Boeing's Starliner plans to launch its first crewed voyage on Saturday, a mission that has been in the works for a decade.
The new spacecraft is expected to lift off atop an Atlas V rocket from the Cape Canaveral Space Force Station in Florida at 12:25 p.m. ET. A livestream of the event will begin at 8:15 a.m. ET on NASA's website.
Weather conditions are 90% favorable for the launch, with the only concerns being wind and cumulus clouds, according to Mark Burger, launch weather officer for the 45th Weather Squadron at Cape Canaveral Space Force Station.
The mission, called Crew Flight Test, is the culmination of Boeing's efforts to develop a spacecraft to rival SpaceX's Crew Dragon capsule and expand U.S. options for transporting astronauts to the space station under the NASA's Commercial Crew Program. The federal agency's initiative aims to promote collaboration with private sector partners.
If successful, the flight would mark just the sixth inaugural voyage of a manned spacecraft in U.S. history, NASA Administrator Bill Nelson noted at a news conference in May. Experienced NASA astronauts Butch Wilmore and Suni Williams ride on board.
"It started with Mercury, then Gemini, then Apollo, the space shuttle, then (SpaceX's) Dragon - and now Starliner," Nelson said.
Williams will also make history as the first woman to fly such a mission.
Objectives of the Boeing Crew Flight Test Mission
After reaching orbit, the Starliner crew capsule carrying Wilmore and Williams will detach from the Atlas V rocket and fire its own engines. Starliner is expected to spend more than 24 hours traveling to the International Space Station, with docking expected to take place at 1:50 PM ET on Sunday.
The astronauts will test various aspects of Starliner's capabilities, including the performance of the spacecraft's thrusters, how their spacesuits function in the capsule and manual controls in case the crew needs to override the spacecraft's autopilot.
The astronaut duo will join the seven astronauts and cosmonauts already on board the space station and spend eight days in the orbiting laboratory.
The astronauts will test Starliner's "safe harbor" capability, designed to provide shelter for the space station crew if there is a problem on the space station, said Steve Stich, manager of NASA's Commercial Crew Program, who spoke at a news conference on Friday.
When it's time to go home, Williams and Wilmore will return on the same Starliner capsule and land at a location in the southwestern United States.
The earliest possible return for Williams and Wilmore is June 10, but other dates are available in case of inclement weather, Stich said.
If the spacecraft does not lift off on Saturday as planned, there are backup options to launch on June 2, June 5 and June 6, according to NASA.
A series of delays
Years of development problems, test flight problems and other costly setbacks have slowed Starliner's path to the launch pad. Meanwhile, Boeing's competitor under NASA's commercial program - SpaceX - has become the transportation provider for the space agency's astronauts.
This mission could be the last major milestone before NASA deems Boeing's Starliner spacecraft ready for routine operations to deliver astronauts and cargo to the space station.
"We look forward to flying this mission. This is a test flight; we know we're going to learn things," said Mark Nappi, vice president and program manager of the Commercial Crew Program at Boeing, in a statement. "We are going to improve, and that improvement starts with the Starliner-1 mission and it will be even better than the mission we are going to fly."
Starliner was only about two hours away from its first crewed launch attempt on May 6 when engineers discovered a problem with a valve on the second stage, or upper portion, of the Atlas V rocket. The entire stack, including the rocket and spacecraft, was rolled back from the launch pad for testing and repairs.
Subsequently, mission teams reported a small helium leak in the spacecraft's service module. The leak was traced to a part called a flange on a single reaction control system thruster, where helium is used to fire the thrusters.
The space agency said the leak did not pose a threat to any mission.
"We looked very carefully at what our options were with this particular flange," he said. "A fuel line, an oxidation line and a helium line all go into the flange, which makes it difficult to work on. It makes it almost unsafe to work on."
Instead of making a replacement to fix the leak, teams decided the helium leak was small enough to be manageable, Stich said.
"When we looked at this problem, it didn't come down to making transactions," Nappi said. "It came down to 'is it safe or not?' And it's safe. And that's why we decided that we could start flying with what we have."
During the launch countdown, mission teams will monitor the leak to see if it gets bigger. The teams have spent the past two weeks assessing acceptable levels for the helium leak and troubleshooting issues, which are laid out in the rulebook that engineers will use when they assess the leak Saturday morning, Nappi said.
While evaluating the helium problem, engineers also discovered a "design vulnerability" in the propulsion system - essentially identifying a remote scenario in which certain thrusters could fail if the vehicle leaves Earth's orbit, without a backup method to get home safely .
NASA and Boeing have since worked with the thruster vendor to come up with a backup plan to perform the deorbit burn, should that situation arise, Stich said at a May 24 news conference.
"We restored that redundancy for the backup capabilities in a very remote set of direct burn failures," Stich said.
Following a flight readiness review meeting on May 29, leaders from NASA, Boeing and United Launch Alliance, which built the rocket, "verified readiness for launch, including all systems, facilities and teams supporting the test flight," the space agency said .
The mission teams also scrutinized Starliner's parachutes after a parachute on Blue Origin's recent suborbital crewed flight failed to fully inflate. Starliner uses components similar to that parachute system, Stich said.
Blue Origin shared flight data with Boeing and NASA, and after assessing Starliner's parachutes, the team deemed them "good to fly."
Last minute packing
The space station experienced an anomaly on Wednesday that Starliner could help resolve, said Dana Weigel, manager of NASA's International Space Station Program.
A pump on the station's urine processor is defective.
"That urine processor takes all the crew's urine and processes it in the first step of a water recovery system," Weigel said. "It is then sent downstream to a water processor, which turns it into drinking water. The station is really designed as a closed loop."
The pump was expected to operate until the fall, and a replacement would fly aboard a cargo resupply mission scheduled for August. But the pump's failure "put us in a position where we had to store an awful lot of urine," Weigel said.
Now the urine must be stored in containers on board. To solve this problem, a replacement pump was quickly swapped into Starliner's cargo. The pump weighs about 150 pounds, so the team retrieved two crew suitcases from Starliner containing clothing and toiletries such as shampoo and soap, selected by Wilmore and Williams.
There is an emergency supply of general clothing and toiletries on the space station that the astronaut duo will use instead for their short stay, Weigel said.
Wilmore and Williams have been in crew quarantine since late April to protect their health ahead of launch, said NASA astronaut Mike Fincke, who will serve as pilot for the upcoming Boeing Starliner-1 mission that would follow a successful test flight .
"Butch and Suni have every confidence in our rocket, our spacecraft and in our operations and leadership management teams, and they are absolutely ready to go," he said.
CNN's Deblina Chakraborty contributed to this report.
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