NASA's Orion spacecraft awaits the U.S. Navy’s USS Anchorage in the Pacific. (AP/AP)

NASA’s $23 billion program to build a powerful new rocket and spacecraft that could carry astronauts to the moon or even Mars faces potential cost overruns and schedule delays, a government watchdog warned Wednesday.

The findings released in a pair of reports by the Government Accountability Office are the latest concerns about the Space Launch System rocket and the Orion spacecraft, and come as NASA “struggles with poor cost estimation, weak oversight and risk underestimation,” the GAO said.

While there are technical difficulties that are expected to be associated with complex vehicles designed to launch humans deep into space and keep them alive for an extended period, the GAO also found management challenges that could create future problems and lead to cost overruns.

The SLS and Orion would be NASA’s first vehicles in more than 40 years capable of sending humans beyond what’s known as low Earth orbit, where the International Space Station circles Earth at an altitude of about 250 miles.

The pair are scheduled to make their maiden flight together at the end of 2018 in a no-crew mission around the moon that would last more than three weeks. Then the first mission with astronauts on board could come as early as 2021, although NASA has warned that could be delayed by two years.

The SLS, being manufactured by contractors Boeing, Orbital ATK and Aerojet Rocketdyne, would be a monster of a rocket, with versions as tall as 365 feet.

In 2014, Orion, manufactured by Lockheed Martin, flew for the first time but launched atop a different heavy-lift rocket. The spacecraft traveled 3,600 miles, more than any other one designed for humans had gone since the last moon landing in 1972.

NASA heralded it as a history-making success. But the GAO warned that NASA’s attempts over the past two decades to develop a system to take astronauts into deep space “have ultimately been unsuccessful.”

The agency has not flown a human beyond low Earth orbit since the Apollo era. And currently, NASA relies on the Russians to fly American astronauts to the space station, although Boeing and SpaceX are developing spacecraft that would restore human flight to the space station from U.S. soil as soon as next year.

The predecessor to the SLS and Orion program, known as Constellation, was killed by the Obama administration when it was over budget and behind schedule.

The GAO said that some challenges are inherent in a program as big and complicated as SLS and Orion but that they merited scrutiny because of past failures.

The GAO found that cost overruns for Orion could be as high as $707 million and that work is “not being accomplished as scheduled.”

It also found challenges with the capsule’s software and heat shield, and said that the space capsule’s cost and schedule “estimates are not reliable.”

At a recent congressional hearing, Sen. Ted Cruz (R-Tex.) said NASA had spent about $13 billion between 2005 and 2010 before the Constellation program was canceled. And he asked what needed to be done to ensure than SLS and Orion “don’t face the same.”

William Gerstenmaier, NASA’s associate administrator for human exploration and operations, said the new program is in a different position, with a lot of the development already completed.

“The teams have made tremendous progress moving forward,” he said. He urged Congress to remain focused on the agency’s “long-term vision” and to resist temptations “to jump to the next shiny object.”

As for the GAO’s findings, he said in a letter attached to the report that many of the concerns are being addressed.

In a podcast accompanying the reports, the GAO’s Cristina Chaplain said the goals of SLS and Orion “are still a little unclear.”

NASA says it plans to fly humans to Mars by the 2030s.

But Chaplain said, “There’s still a lot of debate about is Mars the right place? Or should we be going to the moon? And how firm should these plans be?”