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How long will the 1st astronauts to ride SpaceX's Crew Dragon be in space? No one knows exactly (yet).

NASA astronauts Bob Behnken and Doug Hurley sit inside SpaceX's Crew Dragon spacecraft during a fully integrated test of critical crew flight hardware on March 30, 2020. The two astronauts are scheduled to launch to the International Space Station in the Crew Dragon on May 27, but it's not yet clear how much time they'll spend on board the orbiting laboratory. (Image credit: SpaceX)

Two NASA astronauts will make history this month when they become the first crew to fly to the International Space Station in a private spacecraft. But exactly how long their historic mission will last has yet to be determined. 

On May 27, astronauts Doug Hurley and Bob Behnken will board SpaceX's Crew Dragon spacecraft and launch toward the orbiting laboratory. The test flight is scheduled to lift off on a Falcon 9 rocket at 4:32 p.m. EDT (2032 GMT), and If all goes according to plan, the capsule will dock with the station about 19 hours later. 

While NASA and SpaceX have been planning this Crew Dragon Demo-2 mission for years, there is one key aspect of this mission for which they do not yet have a plan: the amount of time that the astronauts will spend in space before heading back to Earth.  

Related: Take a walk through SpaceX's Crew Dragon spaceship

Behnken and Hurley could spend anywhere from one month to 119 days at the International Space Station, but the exact duration of their mission won't be determined until they are already in orbit, NASA officials said in a mission briefing on Friday (May 1). So, they could be returning to Earth at the end of June or as late as Sept. 23. 

To decide on a date for the return flight, NASA and SpaceX will be evaluating not only the state of the Crew Dragon spacecraft in orbit, but also the progress on SpaceX's Crew-1 capsule. That vehicle will carry the first operational flight of a Crew Dragon spacecraft, which will transport four astronauts of the Expedition 63/64 crew to the International Space Station. (So far, no launch date has been set for the Crew-1 mission; Demo-2 must return safely before Crew-1 will be able to fly.) 

"Really the decision point is, hey, is Dragon healthy? Is the vehicle performing well, the Dragon that's on orbit? And then we'll be looking ahead to that next mission, the Crew-1 flight, and looking at the vehicle readiness and trying to determine what's the smart thing to do relative to the mission duration," Steve Stich, deputy manager of NASA's Commercial Crew Program, said at the mission briefing. 

"It's a little bit of a variable, but it's one that I think we can manage well," Stich said of the Crew-1 capsule status. "We would like to fly a mission that is as long as we need to for a test flight but also support some of the space station program needs and augment their crew capability to do science in other operations at the station."

SpaceX's first Crew Dragon capsule to carry astronauts is seen during final preparations in this image released on May 1, 2020. (Image credit: SpaceX)

Regardless of how the timing works out with the arrival of the Crew-1 mission, the Crew Dragon used for the Demo-2 mission cannot stay docked with the station for more than 119 days, Stich said, because its solar arrays are not designed to withstand degradation in space for more than 120 days. 

"Any solar array in low Earth orbit tends to degrade a little bit over time," Stich said. "It turns out the atmosphere has a little bit of oxygen in it — it's called atomic oxygen — and so there's a little bit of degradation in the ability for the cells itself to generate power. And the particular cells on the trunk for a Dragon, based on analysis capability, kind of a worst-case prediction, we think we can get about 120 days capability out of those."

However, the operational version of the Crew Dragon spacecraft, like the one that will fly the Crew-1 mission, is designed to last 210 days in space. That matches the on-orbit lifespan of Russia's Soyuz crew capsules, which have ferried NASA astronauts to and from the space station since the space shuttle program ended almost a decade ago. 

The Demo-2 mission comes a little over a year after SpaceX successfully launched an unpiloted Crew Dragon spacecraft to the International Space Station for the Demo-1 mission. If all goes well with Demo-2, SpaceX could begin regularly ferrying astronauts to and from the orbiting lab as early as this summer. 

Email Hanneke Weitering at hweitering@space.com or follow her @hannekescience. Follow us on Twitter @Spacedotcom and on Facebook.

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