The launch of NASA astronauts in a Boeing Starliner has been cancelled

December 2019: “High Definition Close Call.”

On December 20, 2019, it appeared that Boeing was on its way to the end.

The Starliner capsule — the same spacecraft that will carry NASA astronauts Butch Wilmore and Sonny Williams to the space station on Saturday — was on the launch pad atop an Atlas V rocket.

The test flight to the space station had no astronauts on board, and its mission was to evaluate the spacecraft’s navigation, propulsion, and docking systems. If the flight passes this final technical hurdle, a flight with astronauts on board could take place within months.

The Atlas V rocket lifted off flawlessly, launching the Starliner.

And then the mission immediately went awry.

The spacecraft’s clock was set to the wrong time, causing the Starliner to believe it was in the wrong location. The pod fired its impulses to try to get to where it thought it should be. Meanwhile, a communications glitch thwarted efforts by flight controllers in mission control to diagnose and fix the problem.

The Starliner spacecraft consumed too much propellant, and the planned docking operation at the space station was cancelled.

During troubleshooting, Boeing engineers discovered another software bug that would have fired the faulty thrusters during the maneuver preceding reentry into the atmosphere. NASA described the incident as a “high-definition close call” that could have destroyed the spacecraft if errors had not been corrected from Earth during flight.

The investigation revealed multiple failures in Boeing’s operations that were supposed to catch errors before launch. A comprehensive audit reviewed 1 million lines of program code.

NASA officials acknowledged that they may have placed too much trust in Boeing, which has decades of experience working with NASA.

Summer 2021: Corrosion on the launch pad.

NASA and the company decided that a second unmanned test was needed before a flight with astronauts on board. The spacecraft was launched on the launch pad in July, but a problem aboard the space station led to a postponement to early August. Then, before the launch attempt on August 4, mission managers discovered corroded pusher valves aboard the Starliner spacecraft that would not open. The test flight was canceled and another long round of troubleshooting ensued.

May 2022: Another launch, more problems.

The second unmanned test was finally launched on May 19, 2022.

During a maneuver to place the Starliner into a stable orbit, two thrusters failed, but the spacecraft was able to compensate. It proceeded to dock with the space station and successfully return to Earth.

July 2023: Parachutes and duct tape.

Before the test flight with astronauts on board, which was scheduled for July 2023, two more problems emerged. The protective tape wrapped around the wire insulation turned out to be flammable, and a key component in the parachute system was weaker than designed and could break if the Starliner’s three parachutes were not deployed properly.

About a mile of tape was replaced, the parachute design was upgraded and strengthened, and then retested.

May 2024: Still not ready to fly.

“We’ve taken our time to go through everything methodically because it’s a test flight, and we want it to go well,” Steve Stich, director of NASA’s Commercial Crew Program, said during a May 3 news conference.

“We are ready to perform the test flight,” said Mark Nappi, Boeing’s Starliner program manager. I have never felt more prepared for any mission I have ever been involved in.

But Starliner wasn’t ready yet.

The May 6 countdown was going smoothly until a malfunctioning valve on the Atlas 5 rocket’s second stage — not attached to the Starliner rocket — began to fire, audibly vibrating at about 40 times per second.

The launch was cancelled, and the rocket had to be removed from the launch pad so the valve could be replaced. This work was completed within a few days.

But a thorny issue emerged.

While draining propellant from the tanks of an Atlas 5 rocket, engineers discovered a small helium leak in the Starliner’s propulsion system.

Helium, an inert gas, is used to force propellant into the thrusters, and if too much helium is lost, the thrusters may not function properly.

The leak was traced to a seal on a helium line leading to one of 28 small motors known as Reaction Control System motors.

“It’s pretty much like any piece of plumbing in your house, like a faucet or something like that,” Mr. Stitch said during a conference call on May 24. “There’s a seal that keeps that interface tight.”

Tests showed no leaks in the seals leading to the other 27 Reaction Control System actuators, and engineers were confident that the individual leak could be controlled. There are no plans to replace the seal, which would require pulling the Starliner off the Atlas V rocket and would result in a longer flight delay.

“We can deal with this particular leak if the leakage rate increases up to 100 times,” Stitch said.

The helium leak prompted NASA and Boeing to take a broader look at the Starliner’s propulsion system, which revealed a “design vulnerability,” Stitch said. If a series of unexpected failures occur, the spacecraft may not be able to safely return the astronauts to Earth.

If there were problems with the larger engines that were to be powered for a maneuver to drop the spacecraft out of orbit, one backup plan was to use eight of the smaller thrusters. However, the analysis showed that any additional failure could mean that there would only be four of the smaller thrusters available.

Engineers then developed another backup plan to deorbit Starliner using only the four thrusters. NASA and Boeing officials said that after weeks of studying the problem, they are confident in their ability to manage problems that could arise from the leak.

On Saturday, Mr. Wilmore and Ms. Williams may travel on the Starliner.