Project Prometheus: Nuclear Propulsion to the Moons of Jupiter

Nuclear power has had a long and complex history in outer space. Starting in the 1960s, both the US and USSR deployed full-on fission reactors aboard Earth-observing satellites; more recently, high-profile probes—Cassini, Curiosity, New Horizons—have all used safer but far less powerful radioisotope thermoelectric generators (RTGs), which extract energy from the waste heat of decaying isotopes. Nuclear energy for propulsion has sadly never progressed beyond conceptual studies and ground tests, hampered by safety concerns1 and limited funding. Nevertheless, the drawing boards are populated by numerous exciting, ambitious schemes—including expeditions to the Outer Planets far exceeding what we’ve been able to do with chemical rockets. In 2003, this was the main thrust of a NASA research study known as Project Prometheus, and engineers at JPL designed a true behemoth, the Jupiter Icy Moons Orbiter (JIMO), which would have sent a thirty-six-ton nuclear spacecraft to orbit Europa, Ganymede, and Callisto. It would have been the adventure of a lifetime, if not for tragic (yet predictable) budget cuts. Today we will explore the ins and outs of Project Prometheus—one of the most audacious of space history’s many unfunded proposals, and a missed opportunity to change the way we do planetary exploration.

The cover of the official NASA report. What could have been!

One of the greatest barriers to exploring Jupiter and beyond is that you can’t “live off the land,” at least as far as energy goes2. There just isn’t a ton of energy to be had. In Earth’s neighborhood, about 1366 watts of solar radiation falls on a given square meter, which makes solar power fairly economical. Out at Jupiter the figure is fifty. At Saturn? Fifteen. You need an energy source you can take with you, and keep using for the length of a years-long mission. This is why New Horizons, Cassini, and Galileo all used RTGs3 for electrical power. They still relied on chemical propulsion, though, which dramatically cut down on payload and maneuvers—imagine what a probe could do with hundreds of kilowatts of nuclear energy, for power and propulsion!

Project Prometheus imagined exactly that. The result was a design study on a substantially greater scale than most others, enabled by a powerful nuclear-electric drive. How many probes mass thirty-six tons and stretch fifty-eight meters from end to end? It was to be a long, spindly creature, with the shielded reactor situated on one end, the engines and scientific equipment on the other, and a vast wedge4 of radiators in between. This contemporary promotional video, replete with dodgy yet charming early-2000s CGI, provides an overview:

My main sources for this post are the Wikipedia page, ever-helpful, and the JPL official report, accessed here, which is a wealth of nitty-gritty details and diagrams. The nice thing about official NASA projects is that a lot of material ends up publicly available on the internet; with some of the other subjects I like to research, like nuclear missiles and submarines, information is much harder to come by.

Back to JIMO, though. Despite the advanced and high-powered energy source, the engines themselves were not particularly revolutionary. They were just scaled-up ion thrusters, like those on the Dawn probe and others, and the science behind them was very well understood in 2003. The probe would have carried twelve tons of xenon propellant to feed the ion drive, resulting in low-thrust but extremely efficient maneuvers. As such, JIMO’s delta-v would have been huge—though I can’t find the exact numbers—allowing it not only to fly more or less directly to Jupiter without gravitational slingshots, but also to enter orbit around three of its moons, which would permit far more detailed studies than Galileo‘s successive flybys.

JIMO’s trajectory, showing all major propulsive burns. Given its low thrust, it was not capable of a direct Hohmann orbit, instead opting for a slow spiral out of the Inner Solar System.

At Jupiter, the main scientific objective would have been to map the icy crusts of the three outermost satellites. Major studies were to fall under a trio of overarching themes—“Oceans,” examining subsurface liquid as well as the ice above it; “Astrobiology,” studying the chemical building blocks of life; and “Jovian System Interactions,” observing the atmospheres of Jupiter and the satellites, as well as the powerful Jovian magnetic field. Together these priorities were intended by the JPL team to build a complete picture of the outer Galilean moons. Beyond pioneering methods for deep-space nuclear propulsion, which was admittedly the main point of Project Prometheus and JIMO, the mission would have been a scientific feast, providing insights into many of the questions we always ask about Europa: how thick is the ice? How deep is the ocean? Could life exist down there, clinging to a hydrothermal vent?

The instruments exploring these questions were to be fairly standard, comprising a suite of cameras, a laser altimeter, and an ice-penetrating radar, plus magnetometers, spectrometers, dust counters, and other odds and ends. But the star of the show, if funded—which the designers weren’t sure about—would have been the Europa Surface Science Package, a tiny 375-kilogram lander for close-up study of Europa’s icy crust. Compare and contrast with a more current Europa lander which may fly late this decade. We’ve only ever seen a handful of planetary surfaces from the ground5—can you imagine the thrill of images streaming back from Europa’s icy crags?

Borrowed from my previous blog post on the subject of Europa! Though I personally imagine it would look less rocky and more like an enormous glacier.

The target date for launching JIMO was 2015, with the probe assembled in orbit by three Delta IV Heavy launches. At the end of that year, it would begin its spiral away from Earth, and it would arrive at Jupiter in 2021. Operations at Jupiter were to continue for four years. I have conflicting information on how this mission was supposed to end—Wikipedia states that the plan was to place JIMO in a stable Europa parking orbit, while the Project Prometheus report proposes instead to smash it into Europa’s surface6. Which one is correct? Well, I’ll take my facts from the horse’s mouth; the claim on Wikipedia isn’t even properly cited. So we have it on good authority that this vast, fifty-meter spacecraft, after a decade of service, would meet its end hurtling at multiple kilometers per second into a gigantic sheet of ice—it sounds extreme, yes, but it’s how I’d want to go, if I were a probe.

To its credit, Project Prometheus was not just a crazy one-shot proposal. It leveraged existing technology and fit well into the scheme of technological development in the early 2000s. The reactor was to be developed in consultation with Naval Reactors, which manages (as you might have guessed) fission systems for the US Navy, and a version of it would eventually have been deployed on the lunar surface, supporting Project Constellation (just recently proposed by George W. Bush). Also buried in the JPL report is a discussion of follow-on flights to various Solar System destinations; similar spacecraft to JIMO might have visited Saturn, Neptune, and the Kuiper Belt, or returned frozen samples from a comet. Nuclear electric technology is very well suited to such missions, requiring immense delta-v’s as well as long-duration, sun-independent power sources—if we accept it, we can perform missions that would be completely impossible otherwise.

NASA artwork for JIMO. The Galilean moons are, of course, arranged more photogenically than they ever would be in real life.

Tragically, JIMO and Project Prometheus got the axe in 2005, two years after the project started. Its downfall was NASA’s decision to allocate funds towards manned spaceflight7 and away from unmanned probes. There were also concerns about two of its more revolutionary aspects, namely the nuclear power source and on-orbit assembly—the mission was too complicated for NASA administrators’ comfort. This more conservative approach kept any use of nuclear power for propulsion strictly on the drawing boards; after JIMO’s demise, Europa exploration languished, too, at least until the recent approval of the Europa Clipper and ESA’s Jupiter Icy Moons Explorer.

JIMO certainly represents a lost opportunity for planetary science, and for spacecraft design more broadly. Yet even the design study advanced the state of the art. It articulated a vision for a probe like none seen before, using nuclear energy to achieve extraordinary feats of endurance and efficiency—my hope is that NASA will one day reinvent Project Prometheus for a new generation, and, by finally accepting a powerful new technology, open the far reaches of the Solar System to unprecedented discoveries.

Further Reading:


  1. This is your daily reminder that nuclear power is far safer than some would have you believe. Chernobyl, by far the worst nuclear incident, still only led to about fifty fatalities, and of the other deaths that have occurred at nuclear power plants, the vast majority of them were from mundane things like high-pressure steam or electrical accidents—radiation had nothing to do with it. Even the much-feared waste products are generally disposed of safely and sometimes reused (as in France). Concerns over the cost of nuclear plants are often valid—new ones certainly aren’t cheap—but environmental and safety objections frequently depend on appeals to irrational, scientifically ignorant fear.
  2. If you need fuel, though, you’re covered—ice is damn near everywhere out there.
  3. But not Juno, surprisingly. NASA was running low on plutonium, so it opted for grossly inefficient solar power.
  4. Why a wedge? Well, the reactor wasn’t going to be shielded all the way around, so there would have been radiation flying everywhere except for a cone towards the front of the ship. The radiators were contoured to fit within that cone. This is a relatively common feature of nuclear spacecraft design, called a shadow shield.
  5. Running tally: Venus, Earth (surprise!), the Moon, Mars, and Titan. Also there were a few landings on asteroids and comets, but those aren’t nearly as interesting.
  6. Planetary protection issue? Probably, but according to the paper’s authors the odds were fairly low. It helped that the probe would have been heavily sterilized on Earth and then soaked in radiation for the entire trip.
  7. And look how that went! Sixteen years later, and the Orion capsule still hasn’t flown with crew inside.

2 thoughts on “Project Prometheus: Nuclear Propulsion to the Moons of Jupiter

Add yours

  1. Would have been an awesome mission but lofting the reactor fuel and assembling in orbit would have been risky. Was the reactor too big to go in one lift? Seems like these missions might have to wait until an asteroid could be mined for fission fuel but not sure how practical that would be. On the plus side, you should have easier access to all those iron-loving elements. On the minus side, you don’t have tectonics and water to concentrate ores. You would still get some magma fractionation but I don’t think that helps for the metallic elements.

    Liked by 1 person

    1. The reactor itself would have gone up in one payload—the whole reactor module was only six tons, well within the capabilities of a Delta IV. And besides, in the event of a launch failure it would almost certainly have plummeted into the ocean as one piece. A rocket explosion, though spectacular, can’t necessarily atomize a well-built plutonium power source.

      Regarding asteroid mining, I definitely agree. We humans on Earth are fortunate to have had billions of years of geological processes concentrate ores in usable quantities. Mining the homogenous rocks of the asteroids and other Solar System locations is going to require sifting through a *lot* of material!

      Like

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