Flying astronauts has always been risky. No wonder NASA and SpaceX are on edge.

UPDATE: NASA scrubbed Wednesday’s launch until Saturday afternoon due to adverse weather conditions on the launch range.

Original story:

CAPE CANAVERAL, Fla. - Spaceflight is a dangerous enterprise. That unrelenting reality underlies every decision as NASA and SpaceX near the hour when they’ll strap two veteran astronauts into a spacecraft that will mark not only the first human launch to orbit from United States soil in nearly a decade but also the first time a private company has performed the feat.

That mission, scheduled for 4:33 p.m. Eastern Time Wednesday, is being celebrated as a historic moment for NASA and the nation. It's also an enormously risky endeavor whose failure could be a major setback for a growing commercial space industry and a devastating blow to SpaceX, which has upended the traditional aerospace pecking order.

That's among the reasons that weather now looms as the most likely obstacle to an on-time launch. It has been rainy and overcast here the last few days with low thick clouds that unveiled a stunning rainbow Tuesday morning.

But forecasters at Patrick Air Force Base still predicted a 40 percent chance that weather would force a postponement of Wednesday's launch, an improvement from Monday's 60 percent, but still casting doubt on the schedule. And conditions at Cape Canaveral may not be the only reason for cancellation: forecasters are watching developments all along the East Coast, where the Dragon capsule might have to ditch in the case of an emergency abort.

Fear of lightning also could force a postponement under complex NASA rules that prohibit a launch if the spacecraft is going to fly within 10 nautical miles of storm clouds that might generate an electrical discharge. Rockets tearing through such clouds can cause a lightning strike, as happened during Apollo 12 when the Saturn V rocket was hit twice, causing damage to some non-essential components. The crew was still able to complete its mission to the moon.

NASA administrator Jim Bridenstine emphasized at a news conference Tuesday his concerns for the safety of the two astronauts, Bob Behnken and Doug Hurley, and said the expected presence of President Trump and Vice President Pence would not increase pressure to launch if conditions are not right.

"We want people to be able to feel free to say, 'No,' and not feel any pressure to launch," he said, adding that he had texted Hurley and Behnken on Monday and told them, "If you want me to stop this for any reason, say so."

There is no way to exaggerate the inherent risk involved any time people are placed atop a rocket filled with thousands of gallons of highly volatile propellants. The danger is compounded by the fact that SpaceX's Dragon spacecraft has never flown humans before.

In an interview, Elon Musk, SpaceX's founder and CEO, called the launch a "crucial step. Can't mess it up."

He said he was optimistic. "The probability of success, you know, knock on wood, I think is high," he said. "But it it is not 100 percent. And so we're just doing everything we can to think of, any possible way, to improve the probability of success, because this would be a big setback if something were to go wrong."

Paul Hill, a member NASA's safety advisory panel and the former director of mission operations at NASA's Johnson Space Center in Houston, warned that human factors can lead to mistakes.

"Now is the time to be on alert for 'go' fever," he said after the panel's recent quarterly meeting. "So much work has gone into being this close to launch, it can be difficult to resist the pressure to accept some risk or trivialize some concern with less rigor."

For all of NASA's accomplishments, human spaceflight remains a relatively rare and exceedingly dangerous enterprise. Since John Glenn became the first American to orbit the Earth in 1962, NASA has launched a total of 164 spacecraft with astronauts to orbit, an average of fewer than three a year.

Two ended in disaster - the Columbia and Challenger shuttle flights that killed 14. And many others narrowly escaped harrowing calamities, like the Apollo 12 lightning strike or the oxygen tank rupture that threw the Apollo 13 mission into chaos.

Maiden flights of spacecraft with humans on board are the scariest. When the space shuttle flew for the first time in 1981, for example, officials estimated the chance of losing the crew was somewhere between 1 in 500 to 1 in 5,000. Later, after NASA had flown the shuttle many times, it found that first flight was far riskier than originally thought - the chance of death was actually 1 in 12.

There are many key differences between the shuttle's first flight and the upcoming launch of what's known as NASA's Commercial Crew Program. The spacecraft and rocket are owned and operated by a private firm, SpaceX, not NASA. The technology has advanced a great deal since the shuttle days; SpaceX's Dragon capsule is outfitted with modern touch screens and safety measures, such as an abort system, not available on the shuttle.

And while the space shuttle's first flight had crews on board, Dragon last year completed what NASA said was a flawless test mission without crews to the International Space Station that returned to Earth safely. Flying astronauts to the station may be a new feat, but SpaceX has flown 21 cargo and supply missions to the station since 2012 in the uncrewed version of the Dragon spacecraft. That's given the company plenty of practice sending spacecraft to orbit, and then chasing down the station and attaching to it as it orbits the Earth at 17,500 mph.

SpaceX's Falcon 9 rocket has also proved to be a reliable work horse that is closing on 100 launches.

But spaceflight is governed by unforgiving physics, requiring extraordinary amounts of energy to escape gravity. Launching a rocket is like detonating a bomb, but with the blast controlled so it flows in a single direction.

"We've done everything we can to make sure that the rocket is safe and the spacecraft is safe," Musk said in the interview. "But the risk is never zero when you're going 25 times the speed of sound, and you're circling the Earth every 90 minutes. It's a speed that's difficult for people to even comprehend."

The mission would also be the first time since the 1975 Apollo-Soyuz flight, when Russian and American spacecraft docked in orbit, that NASA astronauts would return to Earth by landing in the ocean, rather than on land. Water landings have their own risk, such as when Gus Grissom nearly drowned in 1961, after his capsule filled with water and eventually sank.

The space shuttle landed on a runway, and since its retirement in 2011, NASA astronauts have been flying in Russian Soyuz spacecraft that land on the steppe in Kazakhstan. But there have been hair-raising moments with the Soyuz as well. In 2018, one of the side boosters failed to separate properly and slammed into the rocket, triggering the emergency abort system, which sent NASA astronaut Nick Hague and his Russian counterpart, Alexey Ovchinin, on a harrowing ride to the edge of space.

"If you do this business, you're going to get your nose bloodied some," said Gerry Griffin, who served as a NASA flight director during the Apollo era. "Hopefully, you don't kill anybody. But if you do it long enough and fly enough vehicles, you're going to have a failure."

No one knows the risks inherent in human spaceflight better than Behnken and Hurley. Since they were assigned to the Commercial Crew mission in 2015, they've spent thousands of hours training for the flight and are confident, especially since Dragon has a robust abort system.

"Now, I will add that it is the first flight with crew," Hurley said during a recent news conference. "It's the second flight of the vehicle. So the statistics will tell you that's riskier than, say, the 15th flight or the 20th flight of the vehicle."

Still, he said he was "confident in both the SpaceX and NASA teams. We've looked at all the stuff that we need to look at. And when we're ready to launch, we'll go do it."

The whole point of this test flight is to put the spacecraft through its paces and wring out any problems before NASA certifies it for the operational missions the agency hopes will ferry crews to the space station and back for years to come.

Boeing, the other company NASA is paying to develop a new vehicle to fly its astronauts, suffered troubling setbacks late last year during the test flight of its Starliner spacecraft. No astronauts were on board, but the spacecraft encountered trouble almost immediately upon reaching orbit. The onboard computer was 11 hours off, making the spacecraft think it was at a different part of the mission than it actually was.

Crews on the ground scrambled, and then discovered a second software problem that would have caused the wrong thrusters to fire during the spacecraft's return to Earth, when what's known as the service module was to separate from the crew module.

Controllers on the ground discovered the problem while the spacecraft was in orbit and were able to correct it. Had they not, however, it could have damaged the spacecraft's heat shield or sent the crew module tumbling off course.

Since then, NASA officials have said they should have been better at holding Boeing accountable to the agency's rigorous safety standards. And they are confident that after years of working closely together to deliver cargo and supplies to the station and now crew, NASA and SpaceX won't encounter any such problems on the upcoming mission.

But there are always concerns about the unexpected and the overlooked.

The space shuttle Challenger exploded on chilly January day in 1986 after an "O-ring" joint failed amid unheeded warnings about launching in cold weather. In 2003, Columbia disintegrated as it was reentering Earth's atmosphere after a piece of foam became dislodged during the launch and damaged heat resistant tiles on the wing. A subsequent investigation found a "broken safety culture" at the space agency.

In the past several years, NASA and SpaceX have led investigations into a trio of failures. In 2015, a Falcon 9 rocket carrying supplies to the station exploded during flight after the failure of a strut designed to withstand 10,000 pounds of force buckled at 2,000. A year later, another rocket blew up on the launchpad after the failure of a pressure vessel in the second stage liquid-oxygen tank. And last year, during a test of the abort system, the Dragon spacecraft exploded because of a leaky valve. SpaceX also has struggled with the parachute system that would guide the spacecraft to a soft landing as it returns to Earth.

No one was injured in any of those failures. NASA and SpaceX said they have investigated all the problems, fixed them and are now ready to launch humans.

"This endeavor is the culmination of not only years and years of experience, or time and work, but hundreds of thousands of hours of tireless effort to bring us here," said Benji Reed, SpaceX's Commercial Crew Program manager. "And it's all focused on the safety and reliability of the system."

He said that the mission was a "sacred honor" and that the company would do everything possible to fly the astronauts "to the space station and safely bring them back home to their families. Fundamentally, this is what SpaceX was founded for."

Outside experts agree the teams have prepared as best they could.

"I would say the risk is acceptable. But it's not zero. Spaceflight is inherently dangerous, so there is always risk," said Wayne Hale, the former manager of NASA's shuttle program. "But I think that all the appropriate checks appear to have been done. I think appropriate measures have been taken and having an uncrewed test flight was a big step."

That said, everyone will be holding their breath the moment the countdown ticks to liftoff.

“I think we’re all very appropriately nervous,” Reed said.