Boeing said on Monday that one of three parachutes failed to deploy during an otherwise successful safety test of its unmanned CST-100 Starliner crew capsule, being developed to ferry astronauts to and from the International Space Station, reported.

The deployment failure happened during a so-called pad abort test of a system designed to propel the crew to safety in the event of an emergency, Boeing spokesman Todd Blecher said by email.

“It’s too early to determine why all three main parachutes did not deploy,” Blecher said. “However, having two of three deploy successfully is acceptable for the test parameters and crew safety.”

Blecher said overall the test on Monday morning was successful.

Boeing and US space agency NASA said they were still targeting December 17 for the Starliner’s first unpiloted mission to the International Space Station.

NASA has picked Boeing and Elon Musk’s SpaceX as the main contractors to build rocket-and-capsule launch systems to return Americans to the orbiting research lab for the first time since the U.S. space shuttle program ended in 2011.

Parachute deployment has been among the top technical challenges both companies have faced that have raised questions over crew safety and pressured launch schedules. The parachutes are designed to slow down the capsules when they return to Earth at supersonic speeds.

Two main parachutes carried the ST-100 Starliner crew capsule to safety in New Mexico. Credit: NASA.

SpaceX is working to fly its Crew Dragon capsule on a first manned test flight into orbit in the first quarter of next year, after completing its unmanned test in March, the company and NASA have said.

According to Xinhua, at 9:15 a.m. US Eastern Time (1415 GMT), the five-meter, cone-like vehicle in the launch position on White Sands Missile Range in New Mexico had its four launch abort engines ignited, showed the NASA live broadcast.

The abort engines produced a combined 160,000 pounds (nearly 72,600 kilograms) of thrust, pushing the vehicle away from the stand into an altitude of about a mile (1,609 meters) above the ground. Also, the orbital maneuvering and attitude (OMAC) thrusters kicked in simultaneously to maneuver the spacecraft into the proper orientation for parachute deployment, the report said.

The ascent cover and forward heat shield protecting the spacecraft’s parachutes jettisoned roughly 19 seconds into flight, and then, drogue parachutes and two main parachutes deployed to slow the descent of the vehicle.

Finally, the Starliner’s service module separated from the crew module and the latter touched down with a cushion of inflated airbags about a mile north of the test stand, approximately 90 seconds after the launch.

The spacecraft service module, which has a total of 52 engines including those designed to give small directional changes in orbit, did not survive the test and was not planned to.