Watch NASA launch its first astronauts aboard the Boeing Starliner at 10:34 p.m. ET

NASA will try to send two astronauts into space aboard Boeing’s Starliner aircraft for the first time soon, but not today. As the astronauts settled in, authorities rubbed Launch attempt planned for Monday night around 8:30 pm ET due to “a faulty oxygen relief valve observation” on the Centaur second stage of the ULA Atlas V rocket. Another launch window will become available Tuesday night, but it’s unclear whether they’ll be able to reset that quickly.

The launch was scheduled to take place at 10:34 pm ET from Cape Canaveral Space Force Station in Florida, with a live stream available at NASA YouTube Channel.

United Launch Alliance’s Atlas V rocket will propel the Starliner aircraft into space, where it will remain on the ISS for a week. While on board, flight commander Butch Wilmore and pilot Suni Williams will test the Starliner’s capabilities, including launching, docking and its eventual return to Earth.

Boeing built the Starliner as part of its $4.2 billion contract with NASA, which encourages the development of new spacecraft from private companies through the agency’s Commercial Crew Program.

If all goes well during this flight, NASA will certify the Starliner, allowing it to transport NASA astronauts between Earth and the ISS. This could make Starliner a new rival to SpaceX, which has been carrying NASA astronauts into space since 2020.

Update May 6: Added details about Monday night launch attempt.