The ISS has a urine pump problem. Boeing's Starliner astronaut launch will flush it out.

CAPE CANAVERAL, Fla. — Boeing's Starliner spacecraft is poised for its long-awaited first crewed launch. 

That liftoff, which will kick off the Crew Flight Test (CFT) mission to the International Space Station (ISS), is set for 12:25 p.m. EDT (1625 GMT) on Saturday ( June 1) from Cape Canaveral Space Force Station here, atop a United Launch Alliance (ULA) Atlas V rocket.

NASA, Boeing and ULA officials provided updates about the launch during a press call on Friday (May 31), confirming that all systems are "go" for Saturday's launch with NASA astronauts Butch Wilmore and Suni Williams aboard. The mission aims to further the certification of Starliner to carry crew on operational missions to the ISS.

Leading up to this first shakedown passenger flight, Starliner has faced a series of delays, both over the last few years and within the past few weeks. A May 6 attempt was scrubbed about two hours before liftoff, for example, due to a malfunctioning valve in the Atlas V's Centaur upper stage. That led mission managers to roll Starliner and the rocket back to ULA's vertical integration facility (VIF), where engineers discovered a helium leak in Starliner's service module.

Related: Boeing Starliner's 1st astronaut launch: Live updates

Given its location and systems interactions inside Starliner, it would have been "almost unsafe to work on" the leak, NASA Commercial Crew Program Manager Steve Stich said during Friday's call. But launch team members made the determination that the leak was small enough not to pose a serious risk to the spacecraft or to the CFT mission, and the Atlas V/Starliner stack was rolled back to the launch pad on Thursday (May 30). 

"Sometimes for spaceflight, you plan for contingencies, and you design the vehicle to have margin. And, in our case, we have margin in the helium tank," Stich said. "We could handle a leak that's 100 times worse than this. So ... we concluded that the smartest thing to do was to go fly the mission, and we could fly it safely."

Adding to the prelaunch drama, a recent anomaly aboard the ISS created a last-minute manifest change ahead of tomorrow's liftoff: A malfunctioning pump in the station's urine processor assembly has halted the ability to convert the ISS crew's urine back into drinkable water. A replacement for this part was already scheduled to launch on the next Northrop Grumman Cygnus cargo mission in August, but the pump's unexpected failure has necessitated an expedited delivery, NASA officials announced on Friday.

"We're in a position where we have to store urine onboard Station," Dana Weigel, NASA's ISS program manager, said during Friday's press conference. "We've got bags and tanks that we've got up there for this purpose, but we've got limited inventory." 

With the imminent arrival of two new astronauts to the station — Wilmore and Williams on CFT — NASA made the decision to get the needed part on orbit ASAP. But a small sacrifice had to be made.

To maintain a consistent mass for Starliner's mission, cargo roughly equal in weight to the replacement part — about 140 pounds (64 kilograms) — needed to be removed. In this case, Wilmore's and Williams' luggage took the hit, leaving the pair without spare clothes once they reach the space station. Thankfully, they won't be entirely without a change of attire. 

"We do have a lot of generic contingency clothes on board. So, not an issue," Weigel said.

Wilmore and Williams are scheduled to spend eight days aboard the space station, performing systems checks and generally putting Starliner through its paces. Starliner and its two-person crew are expected to land no earlier than Monday, June 10, in Willcox Playa, east of Tucson, Arizona. 

Undocking from the ISS that morning will be around 5:50 a.m. EDT (950 GMT), with landing at approximately 10:16 a.m. EDT (1416 GMT). If weather issues or other delays crop up, there is a backup landing opportunity on June 11 at White Sands Missile Range in New Mexico, with a landing time that day around 6:35 a.m. EDT (1035 GMT).

CFT is scheduled to lift off at 12:25 p.m. EDT (1625 GMT) tomorrow (June 1), with a forecasted 90% chance of favorable weather. NASA's livestream begins at 8:15 a.m. (1215 GMT), which you can watch here at Space.com. Coverage will continue through Starliner's rendezvous and docking with the ISS, set for around 1:50 p.m. EDT (1750 GMT) on Sunday (June 2).