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It’s all systems go for a second attempt to launch the oft-delayed Boeing Starliner crew shuttle Saturday on a long-awaited test flight to the International Space Station, the first capsule with astronauts aboard, NASA managers said Friday.

“From the perspective of the station, from our crew, from our ground teams, we are ready to execute this mission,” said Dana Weigel, NASA’s space station program manager. “We are excited to be on the cusp of this historic mission.”

Liftoff of the Starliner atop a United Launch Alliance Atlas 5 rocket is targeted for 12:25 p.m. EDT Saturday, about the time Earth’s rotation brings Pad 41 at the Space Force Station at Cape Canaveral into alignment with the orbit of the station.

ULA engineers are expected to begin fueling the rocket around 6:30 a.m. Veteran NASA astronauts Barry “Butch” Wilmore and Sunita Williams, serving as commander and copilot, respectively, plan to dock three hours later to await liftoff.

It will take Atlas 5 about 12 minutes to launch the Starliner into orbit, beginning a 25-hour rendezvous with the space station. Docking is expected at 1:50 PM on Sunday. If all goes well, Wilmore and Williams will undock and return to Earth on June 10, landing that morning in a desert location in Arizona or New Mexico depending on the weather.

“I’ve talked to them and they have full confidence in our rocket, they have full confidence in our spacecraft, in our operations teams and in our management teams,” said astronaut Mike Finke, training to command a Starliner mission next year. “They are definitely ready to go.”

Speaking of ready to “go,” the Starliner will carry a urine pump module that was added at the last minute to replace one that failed aboard the station earlier this week. To make room for the 150-pound component, some of the crew’s clothing and other personal items were removed from the ship, but general clothing is being stored aboard the lab and no problems are expected.

“The two specific suitcases that came out had clothes for both Butch and Sonny in them, and also some … of their own unique hygiene supplies,” Weigel said. “Of course we have generic shampoo, soaps, etc. on board that they can simply use from the generic supplies. It’s the same with clothes, so that’s what they’re going to do.”

The long-awaited Starliner flight marks an important milestone in NASA’s Commercial Crew Program, which is funding the development of SpaceX’s Crew Dragon and Boeing’s Starliner shuttles to provide post-shuttle transportation to and from the space station without having to rely on Russia for trips aboard the Soyuz spacecraft.

From the beginning, NASA wanted spacecraft from various vendors to provide continuous access to the space station, even if problems grounded one shuttle for an extended period of time.

SpaceX began flying astronauts aboard Crew Dragon spacecraft in 2020 and has already carried 50 astronauts, cosmonauts and civilians into orbit on 13 flights. Boeing’s Starliner, by contrast, suffered a steady stream of problems that delayed the first manned flight by four years, costing the company more than $1 billion to fix.

Wilmore and Williams finally received permission to launch on May 6. They were in the process of tightening up for liftoff when ULA engineers reported problems with a safety valve used to maintain the correct pressure in the oxygen tank on top of the Centaur rocket. I don’t feel comfortable with the operation of the valve, mission managers ordered a scrub.

Atlas 5 was towed back to the ULA Vertical Integration Facility where a replacement valve was installed, tested and cleared for flight. Meanwhile, Boeing engineers were evaluating data collected after the cleanup, which showed a small leak of helium in the plumbing used to pressurize the Starliner’s propulsion system.

The leak was eventually traced to the plumbing leading to a specific reaction control system jet, one of 28 such thrusters mounted around the Starliner’s drum-shaped service module. After extensive testing and analysis, mission leaders concluded that the spacecraft could safely fly as is without a credible threat to flight safety.

If the leak worsens dramatically during flight, the helium manifold in question will be isolated, disabling the thrusters. This would prevent normal re-entry using more powerful thrusters at the end of the mission, but there are back-up plans to perform a de-orbit ‘burn-up’ using two long firings of unaffected RCS jets.