WASHINGTON — The White House warned Congress in a recent letter that without funding increases for its exploration programs, NASA won’t be able to achieve the goal of landing humans on the moon in 2024.

The Oct. 23 letter from Russell Vought, acting director of the Office of Management and Budget (OMB), to Sen. Richard Shelby (R-Ala.), chairman of the Senate Appropriations Committee, addressed overall issues with appropriations bills that Shelby’s committee had approved in recent weeks, including the Commerce, Justice and Science (CJS) bill that funds NASA.

“The Administration appreciates the Committee’s continued support for space exploration, reflected in the $22.8 billion provided in the bill for NASA,” Vought wrote in the letter, first reported by Ars Technica.

He took issue, though, with the funding provided for exploration research and development, which includes work on lunar landers and the lunar Gateway. “However, the $1.6 billion provided for exploration research and development (R&D) is insufficient to fully fund the lander system that astronauts would use to return to the Moon in 2024,” he wrote. “Funding exploration R&D at the $2.3 billion level requested in the FY 2020 Budget is needed to support the Administration’s goal of returning to the Moon by 2024.”

The $1.64 billion included in the Senate bill for exploration R&D is slightly higher than the original administration request for $1.58 billion released early this year. However, in May the White House requested an additional $1.6 billion in funding for NASA to achieve the goal of returning humans to the moon by 2024. That included $1 billion for lunar landers and $132 million for exploration technology, but a cut in Gateway funding of $321 million. Those changes increase the total exploration R&D request to more than $2.3 billion.

The House, which had already developed its CJS bill by the time the budget amendment was released, did not act on the request and provided only $962 million for exploration R&D, approximately the same as in 2019. Vought wasn’t as specific in a May 21 letter to House appropriators about that bill, saying only that the White House was “very disappointed that the bill provides far less funding than is needed to support the Administration’s goal of a near-term human lunar landing.”

Among the programs affected by shortfall of funding for exploration R&D is a lunar lander program. The Senate bill provides $744 million for that work, less than the requested $1 billion.

Other exploration programs, though, received increases far greater than requested. The budget amendment in May sought to increase funding for the Space Launch System and Orion by $651 million from the original request. The Senate bill, though, provided an combined increase of $951 million for the two programs, including an $810 million boost for SLS alone.

Vought took issue with another SLS-related provision in the Senate bill that directs NASA to launch the Europa Clipper mission on an SLS. “The Administration is deeply concerned that this mandate would slow the lunar exploration program, which requires every SLS rocket available,” he wrote. “Unlike the human exploration program, which requires use of the SLS, the Europa mission could be launched by a commercial rocket.”

The letter also offered an unexpectedly high cost estimate of a single SLS launch. “At an estimated cost of over $2 billion per launch for the SLS once development is complete, the use of a commercial launch vehicle would provide over $1.5 billion in cost savings,” Vought wrote.

NASA has been reticent to provide cost estimates for a single SLS launch, although ballpark figures have been around $1 billion. NASA, in its fiscal year 2020 budget request, said launching the mission on a commercial rocket, such as a Delta 4 Heavy or Falcon Heavy, “is estimated to result in over $700 million in savings compared to use of an SLS rocket.”

A NASA Office of Inspector General (OIG) report in May played down the cost savings between SLS and commercial alternatives largely because it offered a much lower cost estimate for the SLS. That report estimated an SLS launch at $876 million, versus $450 million for commercial alternatives.

However, in a follow-up letter in August, OIG said NASA could save up to $1 billion by launching Europa Clipper on a commercial vehicle versus the SLS, adding that NASA needed to decide how to launch the mission in the next few months in order to procure a launch vehicle in time to support a potential 2023 launch.

While there is disagreement about the cost differential, both NASA and its inspector general agree that an SLS would not be available to launch Europa Clipper in 2023, a date mandated in the House appropriations bill.

“Given our current rate of production, we will have three SLSs available, and third one would be for Artemis 3 that takes us to the moon in 2024,” NASA Administrator Jim Bridenstine said at a House appropriations hearing Oct. 16 when asked about accelerating production of the rocket. He was doubtful a fourth SLS could be produced by then when asked about the availability of the rocket for launching commercial lunar landers. “Adding an additional SLS into the mix? I’m not confident that could happen.”

The Senate did not act on Vought’s letter, making no significant changes to the CJS spending bill when it passed the full Senate Oct. 31.