Mission teams are working to understand what caused an automatic hold of Starliner's launch minutes before liftoff and focusing on safely extracting the crew. Currently, the rocket and spacecraft are in a safe configuration, according to the NASA broadcast.

Later today, the rocket tanks will be drained of propellant, which is called detanking.

Starliner's next launch opportunity is 12:03 p.m. ET Sunday, but mission teams have not yet said whether they will attempt a launch tomorrow.

Meanwhile, mission teams will analyze the countdown sequence to find out what caused the hold, which remains unknown at this time.

The Ground Launch Sequencer, or computer that essentially launches the rocket, performs key actions during the final 10 minutes before launch, like pressurizing the fuel tanks for launch, transitioning the vehicle to its onboard batteries from ground electrical systems and configuring other systems for launch.

Today's historic launch of Starliner has been scrubbed.

An automatic hold triggered by the ground launch sequencer, or the computer that launches the rocket, stopped the countdown clock at 3 minutes and 50 seconds ahead of launch.

The cause of the automatic hold is not clear, but mission teams estimate a 24-hour turnaround time, according to the NASA broadcast.

Now, teams will prepare to safely extract the crew from the capsule.

Just as Starliner entered the final minutes of countdown to launch, an automatic hold was triggered by the ground launch sequencer, according to the NASA broadcast.

There is no indication yet as to what caused the hold, but the countdown clock remains at four minutes to launch.

Just before the hold, all of the mission teams participated in the "go/no go" poll and declared that Starliner was "go" for launch.

"This is a great day to be proud of your nation," Wilmore said from the capsule after the poll.

Although today's event will feature all the trimmings of a standard astronaut launch (SpaceX has carried out 13 of them in the past four years) — this is not a typical mission.

The inaugural Starliner flight is still considered a test — a step on the way to NASA finally deeming the spacecraft ready for regular operations.

As such, astronauts Suni Williams and Butch Wilmore will put the vehicle through its paces while on board.

NASA recently shared a rundown of some of the items on the checklist:

Williams and Wilmore have taken a measured approach when responding to questions about the Starliner spacecraft’s development woes. 

“I understand it when you say ‘setback,’” Wilmore said during a news conference in May. “But honestly, with all the discovery — that's what we would term it — that we've had, it's been steps forward. 

“It's not been a setback, it's been pressing forward,” he said. “And our families have lived that with us.”

Williams added that she is prepared to go into Saturday’s mission with the expectation that small issues may arise.

“We always find stuff — and we are going to continually find stuff,” she said. “Everything's not going to be absolutely perfect as we fly the spacecraft."

NASA chief Bill Nelson also spoke about the historic (and experimental) nature of this mission during a news briefing in May:

"Because it is a test flight, we give extra attention," Nelson added.

Make no mistake: Today's launch is a massive deal for Boeing.

The company's airplane woes are one thing, but Starliner has had its own issues.

The spacecraft was developed under NASA's Commercial Crew Program, which got rolling in 2014.

That's when the space agency awarded Boeing and SpaceX contracts to develop spacecraft capable of getting astronauts to the International Space Station, taking the baton from the retired Space Shuttle Program. (Boeing got $4.2 billion, while SpaceX got $2.6 billion.)

Boeing — a close partner of NASA's practically dating back the agency's inception — was expected to lead the charge.

But SpaceX emerged as the forerunner, finishing its Dragon development in 2020.

Boeing, however, has faced all sorts of setbacks: A botched test flight in 2019, myriad delays and development hang-ups, and embarrassing missteps. At certain points, some have even questioned whether Boeing would continue Starliner development at all.

For its part, Boeing has always publicly maintained that it was committed to Starliner and the company's deal with NASA.

And now that a Starliner is sitting on the launchpad ready to carry two astronauts into the unforgiving void of outer space — it's clear Boeing meant it.

At a recent news conference, Mark Nappi, vice president and Starliner program manager at Boeing, reflected on the spacecraft's journey:

For this mission, NASA sees itself as just a customer, buying the use of Starliner from Boeing.

That's how Boeing's Starliner contract was originally set up. NASA's Commerical Crew Progam — which the Starliner and SpaceX's Crew Dragon operate under — was designed to hand over responsibility to the private sector.

But Boeing is also buying some services back from NASA: Namely, in mission control: Boeing is paying NASA for the use of its flight operations team.

“We buy the service from NASA with mission operations," Mark Nappi, vice president and Starliner program manager at Boeing, said during a news briefing prior to the May 6 launch attempt.

But, he emphasized, Boeing is still the top dog when it comes to calling the shots.

The mission management team is still led by a Boeing flight director, Nappi said, and NASA personnel will report up to company officials.

In total, Starliner is hauling about 759 pounds of cargo to the International Space Station.

In addition to the replacement pump that will help the closed loop of water continue to flow on the station, the crew is bringing food, clothes, medical supplies, exercise gear, supplies and tools for the vehicle, as well as photo and media equipment.

But some special, more personal items are also packed inside the capsule.

One of those items is a thumb drive, which holds 3,500 images of artwork from children living across 35 US states and 66 different countries.

Wilmore brought two gold rings that resemble the US Navy astronaut pilot wing pin, which he had made for his father and brother, according to NASA. And he is bringing shirts from Tennessee Technological University and the University of Tennessee, his alma maters.

Williams also brought shirts from the US Naval Academy and the Sunita L. Williams Elementary School, located in Needham, Massachusetts, which she considers her hometown, as well as a diver pin and two dog tags from her Labrador retrievers.  

After troubleshooting the issue in May, the space agency said a helium leak recently found within the spacecraft service module did not pose a threat to a mission.

“We looked really hard at what our options were with this particular flange (the part where the leak is located),” he said. “A fuel line, an oxidizer line and a helium line all go into the flange, which makes it problematic to work on. It makes it almost unsafe to work on.”

Rather than making a replacement to fix the leak, the teams decided that the helium leak is small enough to be manageable, Stich said.

“When we looked at this problem, it didn’t come down to making trades,” Nappi said. “It came down to, ‘is it safe or not?’ And it is safe. And that’s why we determined that we could go fly with what we have.”

During the launch countdown Saturday morning, mission teams monitored the leak, and so far, no issues have been reported.