NASA says Saturday’s attempt at a historic rocket launch is proceeding as planned, although weather could again postpone the return to space for veteran NASA astronauts Robert Behnken and Doug Hurley.

Minutes before launch, the mission was 70% go for weather conditions and President Donald Trump was on the ground to witness the event.

NASA and SpaceX teams are targeting exactly 45 seconds after the clock strikes 3:22 p.m. EDT to launch from pad 39A at Kennedy Space Center to the International Space Station. It will mark the second attempt for the mission known as Crew Dragon Demo-2, the first of which was scrubbed Wednesday due to poor weather.

The mission is set to be the first time a private company sends humans into orbit – and the first time since 2011 that the United States has launched astronauts into space from U.S. soil. Wednesday’s scrubbed launch drew crowds that braved drizzles and downpours – even a tornado warning.

The weather forecast: SpaceX rocket launch is at a ‘50% go’ for Saturday due to more bad weather

NASA Administrator Jim Bridenstine said Wednesday’s launch was delayed because liftoff could have triggered a lightning storm in an electrically charged atmosphere. “In fact, the rocket itself could become a lightning bolt,” he said.

Teams have developed strict launch criteria to protect the safety of the astronauts: “If the weather is good, you go. If the weather is not good, you don’t go,” Bridenstine said. It’s a difficult challenge to meet, given all conditions must be met in a single instant, he said.

Unfortunately, teams can’t extend a window and wait for weather to improve. The requirement for Falcon 9 to launch exactly on time is due to orbital mechanics, as the Crew Dragon capsule has to “catch up” with the ISS, which is traveling at 17,000 mph about 250 miles above Earth.

But it’s not just weather around the pad that matters – teams have to monitor a massive flightpath and the conditions below it in the event that Behnken and Hurley have to abort, hurtling the spacecraft away from its Falcon 9 rocket and into the Atlantic Ocean. If conditions at sea are too rough, the launch is a “no-go.”

Hurley said he was no stranger to scrubbed launches in a Friday tweet: “On my first flight STS-127 on Shuttle Endeavour, we scrubbed 5 times over the course of a month for technical and weather challenges.”

Meanwhile, the astronauts – both longtime friends and shuttle veterans – have been spending time with their families and continuing preps for the one-to-four-month stay at the ISS, Bridenstine said.

“They’ve started a tradition of launching little rockets from the beach,” Bridenstine said of KSC’s famous “Astronaut Beach House,” a place where crews have historically spent time relaxing with families and friends. “I would imagine they’re probably getting some downtime and thinking about what’s coming.”

Also Saturday, President Donald Trump is expected to return to the Space Coast to watch the liftoff of Demo-2. He’s scheduled to be joined by Vice President Mike Pence on the deck of Operations and Support Building II, which offers an unobstructed view of pad 39A from just over three miles away.

Contributing: Britt Kennerly, Florida Today; The Associated Press.

Follow Emre Kelly on Twitter at: @EmreKelly

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