WASHINGTON — NASA’s inspector general says the agency could save nearly $1 billion if Congress gives it the ability to choose the best launch vehicle for a mission to Jupiter’s moon Europa, rather than mandating the use of the Space Launch System.

In an Aug. 27 letter to the chairs and ranking members of both the full Senate Appropriations Committee and its commerce, justice and science subcommittee, NASA Inspector General Paul Martin said that language in past appropriations bills will delay the launch of the Europa Clipper mission and add to its cost.

“NASA’s renewed focus on returning humans to the Moon on an accelerated timetable means that an SLS will not be available to launch the Clipper mission to Europa before 2025 at the earliest,” he wrote in the conclusion of the seven-page letter. The fiscal year 2019 appropriations bill required that the mission launch by 2023 on an SLS.

“[W]e urge Congress to consider removing the requirement that NASA launch the Europa Clipper on an SLS and allow the Agency to decide whether to use an SLS or a commercial vehicle based on cost, schedule, vehicle availability, and impact on science requirements,” he added.

The letter is a follow-up to an audit of the Europa Clipper program that the Office of Inspector General published in May. That report warned of cost and schedule problems, including the conclusion that an SLS would not be available for a 2023 launch because of production timelines and assignment of the first two SLS vehicles for Artemis missions.

Since that audit, Europa Clipper passed a review known as Key Decision Point C, setting cost and schedule baselines for the mission. Martin stated in his letter that, while the spacecraft could be ready for launch in 2023, NASA established a 2025 launch date as the “Agency Baseline Commitment” for the mission because of the requirement to use the SLS and the lack of a vehicle available for the mission before 2025.

That approach will require — assuming the spacecraft is completed in 2023 as projected — placing Europa Clipper into storage for two years, at an estimated cost of $3 million to $5 million per month. Martin stated in the letter that, at the recent review, NASA allocated $250 million in reserves “to cover storage, personnel, and other associated launch delay costs.”

Launching in 2023 would eliminate the need for those reserves, saving $250 million from the estimated total cost of $4.25 billion established at that review. Martin noted in his letter that NASA estimates using a commercial launch vehicle — either a United Launch Alliance Delta 4 Heavy or SpaceX Falcon Heavy — could save an additional $700 million over an SLS launch. However, the audit published in May concluded that savings could be less than $300 million when accounting for the longer travel time, and thus operations costs, of launching Europa Clipper on a Delta 4 Heavy or Falcon Heavy.

“Congress could reduce risks to both the Europa mission and Artemis program while potentially saving taxpayers up to $1 billion by providing NASA the flexibility in forthcoming fiscal year (FY) 2020 appropriations legislation to determine the most cost effective and timely vehicle to launch the Europa Clipper mission in 2023 or whenever the satellite is completed,” Martin wrote.

A decision, he added, needs to be made “in the next few months” so that NASA has time to procure a commercial launch vehicle in time for a 2023 launch. An added complication is that ULA is planning to phase out the Delta 4 Heavy around that time, with the last of five missions on its remaining manifest slated to launch in 2023.

The letter was sent to Senate appropriators since they are still developing their fiscal year 2020 spending bills. The House passed its version of a spending bill for NASA in June, retaining the language from the 2019 bill that requires Europa Clipper to launch on SLS in 2023.

The Senate has, in the past, been less supportive of Europa Clipper or a follow-on lander mission than the House, not offering significant funding increases or requirements for launching the mission on SLS by a specific date. The House’s past support for the mission was linked to former Rep. John Culberson (R-Texas), who chaired the appropriations subcommittee that funded NASA and was a strong advocate for exploration of Europa. Culberson lost his bid for re-election last November.