"Senior NASA officials told GAO that at current cost levels, the SLS program is unaffordable," the report said on Thursday.
The SLS is the world's most powerful rocket created to enable NASA to return humans to the moon.
NASA should act on prior GAO recommendations to improve transparency and monitoring of the SLS program, including its long-term costs and affordability, the report said.
The report further said the original SLS cost baselines applied to the launch of the Artemis I test flight, but lacks baseline costs for future Artemis missions, which makes tracking future production costs for upcoming missions harder for NASA and Congress to monitor.
NASA plans to spend tens of billions of dollars to produce more SLS rockets, especially stronger versions, for use on Artemis missions into the 2030s, the report added.
NASA requested $11.2 billion in the fiscal year 2024 president's budget request to fund the program through fiscal year 2028, in addition to the $11.8 billion spent developing the initial capability.