WASHINGTON — NASA and Boeing will keep the CST-100 Starliner spacecraft at the International Space Station for at least four more days to perform additional testing of the commercial crew vehicle before it returns to Earth.

NASA announced June 14 that Starliner, previously scheduled to undock from the station as soon as June 18 on the Crew Flight Test (CFT) mission, will now depart no earlier than June 22 with NASA astronauts Butch Wilmore and Suni Williams on board.

The agency and the company will use the extra time “to finalize departure planning and operations,” NASA stated, including some additional tests of Starliner systems not originally planned for the spacecraft’s time at the station.

“We have an incredible opportunity to spend more time at station and perform more tests which provides invaluable data unique to our position,” Mark Nappi, Boeing vice president and commercial crew program manager, said in a statement about the latest extension. “We have plenty of margin and time on station to maximize the opportunity for all partners to learn, including our crew.”

Among the additional tests planned for Starliner is a brief test of aft-facing thrusters on the spacecraft. Seven of the eight thrusters will be fired in two pulses with a total duration of about one second. NASA said that test will demonstrate how the spacecraft will perform when docked to the station on future missions lasting up to six months.

Those thrusters, though, were a source of problems during Starliner’s approach to the station June 6. Five thrusters were “de-selected” by the spacecraft’s computer because of readings that were out of bounds. Controllers were able to restore four of them, allowing the docking to proceed, but officials said at the time they did not know what was causing the thrusters to go offline.

Other work planned during the extended stay includes making cabin air temperature measurements to compare to readings in the spacecraft’s life support system, conducting additional tests of the spacecraft hatch and forward window, and repeating a “safe haven” test to see how well the spacecraft can accommodate four people in the event of an emergency on the station.

“We are continuing to understand the capabilities of Starliner to prepare for the long-term goal of having it perform a six-month docked mission at the space station,” Steve Stich, NASA commercial crew program manager, said in a statement.

The announcement did not provide any updates on helium leaks in Starliner’s propulsion system. NASA confirmed June 11 that a fifth leak was detected hours after the spacecraft docked with the station, after one found weeks before launch and three more during the spacecraft’s approach to and arrival at the station. Helium manifolds in the propulsion system remain shut while the spacecraft is docked to the station, and NASA previously stated the spacecraft has more than enough helium to support undocking and deorbit maneuvers.

Starliner was originally scheduled to spend eight days at the ISS, but NASA postponed its original June 14 departure to June 18 in part to deconflict with a spacewalk scheduled for June 13. Space station managers said at a June 11 briefing that they wanted to avoid having major events — a spacewalk and spacecraft undocking — on consecutive days.

However, the June 13 spacewalk by NASA astronauts Tracy C. Dyson and Matt Dominick was called off shortly before it was scheduled to begin. NASA said that a “suit discomfort issue” caused the spacewalk to be scrubbed, but did not go into details about the issue or which astronaut experienced it.

NASA said late June 13 that the tasks planned for the postponed spacewalk will be carried out on another spacewalk previously scheduled for June 24, but did not disclose who will perform the spacewalk.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...