Watch live: Boeing will launch its spaceship on a do-over flight on Tuesday to prove it can carry astronauts for NASA

• Boeing is set to launch its Starliner spaceship to the International Space Station for NASA on Tuesday.
• Before flying astronauts, Starliner must complete this uncrewed test flight successfully.
• Boeing tried this flight in 2019, but a software issue prevented Starliner from reaching the ISS.

Boeing's Starliner spaceship will attempt to redeem itself on Tuesday, after botching its last major test flight in 2019.

The company's eventual goal is to fly astronauts to the International Space Station for NASA, the way SpaceX already does. Both companies developed their launch systems through NASA's Commercial Crew Program, a competition that awarded funding to private companies in order to develop new astronaut-ready spacecraft.

But before carrying people, the Starliner has to complete an uncrewed test flight to and from the ISS as part of NASA's certification process. That mission, called Orbital Flight Test 2, or OFT-2, is set to launch on Tuesday at 1:20 p.m. ET. Starliner will blast off atop an Atlas V rocket from NASA's Kennedy Space Center in Cape Canaveral, Florida.

Boeing first attempted this flight in December 2019, but it turned out that one of the spaceship's clocks was set 11 hours ahead of schedule. The clock prompted the spaceship's engines to fire too vigorously, too early - a move meant to come at a later stage of the mission.

That caused the spaceship to burn through 25% of its fuel, forcing Boeing to skip docking with the space station in order to save the Starliner from total failure.

Now, the company is confident that it has fixed the problems with its spaceship, so it's time for the do-over.

"Now's the right time. This team is ready to go, this vehicle is ready to go," Kathy Lueders, associate administrator of NASA's human-spaceflight directorate, said in a press briefing last week.

Watch Starliner launch live

NASA plans to broadcast the launch in the stream below starting at 12:30 p.m. ET.

If all goes according to plan, the Atlas V booster should fall away after about four minutes. That leaves the rocket's upper stage to give Starliner one final push into Earth's orbit before it, too, separates from the capsule. Starliner should orbit Earth alone overnight, slowly lining itself up to meet the ISS the next day.

"That's the part of this flight that, to me, is so critical: docking with station and then also, on the back end as well, going through that whole undock sequence," Steve Stich, who manages NASA's Commercial Crew Program, said in a briefing.

If the spaceship successfully latches onto a port on the ISS, astronauts on the station will then open its hatch and unload its cargo - science equipment and supplies. After that, the Starliner is scheduled to stay docked to the ISS to test out its systems and its endurance in space, until it returns to Earth a few days later.

The launch was originally set for Friday afternoon, but had to be delayed after a mishap on the ISS. Russia's new module, Nauka, fired its engines unexpectedly after docking to the station on Thursday, which rotated the ISS 45 degrees. Flight controllers regained control about an hour later.

"We wanted to make sure we had some breathing room to fully assess the situation on station before adding another vehicle," Lueders said in a briefing on Thursday.

Boeing's investigation into the failed flight revealed further problems

During Boeing's test flight in 2019, the company's engineers quickly reviewed the spacecraft's software while Starliner was still orbiting Earth. In doing so, they discovered and patched another issue - not the clock error - that could have been catastrophic.

As Starliner prepares to fall back to Earth, it's supposed to shed its service module - a cylinder containing the spaceship's main engines. That part is meant to fall away from the crew module, which holds the astronauts. But this second software error could have led the service module to bounce back and crash into the crew module. That could have sent the astronauts' capsule tumbling or significantly damaged its protective heat shield, making it unsafe to plow through the atmosphere.

The discovery of that issue prompted a NASA investigation into Boeing's coding and overall safety culture. NASA administrators at the time said the software issue was likely a symptom of larger problems at the company. But now, Stich said, "Boeing has an excellent safety culture."

Boeing has fixed both issues and changed some of the spaceship's communications software.

"There's always a little bit of that trepidation in you," Stich said. "This is spaceflight. The Atlas is a great vehicle. Starliner is a great vehicle. But we know how hard it is, and it's a test flight as well. And I fully expect we'll learn something on this test flight."

Why NASA needs Boeing

Assuming Starliner can make it to the ISS and back without major issues, its next step will be to do that again with astronauts onboard - a crewed test flight. If everything goes smoothly, that flight could launch by the end of this year, Stich said.

NASA is relying on both Boeing and SpaceX to replace the government-developed Space Shuttle, which stopped flying in 2011. After that, NASA relied solely on Russian Soyuz rockets to ferry its astronauts to and from the ISS. Then SpaceX's Crew Dragon passed the agency's tests, flying its first astronauts to the ISS last year. SpaceX has flown two full crews since then.

NASA hopes to add Starliner to its fleet soon so that the agency is no longer reliant on just one launch system.

This story has been updated with new information. It was originally published on July 28, 2021.