Kenneth Chang

KENNEDY SPACE CENTER, Fla. — With gray clouds above that did not move away fast enough, a rocket launch that was to be the first to take American astronauts to orbit from American soil in nearly a decade stayed on the ground, disappointing spectators including President Donald J. Trump and Vice President Mike Pence.

Despite discouragement from top NASA officials, crowds had gathered along Florida’s Space Coast, and the rocket was on the launchpad, ready to head toward orbit — and a transformed era of human spaceflight.

The launch of two NASA astronauts on a rocket built by SpaceX, the rocket company started by billionaire Elon Musk, would be the first launching of people by a private company and not a national space agency like NASA. For this launch, SpaceX was in charge, although in consultation with NASA officials.

“This is not something that I ever thought would actually happen,” Mr. Musk said during an appearance on NASA Television. “So when starting SpaceX in 2002, I really did not think this day would occur. I expected a 90 percent chance we would fail to even get to a low-Earth orbit with a small rocket.”

Over the years, SpaceX has become more and more successful, now capturing about 70 percent of the market for the launching of commercial satellites. Now it is entering the business of sending people to space.

“SpaceX can do things that NASA historically has not done,” said Jim Bridenstine, the NASA administrator.

On Wednesday, however, more mundane considerations like weather intervened.

Although there was no trouble with the rocket, capsule and crew, light but persistent rain fell around the space center throughout the day. Later in the afternoon, the rain stopped and skies began to clear.

About 15 minutes before the scheduled liftoff time of 4:33 p.m. Eastern time, a weather officer, likely a member of the Space Force’s 45th Weather Squadron, informed the SpaceX launch director that the weather conditions would not clear up in time. If the Falcon 9 were to launch just 10 minutes later, the officer said, the mission might have been able to proceed.

But the liftoff time could not be moved. For the spacecraft to be able to meet up with the International Space Station passing overhead, liftoff must occur at a precise moment. Calling off launches in Florida because of unfavorable winds and clouds, even with just minutes left on the countdown clock, is not uncommon, especially with the fast changing weather over Central Florida.

The decision to launch or not is governed by a detailed set of rules gained over years of experience. For instance, when winds exceed a certain threshold or clouds hold electric charge that could discharge as lightning, launches are called off, because the risk of catastrophic failure to crew, or cargo, is too high.

In describing why the launch was called off, Mr. Bridenstine said later in remarks on NASA TV that weather officers were concerned that a liftoff might “actually trigger lightning,” because of the excess of electricity in the atmosphere.

He praised the team that made the decision not to launch.

“Under no circumstances should anybody feel any pressure,” he said. “If we are not ready to go, we simply do not go.”

The next opportunities to launch are Saturday at 3:22 p.m. Eastern time and Sunday at 3 p.m.

President Trump said he would return to Florida on Saturday.

For the astronauts, the day began with breakfast. Mr. Hurley reported he had steak and eggs, answering a question from reporters the day before. Despite uncertain weather conditions, NASA and SpaceX decided to press ahead with the countdown.

In the early afternoon, Mr. Behnken and Mr. Hurley put on their spacesuits. Mr. Musk and NASA’s administrator, Jim Bridenstine, each wearing surgical masks and standing a socially distanced six feet from the astronauts, shared some final words with the men, who then headed to the launchpad in a Model X S.U.V. made by Mr. Musk’s other company, Tesla. Around the same time, NASA shared video of Kelly Clarkson performing the national anthem. The astronauts’ families said goodbye to the men as the car drove away in a convoy.

Upon arriving, the men paused at the launchpad to take in the javelin-like Falcon 9 rocket, nearly as high as a football field is long. Then, they went up an elevator, made some phone calls to loved ones, crossed a bridge and the astronauts boarded the capsule and, after a series of safety checks, the hatch was closed.

A SpaceX mission controller asked “Are you ready?” One of the astronauts replied, “We are ready.”

The astronauts then sat in the capsule during the hours of procedure that needed to be completed before the launch.

Forty-minutes before liftoff, mission managers had to decide whether to start pumping kerosene rocket fuel and liquid oxygen into the tanks of the Falcon 9. With the skies still gray around the launchpad, about 15 minutes before the liftoff time they made the call to stop and try again on Saturday.

Even after the launch was finally called off, Mr. Behnken and Mr. Hurley had to sit patiently in the capsule for about an hour until the fuel was pumped out of the rocket and it was safe for them to get out.

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