As early as Saturday afternoon, American astronauts are set to blast into space aboard an American spacecraft for the first time since NASA decommissioned the Space Shuttle in 2011.

NASA scrubbed an earlier launch on Wednesday afternoon owing to bad weather. The next window after Saturday is on Sunday.

A successful lift-off of SpaceX’s Crew Dragon spacecraft from NASA’s John F. Kennedy Space Center near Orlando will restore the United States’ ability to launch people into orbit. For nine years, NASA has rented rides for its astronauts aboard Russian Soyuz capsules—at a cost of up to $90 million per seat.

The Crew Dragon, a roughly 27-foot tall capsule, also is a rental. In a break from past practice, SpaceX owns the spacecraft even though NASA paid $3 billion to help develop it. NASA handed the California company around $55 million per passenger for Wednesday’s mission.

James Oberg, a NASA mission-controller during the Space Shuttle era, has paid close attention to the Crew Dragon’s development.

He has concerns. Specifically, he worries about the inexperience of the NASA and SpaceX teams that have developed, built and tested the Crew Dragon.

“Much as been made of the long gap–nine years–since the last human launch from U.S. soil,” Oberg said. “However, there is an even longer gap that has not yet been mentioned, even though it’s probably much more significant for the success of this coming launch.”

“It’s been almost 40 years since the last first-of-its-kind U.S. human spacecraft. That was on April 12, 1981, with the Space Shuttle Columbia. Special preparation is always needed for that kind of first.”

“I was there, on console with the Ascent Team, for that event, and the intense preparation that preceded it,” Oberg added. “Aside from being a systems specialist for the Shuttle’s auxiliary propulsion system, my known experience as a technical writer got me picked as the recording secretary of a special team whose responsibility involved thinking ahead to all credible flight failures requiring urgent responses. Every debate was thoroughly documented for all team members to ponder.”

“As many possible in-flight emergencies had to be imagined. Methods by which they could be detected had to be invented and dedicated instruments needed to be developed and validated. Allowable decision time had to be specified and verified in computer simulations.”

“Both flight crews and ground support crews needed training, then testing in practice sessions scripted by an independent team of creative, diabolically-minded doom-scenario creators,” Oberg continued.

“So we imagined our way into that event, and studied, and practiced.”

“Between the practice runs and on-console debriefings, we talked among ourselves,” Oberg recalled. “We new kids were surrounded by men who in the previous 15 years had won the Moon Race, operated the first successful space station and reached across boundaries to link up internationally in orbit, and they were eager to tell war stories to wow the newbies.”

“There were specific tips and hip-pocket checklists to be shared. Even more profound acculturization occurred with the weird human-angle vignettes … We were being prepared at many levels.”

“There was no guaranty that all of that would turn out to be enough,” Oberg stressed. “For the Space Shuttle in 1981, the awesome leap from theory to practice was immense, because the vehicle was never designed to fly without a crew. The very first time it flew into orbit, there had to be astronauts on board. That leap had never been made before or since.”

“Earlier crew vehicles—Mercury, Gemini, Apollo Command Modules, Apollo Lunar Modules—all had initial shakedown flights without astronauts. And the SpaceX Crew Dragon and its Falcon 9 booster have undergone similar pre-crew flights. Boeing’s Starliner crew-carrying craft also has undergone this vetting, and on its intended last-before-crew orbital test flight recently, it suffered two major systems failures, requiring the insertion of a repeat test before NASA can certify it for human occupancy.”

“The Space Shuttle was, alone among all human-carrying spaceships everywhere in the world, different–and much more dangerous. As it turned out in hindsight, we had gotten barely skilled enough to squeak by several near-disasters, some of which we didn’t even know about until long after the landing.”

“This is still not to diminish the challenge of the Crew Dragon flight,” Oberg said. “The primary source of a still-undefined level of hazard is the historical fact that although the Space Shuttle was developed by NASA-contractor teams with personal experience in previous programs, Falcon-Dragon didn’t have direct access to that legacy of experienced specialists. The simple passage of decades meant they had to absorb it from secondary or even more indirect sources.”

“An experienced team, like the 1981-era NASA shuttle team or the [Wernher] von Braun team in the 1950s or [Soviet rocket-designer Sergei] Korolev’s team in the 1960s, has a hard-earned informed judgment of solution-generation and assessment, learned both from failures as well as successes.”

“This is a much more critical factor than simply knowledge of definable facts and formulae. It creates an intuition to see not just what is there, but to imaginatively perceive what should be, but isn’t yet there. It excels not at creating answers but at raising questions.”

“The degree to which the SpaceX team has absorbed this insight and developed and applied this wisdom remains undemonstrated,” Oberg said. “But their track record to date, including vigorous and on-target responses to setbacks, is encouraging. I’ll still be holding my breath.”