NASA flight controllers called off a previously scheduled approach to resolve issues with Starliner’s propulsion system. Starliner has 28 jets, known as its reaction control system, or RCS, engines, that help the spacecraft make small movements in orbit.

The crew on Starliner, NASA astronauts Butch Wilmore and Suni Williams, were told by NASA capsule communicator, or CAPCOM, Neal Nagata, that the 12:15 p.m. docking attempt had to be called off to resolve the spacecraft’s propulsion issue. CAPCOM Nagata noted that the ISS has a zero fault tolerance for a spacecraft control problem.

The agency and Boeing had to troubleshoot five of the RCS jets that were not operating. Four of Starliner’s malfunctioning jets were recovered after Wilmore and Williams worked with flight controllers to test fire the thrusters.

CAPCOM Nagata had the astronauts hold the spacecraft beyond the “keep out sphere,” an invisible boundary around the ISS that serves as a safety measure, while diagnosing the problematic thrusters.