It’s Saturday morning and in just four hours astronauts Bob Behnken and Doug Hurley are expected to become the first humans to ride a Dragon. The veterans of NASA’s space shuttle program are scheduled to catch a lift to the International Space Station inside SpaceX’s Crew Dragon capsule, which will be boosted to orbit by the company’s Falcon 9 rocket. The Demo-2 mission will be the final test before NASA certifies SpaceX’s capsule for human spaceflight and resumes regular crewed launches from the US after a nine-year hiatus. But the weather forecast is not looking good.

Swollen white clouds laze over Kennedy Space Center, portents of thunderstorms predicted to roll across the bay later in the afternoon. The Space Force’s 45th Weather Squadron, the group responsible for monitoring the weather around the launchpad, predicts only a 50 percent chance that the launch will occur today. That’s worse odds than they gave on Wednesday, the first attempt at the Demo-2 mission, which was scrubbed just 17 minutes before launch due to concerns about lightning near the pad.

In less than an hour, Behnken and Hurley will ride to the launchpad in a pair of white Teslas, climb into the crew capsule on the rocket, and start preparing for launch. Neither SpaceX nor NASA want the astronauts to go through the prelaunch procedures again for nothing, so the organizations meet to reevaluate the weather. At the press site, a crew of NASA employees are putting the finishing touches on a stage erected on the lawn for the president and his entourage, who are scheduled to arrive later in the afternoon.