International Space Station program manager. "We're going to fully use this vehicle all the way until it lands."
The space station is well protected and can generally withstand being hit by objects up to 1 centimeter. It's moved away from larger items — and there are plenty of these in space.
It's estimated that there are about 36,000 items circling the globe that measure at least 10 centimeters in diameter. There are 1 million items between 1 centimeter and 10 centimeters, and there are as many as 130 million items between 1 millimeter and 1 centimeter, said John Crassidis, an aerospace engineering professor who studies space debris at the University at Buffalo, State University of New York.
These items include naturally occurring micrometeoroids (which are often fragments from an asteroid), operational satellites and defunct satellites. There is also orbital debris caused as humans launch rockets, missiles destroy satellites in space and defunct satellites explode or collide into one another.
NASA and Roscosmos believe the Soyuz was damaged by a micrometeoroid rather than space debris. But Crassidis said it's hard to definitively say the damage was not caused by debris, partly because it's impossible to track items of 1 millimeter.
Krikalev, with Roscosmos, said the strike was an "unlikely hit." He also said the governments are planning for other unlikely scenarios. For instance, how would the crew get home if an emergency forced them to evacuate the station before the replacement Soyuz arrives?
In this extreme case, the damaged Soyuz could be used to return crew home. NASA is also talking with SpaceX about using the Crew Dragon to potentially bring home one (maybe more) of the Soyuz crew members, which could help reduce temperatures for those traveling in the Soyuz.
Emergencies do happen in human spaceflight. On the Apollo 13 mission in 1970, an oxygen tank exploded and forced crew members to take refuge in the self-sustaining lunar module as they swung around the moon to return home. Three cosmonauts suffocated a year later as their capsule returned from a Soviet Union space station during the Soyuz 11 mission. Space shuttle disasters in 1986 and 2003 killed a combined 14 people.
But the current plan put together by NASA and Roscosmos highlights the robust transportation system that has been created for low-Earth orbit, said John Logsdon, founder of George Washington University's Space Policy Institute. He said it's good to have multiple systems that can transport crew.
And in some ways, Logsdon said, this is like swapping out planes at an airport when one breaks down.
Cosmonauts and astronauts typically rotate on and off the space station every six months. The replacement capsule launching in February would normally be filled with crew. Since it's arriving empty, cosmonauts Prokopyev and Petelin and NASA astronaut Frank Rubio will stay onboard until another Soyuz can be launched with their replacements.
"The awesome thing about our crews is they're willing to help wherever we ask," Montalbano said. "I may have to fly some more ice cream to reward them."
The station's other occupants — NASA astronauts Nicole Mann and Josh Cassada, Japan Aerospace Exploration Agency astronaut Koichi Wakata and Roscosmos cosmonaut Anna Kikina — will return home in the SpaceX Crew Dragon that carried them into space. NASA said it will need a few more weeks to determine the timing of their return and the SpaceX Crew-6 mission carrying their replacements.
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