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

The launch was scheduled to take place at 10:34PM ET from the Cape Canaveral Space Force Station in Florida, with a livestream available on NASA’s YouTube channel.

The United Launch Alliance’s Atlas V rocket will propel the Starliner aircraft to space, where it will stay at the ISS for one 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 Starliner, allowing it to ferry NASA astronauts between Earth and the ISS. This could make the Starliner a new rival for SpaceX, which has been carrying NASA astronauts to space since 2020.

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