The first crewed test of the Orion space capsule, intended to end America's reliance on Russian spacecraft to get U.S. astronauts to the International Space Station, has been pushed back two years to 2023, NASA has announced.

Although they haven't given up completely on the original test date of 2021, the space agency says a thorough review of the entire Orion program has suggested it may have to be delayed, with a new launch date for a manned test "no later than April 2023."

While NASA scientists and engineers will continue to work to try and make the original August 2021 goal, at this point, that's unlikely, space agency officials have acknowledged.

"It's not a very high confidence level, I'll tell you that, because of the history — the things we see historically pop up," said Robert Lightfoot, NASA Associated Administrator.

The necessity of ensuring that equipment can be reused from mission to mission and the development of flight software were both mentioned as possible complicating factors.

"We're not seeing any issues in those areas, but we have to account for those, because we have a lot of runway in front of us," Lightfoot said.

The Orion capsule, manufactured by Lockheed Martin, in tandem with NASA's new launch rocket, the Space Launch System, are intended to take crews beyond Earth orbit and perhaps to Mars.

In an unmanned test in 2014, the Orion capsule was sent to 3,600 miles above Earth, but the first unmanned test with the SLS has already been pushed back to 2018.

Development of Orion, designed to hold a crew of four astronauts, has already cost $4.7 billion, with NASA committing another $6.7 billion intended to carry the project to the 2023 first crewed flight.

That flight will stay close to Earth, while the crew systems, particularly life support systems, are checked out, NASA says.

Getting Orion into space with a crew aboard remains one of NASA's highest priorities, space agency officials say.

A second crewed flight may circle the far side of the moon, which could become a testing ground for a number of missions, says Bill Gerstenmaier, associate administrator for NASA's Human Exploration and Operations Directorate.

"Orion is a predominant, first key player in really allowing us to move human presence out of low Earth orbit," he says.

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