WASHINGTON — The first flight of NASA’s Orion spacecraft with people on board could be delayed by more than a year to early 2023, agency officials said Sept. 16.

NASA announced that the Orion program had achieved a milestone known as Key Decision Point C (KDP-C), completing a technical and programmatic review of the spacecraft designed to carry astronauts beyond Earth orbit. That review was similar to one completed by Orion’s launch vehicle, the Space Launch System, in August 2014.

The KDP-C review found that there is a 70-percent chance Orion will be ready for its first crewed mission, Exploration Mission 2 (EM-2), no later than April 2023. The review also set a cost baseline for Orion from October 2015 through EM-2 of $6.77 billion. That figure excludes the funding spent on Orion to date, including several billion dollars during the Constellation program prior to its 2010 cancellation.

The April 2023 date is a delay of more than 18 months from the earlier target date of August 2021. NASA is keeping that 2021 date as an “aggressive” internal goal for that mission, even while acknowledging the chance of being ready to fly then is low.

“I’ve asked the team to keep working towards the August 2021 date,” NASA Associate Administrator Robert Lightfoot said in a call with reporters. “They’re planning for that and working to that date, recognizing that we have a much lower confidence in that date.” He said NASA did not calculate a specific confidence level for August 2021, but admitted that “it’s not a very high confidence level.”

There was no specific issue with Orion’s development that is causing the delay, and both Lightfoot and William Gerstenmaier, NASA associate administrator for human exploration and operations, praised the program for the progress it has made. Instead, they believe that “unknown unknowns” will occur during the development of the vehicle, based on experience with past programs, that will cause the schedule to slip.

Lightfoot specifically mentioned potential issues reusing hardware developed for testing of Orion, construction of a structural test article, and software development as all key areas that could take longer than currently anticipated. “Right now we’re not seeing issues in those areas, but we have to account for those because we have a lot of runway in front of us,” he said.

One example of potential slips Gerstenmaier mentioned was the redesign of panels that form the conical portion of Orion. NASA decided to reduce the number of panels to reduce weight and save production time. Instead, the panels, now larger than in earlier designs, have become harder to handle because they tend to “unbend” from their desired curved shape.

“If you look at the complexity of what we’re doing in building this spacecraft, there will be some unknowns that show up,” Gerstenmaier said. “To protect for those, we went with the later date of 2023.”

The likely delay of EM-2 will not affect the first, uncrewed flight of Orion on SLS, designated EM-1 and tentatively scheduled for the fall of 2018. Lightfoot said a formal date for the mission will be made after the SLS and Orion programs, as well as associated ground systems, complete separate critical design reviews by the end of this year. “There’s nothing right now that gives us any indication different from the fall of ’18 for everybody being ready,” he said.

Lightfoot said the KDP-C cost and schedule estimates were based on the Obama administration’s budget projections. However, Congress has added funding to Orion in the past: NASA requested $1.053 billion for Orion in its fiscal year 2015 budget proposal, but Congress appropriated $1.194 billion for the program in the final 2015 omnibus spending bill.

Lightfoot said additional funding could accelerate Orion’s schedule, but did not specify by how much when asked about it at the briefing. He added that it also depends when the funding shows up. “I can’t get it all in the last year,” he said.

One key member of Congress swiftly criticized the administration for not providing Orion with enough funding to support a 2021 launch. “Once again, the Obama administration is choosing to delay deep space exploration priorities such as Orion and the Space Launch System that will take U.S. astronauts to the Moon, Mars, and beyond,” said Rep. Lamar Smith (R-Texas), chairman of the House Science Committee, in a Sept. 16 statement.

Smith called for funding NASA at the level specified in a NASA authorization bill that his committee approved earlier this year. That bill authorized $1.2 billion for Orion in fiscal year 2016, $104 million more than what the administration requested. An appropriations bill passed by the House in June funded Orion at the administration’s level, while a Senate version provides $1.2 billion for Orion.