The Cost of a PROMISE

In a surprise announcement, NASA Administrator Jared Isaacman shared that the agency may send an engineering duplicate of the Perseverance Mars rover to the Moon.

“We've got this hardware that the taxpayers invested a lot in,” said Isaacman. “So the question was posed, what if we sent it to the Moon?”

NASA did not clarify which scientific priorities this project would address or how it would be funded given the 46% cuts currently proposed to the agency’s Science Mission Directorate.

The concept, called PROMISE, was presented as a sensible attempt to repurpose existing hardware for lunar exploration that would save both time and money. It’s a worthy experiment, but the outcome is far from certain. Qualifying a 5-year-old engineering backup for the rigors of spaceflight will take time and money (particularly if some parts need to be added or replaced), and the safety review process required for a nuclear-powered spacecraft usually takes many years. If NASA decides to pursue this project, PROMISE is likely to cost somewhere between $700 million and $1.3 billion and launch sometime in the early 2030s.

To reach this conclusion, I analyzed historical spending on prior missions, NASA procedural requirements, and detailed cost estimates for nuclear-powered lunar rovers done for the decadal survey. This is, emphatically, not a replacement for a detailed engineering cost assessment. That effort is currently underway at NASA's Jet Propulsion Laboratory. Instead, this estimate attempts to outline a reasonable cost range based on known requirements: launch costs, spacecraft processing and assembly, mission operations, nuclear power compliance, and science payload development. These quickly add up to hundreds of millions before any hardware upgrades are considered.

Regardless if one agrees with every decision below, it’s clear that this mission is far from a “freebie”, and will consume significant resources that could otherwise be allocated toward higher-priority scientific exploration.

OPTIMISM, and why it's not flight-ready

NASA maintains an engineering duplicate (not a one-to-one "copy" of flight hardware) of both its Curiosity and Perseverance Mars rovers, each with access to a simulated Martian landscape (the "Mars Yard" at JPL). These are used to test procedures and drive paths, and to help troubleshoot any hardware issues with the active spacecraft. 

Perseverance's testbed rover, named OPTIMISM, was deployed in 2021 and has the same size and characteristics as the Martian variant: it carries avionics, a computer, drive cameras, and a sampling system. But it was built to work on Earth, not Mars, and it lacks many components critical to operating in space. For example, according to a paper published by JPL engineers in 2022, the testbed has no sensors or failsafes for controlling its onboard temperatures. These sensors are normally embedded throughout a spacecraft's inner components; adding them after-the-fact is no simple feat. A space-qualified update of OPTIMISM would need to account for the huge temperature swings on the lunar surface, eclipses, heater control systems, heat conduction, and rejecting heat from the nuclear power source, and all these thermal issues interact with each other in complex ways.

The testbed rover carries no flight-ready scientific instruments, and it has no plutonium power source or communications system. Engineers simply plug it into the wall and communicate via cables. Further, many electronic parts may not be rated for, or shielded against, expected space radiation levels. Other components may not be rated for operation in a vacuum or in the presence of lunar dust (basically, microscopic shards of rock and glass that cling to everything via static electricity), or verified for electromagnetic compatibility with the other components around them.

Any new or updated items would need to be procured, integrated, and qualified for flight. Components that fail this qualification would have to be replaced — no easy task for older hardware that may no longer be actively made. All new hardware would have to be tested and integrated into the spacecraft, which could trigger extensive redesign to accommodate the weight, thermal dissipation, and other changed requirements necessary for use in space.

Historical examples are rare

Repurposing old hardware is not uncommon at NASA, even for space science missions. The Phoenix spacecraft, which was selected through an open and competitive scientific selection process, landed in the polar regions of Mars in 2008, and reused 70% of its mission hardware from an earlier lander mission canceled in 2000. But, unlike the OPTIMISM testbed rover, Phoenix reused hardware originally built to fly in space that had been stored in a cleanroom. According to NASA's own reporting, reuse succeeded because the hardware had been stored in a clean and safe environment with detailed documentation describing its fabrication and testing. Even then, the mission cost approximately $640 million when adjusted for inflation to fiscal year 2025 dollars, exceeding the project's original cost cap by 40%.

PROMISE is proposing to reuse engineering hardware not intended for spaceflight. There are no examples of NASA using non-flight-qualified hardware for a major science mission on this scale.

Cost estimates

Launch and landing: $234 million - $320 million [confidence: medium]

PROMISE would weigh about 1,000 kilograms, much larger than any other lunar rover and a much heavier payload mass than anything currently supported by NASA's commercial lunar delivery partners. Blue Origin's forthcoming Blue Moon Mark 1 lander advertises a comparable landing capacity when uncrewed (Lunar Starship and Blue Origin's crewed lunar landers would presumably be capable of delivering this as well, but both would likely cost significantly more than uncrewed variants). NASA's contract with Blue Origin to deliver a 950 kg Lunar Terrain Vehicle is used as our low estimate. That contract is valued at up to $234 million.

For comparison, SpaceX's Falcon Heavy launch contract for NASA's plutonium-powered Dragonfly spacecraft is worth $256 million. Astrobotic's Griffin CLPS contract sets the high-end potential delivery cost at $320 million.

These estimates may still be low. Since plutonium decays continuously, it continuously gives off heat. That heat has to go somewhere while the spacecraft is en route to the Moon. The Mark 1 may require modifications to handle this situation. Other ground equipment modifications may be required to facilitate loading of the plutonium power source and to meet additional safety requirements for payload processing.

It's important to note that New Glenn, the only rocket designed to launch Blue Origin's Mark 1 lander, is currently grounded due to a pad explosion earlier this year. Absent a flying rocket, NASA cannot complete the safety analysis required to launch nuclear material, a process that can take years.

There have been some rumors that the Mark 1 could be retrofitted onto a Starship booster or Falcon Heavy, but that would require significant engineering work and likely require new ground support structures for SpaceX hardware, at a minimum. And this novel launch configuration would still need to complete its own safety review before launching PROMISE. The costs of this are unknowable, but certainly far from zero.

Radioisotope power system and compliance: $78 million - $117 million [confidence: high]

The Perseverance rover is powered by 4.8 kilograms of Plutonium-238 (Pu-238) oxide, a radioactive material that generates heat, which the power system turns into electricity. Pu-238 does not occur naturally in any useful quantity and decays with a half-life of 87.7 years. It must be actively and continuously produced.

The United States has a limited quantity of Pu-238 (the exact amount is classified). The Department of Energy (DOE) manages this stockpile. For the past 15 years, NASA has paid the DOE to restart Pu-238 production and grow its stockpile, and now faces cancellation in the White House's FY 2027 budget request.

The OPTIMISM testbed rover doesn't need to worry about nuclear safety; it gets its power from the electrical grid. But PROMISE would require Plutonium-238 to explore the Moon and survive the bitter cold environment of two-week-long lunar nights.

Using Plutonium power, however, triggers a myriad of expensive and time-consuming environmental and safety reviews. According to a 2019 study by the Science Technology & Policy Institute, the reviews themselves add approximately $40 million to the cost of a mission and take, on average, 6.5 years to complete. The actual Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) hardware costs tens of millions more and takes years to procure.

Conveniently, NASA contracted to build two flight-ready MMRTGs back in 2021. One is assigned to the upcoming Dragonfly mission to Titan. The other was intended to be made available for a potential outer planets mission as recommended by the planetary science decadal survey. Based on NASA's public comments, this may be the "spare" RTG.

So, how much does NASA save by not having to procure a new MMRTG? It's hard to say, the contracts are not public. A 2017 report by the U.S. Government Accountability Office stated that using a single MMRTG added $77 million to a mission's cost; using two added $94 million. A rough decomposition implies a marginal hardware cost of around $17 million for the MMRTG and fuel itself, with the remaining $60 million being the overhead of managing nuclear material. Adjusted for inflation to 2025 dollars, that amount comes to $78 million, which we use as our low-end estimate.

A Presidential memorandum in 2019 attempted to streamline some of the review process for nuclear material, but it still requires an extensive safety review before a spacecraft is approved to launch. Dragonfly, the first nuclear-powered NASA mission to be developed after the streamlined process, has still taken nearly 6 years to complete this work.

For the high end, I add 50% to the baseline cost to capture the uncertainty of a novel review process for a new commercial launch system and potential ground system safety and processing upgrades.

New science payload: $50 million to $112 million [confidence: medium]

While no science goals were discussed for the PROMISE mission in the announcement or press release, we got a hint of NASA's thinking from NASA's Moon base program manager, Carlos García-Galán, highlighting the value of "long traverses getting into those very hard-to-reach areas" on the lunar surface.

The planetary science decadal survey recommended a lunar rover mission for this very purpose, called Endurance-A. The concept was to design a mid-sized rover that could travel hundreds of kilometers into dangerous areas of the Moon. It would collect lunar samples and then drive them back to a lunar base to be unloaded and prepared for Earth return by astronauts.

As part of its consideration for the last decadal survey, the Endurance-A mission concept was built out in more detail, assigned as a science payload, and subjected to rigorous cost assessments.

We use the reported Endurance-A mission payload and science team costs as our high estimate. To get a low estimate, we averaged comparative payload contract costs for CLPS science instruments as reported in public contract data, which came out to roughly $50 million. Both include multi-year funding for a small- to mid-sized science team.

Flight-qualification, refurbishment, and integration: $235 million - $531 million. [confidence: low]

This is, admittedly, the most uncertain variable, and my estimate is appropriately broad. PROMISE currently has no mission goals or operational requirements that allow us to constrain this work. The actual engineering assessment itself will provide details on what's usable, what's not, what needs to be replaced, and what needs to be tested in order to qualify for flight.

So we took a different approach and compared the integration and assembly portions of recent lunar missions to set our high and low bounds. For the low-end scenario, we can look to the VIPER mission, NASA's smaller, simpler, solar-powered lunar rover awaiting launch next year.

VIPER entered its final assembly, integration, and testing phase in June 2023. NASA planned to spend $235 million in this period (annual expenditures, adjusted for inflation, during this phase, plus an additional $84 million to prepare for a late 2025 launch). This represents a best-case scenario, in which little to no new engineering design is required, the necessary pieces are functionally assembled together without incident, and testing reveals no serious issues.

For the high-end estimate, I use the Phase A-D (design, build, integration, and test) estimates from the Endurance-A mission, minus the radioisotope and science instrument costs already accounted for earlier. These are adjusted to 2025 dollars.

Conclusions

Based on this analysis, I believe we can draw the following conclusions from the PROMISE concept at this early stage:

• Repurposing the OPTIMISM testbed for the Moon would cost many hundreds of millions of dollars. There is no plausible version of this mission that is "free," and even the low-end cost is equivalent to a mid-size planetary exploration mission.
• The use of radioisotope power adds monetary and schedule cost. Nuclear launch safety reviews average $40 million in compliance costs and take more than 6 years. Even an expedited safety review faces significant hurdles given the current state of Blue Origin's New Glenn rocket.
• PROMISE would consume an MMRTG currently reserved for a future science mission to the outer planets. It will also draw plutonium from a dwindling national stockpile.
• A 2028 launch is not credible. The nuclear paperwork alone suggests that the early 2030s are more reasonable. The technical uncertainties of converting an engineering test article into a spaceflight vehicle also add uncertainty to the schedule.
• Should PROMISE move forward, the best-case science scenario is to adopt the goals of the Endurance-A mission concept. This would address decadal priorities and leverage the unique capabilities of this hardware. Anything else makes it hard to justify the cost.