After years of delay, another four weeks wasn’t much of an ask for a pair of NASA astronauts set to finally fly on the first human spaceflight of Boeing’s CST-100 Starliner.

Butch Wilmore and Suni Williams are set to return to the launch pad Saturday to climb on board the spacecraft sitting atop a United Launch Alliance Atlas V rocket at Cape Canaveral Space Force Station’s Space Launch Complex 41. Liftoff of the Crew Flight Test mission is targeted for 12:25 p.m., taking the duo on a trip to the International Space Station, with a backup opportunity on Sunday.

Space Launch Delta 45’s weather squadron forecasts a 90% chance for good conditions at the launch site for both Saturday and Sunday. Further backup opportunities fall to next Wednesday and Thursday.

NASA astronauts return to Space Coast ahead of Starliner launch

The duo had already suited up once and were strapped into their seats at the pad on May 6, but an issue with a fluttering valve on the Atlas V’s upper Centaur stage forced mission managers to scrub the launch and ultimately roll the rocket back to the nearby Vertical Integration Facility to have the valve replaced.

During that work, teams with Boeing and NASA also addressed a small helium leak found on the Starliner’s propulsion module. That leak was ultimately deemed safe enough for launch and teams did not fix it.

“It’s really just a matter of safety. When we looked at this problem. It didn’t come down to making trades, it came down to, is it safe or not. And it is safe,” said Boeing’s Mark Nappi, vice president and program manager for the company’s commercial crew program. “That’s why we determined that we could go fly with what we have.”

Teams also worked out an operational workaround to ensure a safe flight home after discovering a potential issue with the propulsion system related to the helium leak and if more than one the systems four banks of engines were to shut down.

“We certainly learned a lot more about this system since our first launch attempt,” said NASA Associate Administrator Jim Free during a pre-flight press conference Friday. “The teams … as always continue to work diligently to ensure that when we do launch this test flight, we’re ready to conduct it safely.”

While teams worked through the issues, the pair of former Navy test pilots, each a veteran of two spaceflights, returned to Houston, but made their return to KSC this week. They are set to suit back up at KSC’s Neil Armstrong Operations & Checkout Building early Saturday with NASA live coverage set to begin at 8:15 a.m. on NASA TV and its social media channels.

They’ll take an updated version of the Astrovan used to drive astronauts to the pad during the space shuttle era and ascend the tower at SLC-41. If it flies, it will be the first launch from Cape Canaveral Space Force Base since the Apollo 7 launch in 1968. All subsequent Apollo, space shuttle and SpaceX Crew Dragon flights have come from KSC.

One change since May 6 is some new cargo loaded on board to replace a pump essential to the ISS’ urine-to-drinking-water system that recently broke down, forcing astronauts on orbit to store urine. The 150-pound urine process assembly takes the place of the astronauts’ suitcases, meaning they won’t have personal clothes along for the ride.

“We do have a lot of generic contingency clothes on board, so not an issue,” said NASA ISS program manager Dana Weigel. “But huge kudos goes to the whole team for the quick response. I mean, they turned this around rapidly. … Obviously we want to get that capability up and running again.”

The CFT mission, if it leads to certification, would be the final piece needed for Boeing to play catchup to SpaceX as part of NASA’s Commercial Crew Program. While SpaceX’s Crew Dragon just celebrated the four-year anniversary of its first human spaceflight on Friday, Boeing has had to pivot through a series of hardware and software hiccups to get to the same point.

That included another year-long delay since 2023 to replace miles of flammable tape teams realized was used within the Starliner and fixing an issue with the parachute system, issues that compounded the already 2 1/2 year-delay between the first and second uncrewed test flights of Starliner because the first try wasn’t able to rendezvous with the ISS.

“We are continuing to learn. That’s what spaceflight is about,” Free said. “Every day we learn, both on our vehicles like Starliner, on space station, with the ultimate goal of keeping our crews safe going to, living there and returning.”

Wilmore and Williams will test backup manual maneuvers on the mostly automated capsule both on the way up and back from the ISS. The duo are slated to arrive at the station at 1:50 p.m. Sunday and stay on board for eight days before the return flight to Earth with a parachute- and airbag-assisted desert landing in the southwestern United States as early as June 10.

The test mission would pave the way for Boeing to begin sharing regular duties of ferrying astronauts to and from the ISS as soon as February 2025. SpaceX has flown is fleet of four Crew Dragon spacecraft 13 times since 2020 carrying 50 people to space, including the four members of Crew-8 who await the arrival of Starliner aboard the ISS.

“They’re veteran astronauts and my dear friends,” said NASA astronaut Mike Fincke, the backup for CFT who is slated to fly on Starliner-1. “They’re test pilots, and they are looking very forward to getting flight test data that will help continue the certification process for Starliner, so that Starliner-1, 2, 3 and to infinity and beyond can fly and provide the services to the International Space Station.”