Indian-origin astronaut Sunita Williams was suited and ready to launch on her third flight to space on Tuesday when Nasa and Boeing scrubbed the flight just hours before lift-off.
Starliner's maiden human spaceflight, dubbed the Crew Flight Test, to the International Space Station, was scrubbed due to a faulty oxygen relief valve on the second stage of the United Launch Alliance Atlas V rocket.
Minutes later, astronauts Butch Wilmore and Suni Williams exited the Starliner spacecraft at Space Launch Complex-41 at Cape Canaveral Space Force Station in Florida and returned to the astronaut crew quarters.
Nasa said that the launch was scrubbed as teams evaluated an oxygen relief valve on the Centaur Stage on the Atlas V.
The Atlas V rocket is manufactured by the United Launch Alliance, which said that the decision was taken due to observation on a liquid oxygen self-regulating solenoid relief valve on the Centaur upper stage. "The team needs additional time to complete a full assessment," ULA said in an update.
The abundant caution is owing to ULA launching astronauts for the first time on its rocket to space. It has a 100 per cent success rate with cargo and payload launches.
Nasa has announced that Boeing Starliner with the two astronauts will attempt to launch atop the ULA rocket on May 10.
"We are targeting no earlier than Friday, May 10, for the launch of the agency’s Boeing Crew Flight Test to the International Space Station, pending resolution of the technical issue that prevented the May 6 launch attempt," Nasa said.
It added that the delay allows teams to complete data analysis on a pressure regulation valve on the liquid oxygen tank of the Atlas V rocket‘s Centaur upper stage and determine whether it is necessary to replace the valve.
The United States is looking to certify the second spacecraft with orbital capabilities to launch and return humans to and from space. So far, SpaceX's Crew Dragon has been leading the commercial space race.
The Starliner spacecraft, named Calypso, can fly autonomously or be steered manually and is expected to rendezvous and dock with the space station on Wednesday.