There were glitches with its propulsion system, but Boeing’s Starliner spacecraft and the two NASA astronauts it carried successfully docked at the International Space Station on Thursday afternoon.
The docking, at 1:34 p.m. Eastern time, was more than an hour later than planned, after the troubleshooting of several malfunctioning thrusters.
Starliner’s arrival came one day after the vehicle launched from Cape Canaveral Space Force Station in Florida. The docking was a major milestone for the test flight, which is to provide a final check that Starliner is ready to begin once-a-year operational flights to ferry NASA crews for six-month stays at the space station.
NASA hired Boeing to build the spacecraft as one of a pair of replacements for its retired space shuttles, but the company experienced years of costly technical problems and delays that kept it from flying Starliner with people on board.
Engineers expected to encounter problems during this flight, and they did.
Even before launch, a small helium leak was discovered in Starliner’s propulsion system. That led to several weeks of investigation.
Helium, an inert gas, is used to push propellants to the spacecraft’s thrusters. If too much is lost, the thrusters may not work properly.
Engineers determined that the leak appeared to be limited to one seal, but then they uncovered a “design vulnerability.” If a series of unlikely failures occurred with the propulsion system after undocking, Mr. Wilmore and Ms. Willians could conceivably have been stranded in orbit.
Boeing developed a backup procedure for Starliner to return to Earth if the unlikely failures did occur. Officials at Boeing and NASA decided that the helium leak did not need to be fixed and that the spacecraft could launch.
However, last night, two more helium leaks popped up.
Flows of helium to the leaking parts of the propulsion system were turned off, and engineers analyzed the problem while Mr. Wilmore and Ms. Williams slept. In the morning, mission managers decided to continue with the docking. The flow of helium was turned back on for docking maneuvers.
“Starliner is currently maintaining plenty of helium reserves,” Jim May, a Boeing engineer, said during NASA’s coverage of the Starliner mission. “We expect over 90 hours of free-flight propellant capability after undocking. Currently, the helium leak is not a safety issue for the crew or the mission.”
As Starliner approached space station, four of Starliner’s 28 maneuvering jets appeared not to be working correctly. That led to more troubleshooting, and Starliner missed its first docking opportunity.
The spacecraft and the astronauts waited for the next one, and then made their slow approach without further difficulties.