WASHINGTON – NASA Administrator James Bridenstine said he still expects astronauts will fly from U.S. soil to the International Space Station by the end of next year even though an uncrewed test flight scheduled for Jan. 7 now could slip into the spring.

Bridenstine's acknowledgment that January is a "very low probability" window is the first time the agency has publicly cast doubt on the timing of the scheduled launch from Kennedy Space Center in Florida. The test flight of the SpaceX rocket and capsule is a key step in NASA's efforts to resume U.S. transport to Earth's orbit nearly a decade after the space shuttle was mothballed.

The administrator attributed the delay to challenges with several components, including landing parachutes. Some of those systems could be tested without flying them on the initial flight.

It's a matter of determining "what configuration are we willing to accept as an agency and are we willing to waive certain items (and) how do we test those items," Bridenstine told reporters at NASA headquarters.

But he said the test flight "will certainly be in the first half of 2019," a schedule that still would accommodate a crewed flight by the end of the year.

Earlier this year, Bridenstine said that "without question," such launches would resume in 2019.

NASA's multi-billion-dollar program to resume rocket launches to the ISS, known as Commercial Crew, has been a priority since Atlantis completed the last Space Shuttle mission in 2011.

Since then, U.S. taxpayers have been paying Russia to transport astronauts to the orbiting lab on Soyuz rockets at a cost that now exceeds $80 million per seat. At the same time, two companies have been working with NASA – Boeing and SpaceX – to develop a shuttle replacement at a cheaper price.

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Boeing also is working on a test flight next year.

The test flight of SpaceX's Falcon 9 rocket and its Crew Dragon spacecraft is designed to assess ground systems, docking and landing operations. It also will provide valuable data toward NASA certifying SpaceX’s crew transportation system for carrying astronauts to and from the space station.

The program to replace the space shuttle has not gone smoothly.

Even if a crewed launch arrives at the space station in 2019, it would be four years behind schedule.

When Commercial Crew was unveiled in 2010 under the Obama administration, the target date was 2015. But a lack of full funding from the Republican-controlled Congress led to delays. By the time Boeing and SpaceX won contracts in 2014, the date was pushed back to 2017. Further delays pushed schedules into next year.

Bridenstine also said he's confident the space station is "absolutely" safe.

On Aug. 29, ground teams detected what NASA said was a “minor” drop in pressure aboard the laboratory complex orbiting 250 miles up and housing three NASA astronauts, two Russian cosmonauts and a European.

An investigation found the leak to be an isolated incident.

"This has been evaluated many different ways," Bridenstine said. "It is not leaking now.it has been sealed up and we're all in agreement that it is safe."