Update: Today's launch of the SpaceX Crew Dragon was postponed until May 30 due to concerns about the weather.

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Mission managers have cleared the final paperwork for SpaceX's first-ever crewed launch, aimed at sending two NASA astronauts to the International Space Station aboard a SpaceX Crew Dragon capsule.

The stage is now set for the first NASA mission to send humans into orbit from U.S. soil since the retirement of the space shuttle fleet in 2011.

Only one big question remained after today's launch readiness review, which looked at all the technical issues surrounding Wednesday's scheduled liftoff from NASA's Kennedy Space Center in Florida.

"We're burning down the final paper," Kathy Lueders, manager of NASA's commercial crew program, told reporters during a teleconference. "All the teams were 'go,' and we're continuing to make progress toward our mission. Now the only thing we need to do is figure out how to control the weather."

The weather forecast called for a 60% chance of scrubbing the launch due to concerns about rain and clouds at the launch site. The weather was rainy at the Cape, but Mike McAleenan, launch weather officer for the 45th Weather Squadron at Patrick Air Force Base, said the outlook was improving.

"If I was to issue the forecast today, right now, we would probably be down to a 40% chance of violation. … So we have some hope for launch day,"

SpaceX Crew Dragon Is the Most Anticipated Launch of the Year youtu.be

McAleenan said.In addition to the conditions at the launch site, mission managers have to consider what the weather is like at sea, in case an emergency comes up and the astronauts have to abort the flight. That part of the forecast is also iffy, McAleenan said. He said the backup days for launch, May 30 and 31, had a "better probability of launching, and lower risk numbers across the Atlantic."

The countdown clock officially begins running on Tuesday morning — but the two astronauts who are due to ride the Dragon, Doug Hurley and Bob Behnken, will have a relatively light day, consisting of technical briefings mixed with rest and relaxation.

On Wednesday, they'll get a wakeup call at 9 a.m. ET, have breakfast and then receive a weather briefing at about 12:20 p.m. ET. After the briefing, they'll put on their custom-fitted SpaceX spacesuits, walk out from the Armstrong Operations and Checkout Building at about 1:15 p.m., climb into the Dragon capsule at 2 p.m., and close the hatch at 2:38 p.m.

Lueders said launch-pad workers have been "nose-swabbed and tested" to check for COVID-19, and they'll be outfitted in a way that's meant to protect Hurley and Behnken from infection. "They look like ninjas there — because, you know, with the SpaceX folks, you've got to have it be looking cool," she joked.

Hans Koenigsmann, SpaceX's vice president for build and flight reliability, said the launch team will be closely monitoring the weather forecast for the launch site as well as the ocean recovery zones as the clock ticks down. A crucial decision point will come at T-minus-45 minutes, when the go-ahead is due to be given for arming the Dragon's launch escape system and fueling up the Falcon 9 rocket with highly refined kerosene and liquid oxygen.

If all goes as planned, the Falcon 9 will light up its engines at 4:33 p.m. ET (1:33 p.m. PT), kicking off what's expected to be a 19-hour flight to the space station. Along the way, the astronauts will get a couple of chances to steer the Dragon manually using touchscreen panels. But for most of the flight, including the station docking, the Dragon will be under autonomous control.

Assuming that the weather cooperates, the next big uncertainty has to do with how long SpaceX's first spacefliers will stay at the station.

The primary purpose of the mission is to demonstrate the Dragon's performance for future crewed flights. But right now there are only three other crew members at the space station, which typically has a crew of six. Hurley and Behnken may be called upon to put in a tour of duty that's longer than originally scheduled — and help out with tasks including a couple of spacewalks.

"That schedule is a little bit in flux," Koenigsmann acknowledged. "Obviously NASA will tell us when they're ready to go, anywhere between … six weeks and 16 weeks."

The flight back will bring the astronauts down to an Atlantic Ocean splashdown and recovery. In an interview with Aviation Week & Space Technology, SpaceX CEO Elon Musk said the return was the part of the mission that he was most worried about — due to the asymmetry of the Dragon's back shell.

"We've looked at this six ways to Sunday, so it's not that I think this will fail," Aviation Week's Irene Klotz quoted Musk as saying. "It's just that I worry a bit that it is asymmetric on the backshell, and you could have a strange sort of roll coupling as you come in if you turn too much. I think this is low risk, but that's what I would put as my biggest concern."

When Koenigsmann was asked about Musk's comments, he said he didn't have any special concerns in advance of the launch. "I'm at the point right now where I'm actually worried about the weather, and that's a good sign," Koenigsmann said.

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