On Tuesday, NASA Inspector General Paul Martin wrote a rather extraordinary letter to the US senators who determine the budget for the space agency. In effect, the independent NASA official asked Congress to kindly not meddle in decisions that concern actual rocket science.

The letter addressed which rocket NASA should use to launch its multibillion dollar mission to explore Jupiter's Moon Europa, an intriguing ice-encrusted world that likely harbors a vast ocean beneath the surface. NASA is readying a spacecraft, called the Europa Clipper, for a launch to the Jupiter system to meet a 2023 launch window.

Congress, in appropriations legislation, has for several years mandated that the space agency launch the Clipper mission on the Space Launch System rocket—the large, powerful, and very costly heavy-lift rocket that has earned the sobriquet Senate Launch System because its design and construction was mandated by senators nearly a decade ago. However, the rocket remains under development and probably will not fly for the first time until mid- or late 2021 at the earliest. And NASA has said that if it is to have any chance of landing humans on the Moon by 2024, the goal set by US Vice President Mike Pence, it must have the first three SLS rocket launches for the Artemis Moon program.

"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," Martin wrote. "We 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."

Those commercial launch vehicles include United Launch Alliance's Delta IV Heavy rocket, and SpaceX's Falcon Heavy booster. As Ars Technica first reported last year, mission scientists have said the Falcon Heavy with a kick stage could get the Clipper to Jupiter without the need for an inner Solar System gravity boost.

“I will follow the law”

What's remarkable about this letter is that Martin is essentially having to tell Congress to stay in its lane—Congress sets the budget for NASA, certainly, but actual rocket scientists should be deciding the best way for the agency to get its valuable payload safely to Jupiter, on time, for optimal science.

NASA's chief, Jim Bridenstine, struck a tone of neutrality in a comment to Ars. "As the NASA Administrator, I will follow the law," he said.

As the agency's inspector general, Martin is independent of NASA. It is his job to both investigate NASA for potential wrongdoing or problems and to protect the agency from outside influences. If he sees that NASA is not spending money in an efficient or productive way, it is Martin's job to report it. He has been in the position for about a decade now, so Martin has enough experience to know when to speak up.

In this case, Martin highlighted concerns about the Europa mission launch date slipping to 2025 (or, more likely, later); and the mission's cost increasing by $1 billion due to using the more expensive SLS launch vehicle, as well as storage costs for the spacecraft during the interim.

A decision must be made fairly soon. According to Martin, NASA must begin the procurement process in the next few months if it is to ensure delivery of a commercial launch vehicle for a potential Clipper launch in 2023. We should get some answers from the Senate within a few months, as the Appropriations Committee produces its fiscal year 2020 budget.