MMMmmmm….Plutonium cake… (Credit: Department of Energy).

(Editor’ s Note: We’d like to thank fellow backyard astronomer Clay M. Davis with giving us the “Nuclear Physics 101″ help embedded in this post!)

Amidst the impending decade of transition for the United States space program, a quiet fact is slowly rearing its ugly head, one that will have wide implications for the future of manned & unmanned space exploration. Specifically, NASA is running out of juice to explore the solar system. And that’s not a figurative or political metaphor, but we mean “juice” in terms of real, honest-to-goodness fuel in the form of plutonium-238. But first, a little background/history of this often maligned substance and its role in space;

When leaving our fair planet, mass is everything. Space being a harsh place, you must bring nearly everything you need, including fuel, with you. And yes, more fuel means more mass, means more fuel, means… well, you get the idea. One way around this is to use available solar energy for power generation, but this only works well in the inner solar system. Take a look at the solar panels on the Juno spacecraft bound for Jupiter next month… those things have to be huge in order to take advantage of the relatively feeble solar wattage available to it… this is all because of our friend the inverse square law which governs all things electromagnetic, light included.

To operate in the environs of deep space, you need a dependable power source. To compound problems, any prospective surface operations on the Moon or Mars must be able to utilize energy for long periods of sun-less operation; a lunar outpost would face nights that are about two Earth weeks long, for example. To this end, NASA has historically used Radioisotope Thermal Generators (RTGs) as an electric “power plant” for long term space missions. These provide a lightweight, long-term source of fuel, generating from 20-300 watts of electricity. Most are about the size of a small person, and the first prototypes flew on the Transit-4A & 5BN1/2 spacecraft in the early 60’s. The Pioneer, Voyager, New Horizons, Galileo and Cassini spacecraft all sport Pu²³⁸ powered RTGs. The Viking 1 and 2 spacecraft also had RTGs, as did the long term Apollo Lunar Surface Experiments Package (ALSEP) experiments that Apollo astronauts placed on the Moon. An ambitious sample return mission to the planet Pluto was even proposed in 2003 that would have utilized a small nuclear engine.

New Horizons in the laboratory with the RTG (on left) attached. (Credit: NASA).

This is not without risks; for example, the aborted Apollo 13 mission had to ditch their Lunar Module Aquarius in the Pacific Ocean near Fuji along with its nuclear fueled payload on return to Earth reentry (which by the way survived intact as intended and is somewhere in the deep ocean where it can do no harm). Pu²³⁸ has a half life of 87.7 years and a 55 kg mass can power a spacecraft like the Pluto-bound New Horizons mission for decades. The next spacecraft headed for Mars, the SUV sized Mars Science Laboratory, will also contain an RTG as it explores the environs of Gale crater, and doubtless this launch slated for late 2011 will draw a scattering of protesters as did the Galileo, New Horizons, and Cassini missions…

Yes, plutonium is nasty stuff. It is a strong alpha-emitter and a highly toxic metal. If inhaled, it exposes lung tissue to a very high local radiation dose with the attending risk of cancer. If ingested, some forms of plutonium accumulate in our bones where it can damage the body’s blood-forming mechanism and wreck havoc with DNA. NASA had historically pegged a chance of a launch failure of the New Horizons spacecraft at 350-to-1 against, which even then wouldn’t necessarily rupture the RTG and release the contained 11 kilograms of plutonium dioxide into the environment. Sampling conducted around the South Pacific resting place of the aforementioned Apollo 13 LM re-entry of the ascent stage of the Lunar Module, for example, suggests that the reentry of the RTG did NOT rupture the container, as no plutonium contamination has ever been found. The same went for another failure, that of the nuclear powered Nimbus B-1 weather satellite in 1968, in which case the RTG was recovered intact. Yes, the Soviets have had a few release failures historically (see below), but NASA knows its business and has a long standing track record of safely handling nuclear material. RTG’s are designed to withstand intact uncontrolled re-entry, spacecraft explosion, booster explosion, and a host of other high energy events without releasing the contents of the fuel package.  Of course, all stats are highly speculative. The black swan events such as Three Mile Island, Chernobyl and Fukushima have served to demonize all things nuclear, much like the view that 19^th century citizens had of electricity. Never mind that coal-fired plants put many times the equivalent of radioactive contamination into the atmosphere in the form of lead²¹⁰, polonium²¹⁴, thorium and radon gases, every day. Safety detectors at nuclear plants are often triggered during temperature inversions due to nearby coal plant emissions… radiation was part of our environment even before the Cold War and is here to stay. To quote Carl Sagan, “Space travel is one of the best uses of nuclear weapons that I can think of…” Whether it is as use as a thermal electric power plant, a nuclear propulsion engine, or even an Orion style bomb-propelled spacecraft, nuclear fission and the energy it produces provides us a way to get out into the solar system, now. Ideas such as fusion engines and Bussard ramjets are all well and good and should be researched, but for now are on the drawing board only. The joke is that contained fusion capability is always “20 years down the road” and may remain there for some time.

Any science fiction “space ark’ will likely include an RTG or two…(Amazing Stories cover/In the Public Domain).

And therein, as they say, lies the rub. But first let’s look at some basic nuclear physics. We promise, it will only hurt for a little bit…

Plutonium is an artificial element that does not occur in nature. First produced by Glen Seaborg and friends in 1940, plutonium is created in the modern day laboratory by the beta decay process which occurs when uranium²³⁸ is bombarded with neutrons and decays into unstable neptunium and then plutonium²³⁹, the “weapons grade” isotope of the stuff. If neptunium-237 is used as target (fertile material) instead of U²³⁸ in a “fast” reactor the product is plutonium²³⁸. Likewise, bombard uranium-238 with deuterium (2x hydrogen nuclei) in an accelerator and the decay result is Plutonium-238 Pu-238 produces 560 watts per kg of decay heat, 280x times that of Pu239. The United States ceased production of plutonium in 1989 as the Cold War ground to an end, (more on the political aspect in a moment) and starting that production train back up would be no easy process.  The United States and Russia have tiny dwindling reserves, and at best NASA has enough for one more Cassini-style mission and perhaps a small scout style mission like New Horizons past the launch of the Mars Science Laboratory. And as you can see, utilizing the pre-existing weaponized Pu^239/240 would do little good beyond perhaps as part of an Orion-style propulsion system, as the energy of decay or the specific power yielded is just too low.  Reading the writing on the cosmic wall, things look pretty grim for the recent Planetary Exploration Decadal Survey published earlier this year; a Uranus probe, Titan blimp, and Enceladus or Europa orbiter plus lander would all require RTGs, as would the shelved Jupiter Icy Moons Mission. Contrast the problems the spunky Mars Rovers had with “dusty solar panels,” as well as the eventual lack of solar power that did the Mars Phoenix polar lander in…

One of the RTGs that flew on the Voyager spacecraft. (Credit: NASA).

Are there alternatives in the nuclear area? Yes, but not without cost; for example, there are difficulties with the use of thorium isotopes. Relatively abundant in the Earth’s crust compared to uranium, the preferable thorium²³² & thorium²³⁰ isotopes have a low abundance and a relatively low specific power in comparison to plutonium, again, making it a very poor heat source. In addition, thorium²³² is bread to uranium²³³, which is nasty stuff and emits a very penetrating dose of gamma radiation as it decays further to thallium²⁰⁸. (Remember the Hulk?) Weaponized plutonium 239/240 also has too low a specific power, creating a huge mass penalty for outgoing spacecraft with its very short 30 year half-life. Strontium⁹⁰ can be used as a RTG, but also at a great mass penalty. Same goes for any prospects of a pulsed fission reactor. In the 50’s through 70’s, NASA and the Department of Energy looked into the possibility of building a nuclear engine via Project Rover. This phases included Kiwi, Phoebus, and Pewee engines which were tested at the Nevada Test Site Area 25 desert complex facility… several extreme high altitude nuclear detonations where also conducted, most notably the Starfish Prime project in 1962. The Limited Test Ban Treaty of 1967 put the lid on further weapons testing in space, along with the prospects for a development of a nuclear propulsion engine.

Small high power solid core fission reactors have been used in space as the heat source for turbo-electric high power applications (primarily Soviet radar-satellites for intelligence purposes). One accidently returned to Earth landing in Canada in the 1970’s causing much political uproar and very little environmental damage. Solid core propulsion reactors have been designed and tested in both the United States (NERVA) and Russia and have a solid theoretical and practical engineering foundation. None have been tested in space. This concept still stands as our best bet to get humans quickly to Mars.

Nuclear-fueled & ready to roll; the Mars Curiosity Rover. (Credit: NASA JPL/CalTech).

Currently, NASA faces a dilemma that will put a severe damper on outer solar system exploration in the coming decade. As mentioned, current plutonium reserves stand at about enough for the Mars Science Laboratory Curiosity, which will contain 4.8kilograms of plutonium dioxide, and one last large & and perhaps one small outer solar system mission. MSL utilizes a new generation MMRTG (the “MM” stands for Multi-Mission) designed by Boeing that will produce 125 watts for up to 14 years. But the production of new plutonium would be difficult. Restart of the plutonium supply-line would be a lengthy process, and take perhaps a decade. Other nuclear based alternatives do indeed exist, but not without a penalty either in low thermal activity, volatility, expense in production, or short half life.

The implications of this factor may be grim for both manned and unmanned space travel to the outer solar system. Juxtaposed against at what the recent 2011 Decadal Survey for Planetary Exploration proposes, we’ll be lucky to see many of those ambitious “Battlestar Galactica” –style outer solar system missions come to pass. A mission like Juno headed to the environs around Jupiter gets around this somewhat by utilizing huge solar panels; Juno is scheduled to leave the pad at Cape Canaveral next month on August 5^th and this will mark the first non-nuclear powered mission to the outer solar system. This will occur, however, at a huge cost; Juno must drag its panels along for the ride and will only operate in a wide 11-day Jovian orbit. This is necessary to keep Juno exposed to the Sun and will preclude exploration of the Jovian moons during its projected 32-orbit life span. The three solar arrays on Juno also equal an area of 650 sq ft, a large target for any debris in the Jovian system that makes engineers cringe. Solar cells are also sensitive to high radiation fields such as those encountered in Jupiter space. This is one of the factors behind Juno’s short mission life.

Landers, blimps and submersibles on Europa, Titan, and Enceladus will all operate well out of the Sun’s domain and will need said nuclear power plants to get the job done… contrast this with the European Space Agency’s Huygens probe, which landed on Titan after being released from NASA’s Cassini spacecraft in 2004, which operated for scant hours on battery power before succumbing to the -179.5 C° temps that represent a nice balmy day on the Saturnian moon.

One of the “other” uses for plutonium; Fat Man on Tinian. (Credit: US Govt image in the Public Domain).

Part of what has always complicated matters is what is known as the Outer Space Treaty, or in its long-form, The Treaty on Principles Governing Activities of States in the Exploration and the Use of Space. Signed and ratified by the U.S., U.K. and the Soviet Union on January 1967, this treaty seeks to curb the militarization of space and specifically the use of space-based nuclear weapons as well as nuclear detonations in space.  This has formed the basis of a broad amalgam of what been termed over the years as “Space Law” which covers such things as the international use of space, salvage rights and claims, and the non-recognition of any territorial claims on a celestial body. And while “Space Court” hasn’t become filler for afternoon or late-night cable TV, the Treaty did largely keep nuclear weapons out of space during the Cold War. Some of the ideas for an “EMP shield” over the US from the 50’s are slightly frightening to read about today, as we would be now reaping the environmental consequences. While said treaties never specifically limited the use of fissile material for deep space exploration, the very concept and stigma of “Nukes in Space” made it suffer by extension. Whenever a launch with an RTG occurs, a small band of protesters gather outside the gates and grab the media spotlight until the payload has cleared Earth orbit. Modern day fears of all things nuclear can be likened to the 19^th century suspicion of electricity, which to date has taken far more victims than the peaceful use of radioactive isotopes in space.

So, what’s a space-faring civilization to do? Certainly, the “not going into space” option is not one we want on the table, and warp or Faster-Than-Light drives ala every bad science fiction flick are nowhere in the immediate future. In our highly opinionated view, NASA has the following options;

3. Exploit other RTG sources at penalty. As mentioned previous, other nuclear sources in the form of Plutonium, Thorium, and Curium isotopes do exist and could be conceivably incorporated into RTGs; all, however, have problems. Some have unfavorable half-lives; others release too little energy or hazardous penetrating gamma-rays. Plutonium²³⁸ has high energy output throughout an appreciable life span, and its alpha particle emissions can be easily contained.

MER’s curium-containing spectrometer. (Credit: NASA/JPL).

2. Design innovative new technologies. Solar cell technology has come a long way in recent years, making perhaps exploration out to the orbit of Jupiter is do-able with enough collection area. The plucky Spirit and Opportunity Mars rovers (which did contain Curium isotopes in their spectrometers!) made do well past their respective warranty dates using solar cells, and NASA’s Dawn spacecraft currently orbiting the asteroid Vesta sports an innovative ion-drive technology. Solar sails have made their debut on JAXA’s IKAROS spacecraft in the inner solar system, and perhaps a technology employing the use of space-based lasers could do double duty propelling spacecraft through the outer solar system like something out of a Larry Niven novel. Fusion of deuterium or helium³ resources could also provide a powerful light weight energy source, but of course this is all strictly drawing board stuff… the standing joke is that controlled fusion stated above is that its always “20 years away,” which leads us to option #3;

1. Push to restart plutonium production. Again, it is not that likely or even feasible that this will come to pass in today’s financially strapped post-Cold War environment. Other countries, such as India and China are looking to “go nuclear” to break their dependence on oil, but it would take some time for any trickle-down plutonium to reach the launch pad. Also, power reactors are not good producers of Pu²³⁸. The dedicated production of Pu²³⁸ requires either high neutron flux reactors or specialized “fast” reactors specifically designed for the production of trans-uranium isotopes. Going along with such specialized reactors are adequate safe facilities for the separation, concentration, and preparation of the final product. Since the end of the Cold War, the United States and Russia have closed and decommissioned the vast majority of their plutonium production facilities and reactors. The reconstitution of these cold war process plants is as of this writing beyond reasonable consideration. The huge plants at Hanford Washington and Savanna River South Carolina made sense during the Cold War, where Pu²³⁸ was a minor byproduct in the production of many tons of weapons grade plutonium. Practically, specialized research reactors at Oak Ridge Tennesse and Idaho National Laboratory can breed Pu²³⁸ and special separation and processing facilities there could produce gram per cycle quantities. However about 5kg/yr would be required to meet anticipated needs requiring a retool of currently available reactors and processing labs. Such a mission deviates critical research facilities from their primary missions that are themselves vital to understanding materials for spacecraft. Construction of new reactors and facilities for the production and processing of fissile materials is also fraught with significant funding, environmental, treaty, and local / national opposition hurdles. This can lead to very significant increases in cost over initial estimates and multi-decade delays prior to construction or production. Based on the realities of nuclear materials production the levels of funding for Pu²³⁸ production restart are frighteningly small. NASA must rely on the DOE for the infrastructure and knowledge necessary and solutions to the problem must fit the realities within both agencies.

And that’s the grim reality of a brave new plutonium-free world that faces NASA; perhaps the solution will come as a combination of some or all of the above. The next decade will be fraught with crisis and opportunity… plutonium gives us a kind of Promethean bargain with its use; we can either build weapons and kill ourselves with it, or we can inherit the stars.