NASA’s planned launch of astronauts aboard the Boeing Starliner spacecraft was called off Monday night due to an issue with the Atlas V rocket, further delaying the highly anticipated mission to the International Space Station.

Engineers worked tirelessly through the night to evaluate the problem, leaving astronauts Butch Wilmore and Suni Williams uncertain about when they might take to the launchpad again. The setback marks yet another delay for the Starliner programme, which has faced a series of costly setbacks in recent years.

The trouble began just hours before the scheduled liftoff, as a valve regulating pressure in the rocket’s oxygen tank began to buzz unexpectedly. Although this phenomenon had been observed in previous launches, safety protocols dictated no adjustments be made while astronauts were present, leading to the mission’s cancellation.

Tory Bruno, CEO of United Launch Alliance (ULA), emphasised the commitment to safety in a press conference Monday night. Despite the non-threatening nature of the valve issue, ULA opted not to take any chances, prioritising the astronauts’ well-being.

Following the cancellation, attention turned to assessing whether the valve requires replacement, a process that could further delay the mission. A decision on proceeding with a Tuesday night launch must be made promptly, with NASA and ULA closely monitoring the situation.

Boeing, which alongside SpaceX won the contract to transport astronauts to the ISS after the retirement of NASA’s space shuttles, has faced significant hurdles in bringing Starliner to fruition. Technical challenges, including software issues and hardware defects, have plagued the programme, resulting in substantial unexpected costs.