NASA astronauts Butch Wilmore and Suni Williams returned to Kennedy Space Center on Tuesday as their much-delayed flight to the International Space Station aboard Boeing’s CST-100 Starliner looks like it may finally happen Saturday.

The duo are slated to climb back aboard the capsule and fly atop a United Launch Alliance Atlas V rocket from Cape Canaveral Space Force Station’s Space Launch Complex 41 at 12:25 p.m., with a backup launch opportunity on Sunday. ULA said the weather forecast calls for 90% good conditions at the launch site for both days, although the weather also has to be clear along the launch corridor in the event of an emergency abort.

Dubbed the Crew Flight Test, the mission marks the first human spaceflight for Starliner as Boeing tries to play catchup to SpaceX as part of NASA’s Commercial Crew Program. SpaceX launched its first crewed test flight of its Crew Dragon on May 30, 2020, so Starliner’s launch will come just over four years since that flight.

Welcome back! @NASA_Astronauts Butch Wilmore and Suni Williams arrived back at @NASAKennedy‘s Launch and Landing Facility aboard T-38 jets, ahead of the agency’s @BoeingSpace Crew Flight Test. pic.twitter.com/qmQye8RKmQ

— NASA Commercial Crew (@Commercial_Crew) May 29, 2024

Wilmore and Williams came within two hours of launch back on May 6. The pair had suited up, ridden out to the launch pad and climbed aboard the spacecraft when teams scrubbed the attempt because of an issue with a fluttering valve on the rocket’s upper Centaur stage.

The decision was made to roll the rocket back from the launch pad to ULA’s nearby Vertical Integration Facility, where teams switched out the valve. A second issue involving a small helium leak on the propulsion system of the Starliner spacecraft led to more delays, so Wilmore and Williams returned to Houston where they remained in quarantine.

Wilmore and Williams flew back on their T-38 jets arriving at the former shuttle landing facility at KSC with as little as four days to go before they will try again.

Teams with NASA, Boeing and ULA just last week announced they were OK with the helium leak not threatening the safety of the crew while also working a related issue that addressed the unlikely potential for multiple engines failing on the return trip home. They then polled “go” for launch at a combined Delta-Agency Flight Test Readiness Review at KSC on Wednesday.

ULA plans to roll the rocket back out to the pad Thursday.

The Commercial Crew Program was designed to break NASA’s reliance on Russian Soyuz launches after the 2011 end of the Space Shuttle Program, providing ferry service from the U.S. to and from the ISS.

SpaceX’s fleet of four Crew Dragons over the last four years have launched 50 humans into space across 13 flights. Its most recent, Crew-8, took four crew members to the ISS in February. They await the arrival of Williams and Wilmore on Sunday afternoon. The duo will spend about eight days on board before returning on Starliner to Earth with a desert landing planned in western United States.

If all goes well, Boeing’s first contracted mission for crew rotation to the ISS could come as early as next February.