The first crewed launch of Boeing's Starliner spaceship to the International Space Station has been postponed again due to a technical snafu, the company said Tuesday.

The next attempt will be May 21 at the earliest.

The high-stakes mission was already called off last week, with two astronauts strapped in and ready to fly.

That was because engineers detected a problem with a valve that regulates liquid oxygen pressure on the Atlas V rocket meant to propel Starliner into orbit.

The valve was replaced and a new launch date of May 17 was set.

But a new problem emerged—a small helium leak in a part of Starliner called the service module, which holds instruments used to control and operate the spacecraft.

To prepare for exploring the moon during NASA's Artemis campaign, the agency is conducting a week-long field test in the lunar-like landscape of San Francisco Volcanic Field near Flagstaff, Arizona to practice moonwalk scenarios.

NASA astronauts Kate Rubins and Andre Douglas are serving as the crewmembers and wearing mockup spacesuit systems as they traverse through the desert, completing a variety of technology demonstrations, hardware checkouts and Artemis science-related operations.

During the test, two integrated teams will work together as they practice end-to-end lunar operations.