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The world will have to wait at least another week before the long-awaited first crewed mission of Boeing’s Starliner spacecraft.

Launch was expected on May 17 after a previous delay, but teams discovered a small leak of helium in the spacecraft’s service module, according to a Boeing statement. The Starliner crews traced the leak to a thruster reaction control system flange where helium is used to allow the thrusters to fire.

“Teams are now targeting a launch date no earlier than 4:43 p.m. EDT on Tuesday, May 21 to complete additional testing,” the release said.

That mission, called the Crew Flight Test, could be the last major milestone before NASA judges Boeing’s spacecraft ready for routine operations as part of the federal agency’s Commercial Crew Program.

Both Boeing and NASA are developing tests and solutions for the leak, the release said. Boeing plans to bring the propulsion system to the pressurization it would reach just before launch and then allow the helium system to vent naturally.

A review of data from the May 6 launch attempt did not show any other problems, Boeing said.

NASA astronauts assigned to participate in the week-long mission to the International Space Station, Sunny Williams and Butch Wilmore, were quarantined before the flight, but returned to Houston on May 10 to spend time with their families during the operations before flights, Boeing said.

Williams and Wilmore will fly back to NASA’s Kennedy Space Center in Florida in the coming days, according to the release.

The occasion is a decade in the making — the culmination of Boeing’s efforts to develop a spacecraft worthy of carrying astronauts to and from the International Space Station under NASA’s commercial program.

The launch will mark just the sixth maiden voyage of a crewed spacecraft in U.S. history, NASA Administrator Bill Nelson noted at a news conference earlier this month.

“It started with Mercury, then Gemini, then Apollo, the space shuttle, then (SpaceX’s) Dragon — and now Starliner,” he said.

Boeing is designing the Starliner to rival SpaceX’s prolific Crew Dragon capsule and will join NASA’s drive to collaborate with private industry partners, expanding the US’s ability to transport astronauts to the International Space Station.

On board, Williams will also make history as the first woman to join such a mission.

Development delays, test flight problems and other costly setbacks delayed the Starliner’s path to the launch pad. Meanwhile, Boeing’s rival in NASA’s commercial crew program, SpaceX, has become the primary transportation provider for the space agency’s astronauts.

The launch was scheduled for May 6, with Williams and Wilmore already in their seats aboard the Starliner capsule when engineers discovered a problem and aborted the launch.

The United Launch Alliance team building the Atlas V rocket has identified a liquid oxygen tank pressure control valve that needs replacing. The valve has since been replaced, but the new issue of helium leaking into the Boeing spacecraft, which sits atop the rocket causing additional delay.

If the spacecraft launches next week as planned, it and the astronauts inside will separate from the Atlas V rocket after reaching orbit and begin firing their own engines. Starliner will likely spend more than 24 hours gradually making its way to the space station.

Williams and Wilmore will spend about a week aboard the orbiting laboratory, joining the seven astronauts and cosmonauts already aboard while the Starliner remains docked outside.

The two will then return home aboard the same Starliner capsule, which is expected to parachute to a landing in one of several designated locations in the southwestern United States.