• Boeing's Starliner capsule made a delayed docking with the International Space Station on Thursday.
• There are now two U.S.-built crew spacecraft docked with the ISS for the first time, as Boeing's capsule joined SpaceX's Dragon at the space station.
• NASA delayed an initial attempt as five of the 28 thrusters that help control Starliner's movement in space were not operating, but four were recovered to clear a second docking attempt.

Boeing's Starliner capsule docked with the International Space Station on Thursday, a milestone for the company's crew spacecraft in a crucial test flight.

The spacecraft docked with the ISS at 1:34 p.m. ET after NASA called off a previous attempt to fix a thruster problem with Starliner. The docking marks the Boeing spacecraft's inaugural delivery of astronauts to the orbiting research laboratory.

There are now two U.S.-built crew spacecraft docked with the ISS for the first time. Boeing's Starliner joined SpaceX's Dragon capsule "Endeavour," which arrived in March.

Boeing's Starliner launched successfully on Wednesday to begin the crew flight test. The mission represents a final major step before NASA certifies Boeing to fly crew on operational missions.

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The spacecraft was once seen as a competitor to SpaceX's Dragon. However, various setbacks and delays have steadily slipped Starliner into a backup position for NASA, with the agency planning to have the companies fly astronauts on alternating flights.

Fixing the propulsion problem

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.