The cold war plutonium reserves that fuel NASA’s deep space probes are running low. How will we power our way to the outer solar system in future?
MORE than 18 billion kilometres from home, Voyager 1 is crossing the very edge of the solar system. If its instruments are correct, the craft is finally about to enter the unknown – the freezing vastness of interstellar space. It is the culmination of a journey that has lasted 35 years.
NASA’s most distant probe owes its long life to a warm heart of plutonium-238. A by-product of nuclear weapons production, the material creates heat as it decays and this is converted into electricity to power Voyager’s instruments. Engineers expect the craft will continue to beam back measurements for another decade or so, before disappearing into the void.
Since the 1960s, this plutonium isotope has played a crucial role in long-haul space missions, mainly in craft travelling too far from the sun to make solar panels practical. The Galileo mission to Jupiter, for instance, and the Pioneer and Voyager probes all relied on it, as does the Cassini orbiter, which has revealed the ethane lakes and icy geysers on Saturn’s moons, among other wonders.
Yet despite many successes, this kind of mission may soon be a thing of the past. The production of plutonium-238 halted decades ago and the space agency’s store is running perilously low. Without fresh supplies, our exploration of the outer solar system could soon come to a grinding halt.
The problem is that plutonium-238 is neither simple nor cheap to make, and restarting production lines will take several years and cost about $100 million. Though NASA and the US Department of Energy (DoE) are keen, Congress has so far refused to hand over the necessary funds.
But there could be a better way to make it. At a NASA meeting in March, physicists from the Center for Space Nuclear Research (CSNR) in Idaho Falls proposed a radical approach that they claim should please all parties. It will be quicker, cleaner and cheaper and could offer a production line run in a commercial fashion that not only meets NASA’s needs, but also turns a tidy profit into the bargain.
So what to do? Putting the production of this material on a commercial footing, as CSNR suggests, might prove easier on the public purse, but critics are concerned this could compromise safety. Plutonium is one of the most poisonous substances known – the isotope is a powerful emitter of alpha particles and deadly if inhaled. They argue that the time and money needed to restart production would be better spent developing safer alternatives. So is this the perfect opportunity to say farewell to this cold war technology and devise new, cleaner sources of space power that could benefit us earthlings too?
Plutonium-238 has played a key role in almost all of NASA’s long-duration space missions for good reason: it produces heat through the emission of alpha particles, and with a half-life of about 87 years, the material decays slowly. Sealed into a device called a radioisotope thermoelectric generator, the decaying plutonium heats a thermocouple to create electricity. Each gram of plutonium-238 generates approximately half a watt of power. On average, NASA has used a couple of kilograms of the isotope each year to power its various craft.
It does not occur naturally. Like its weapons-grade cousin, plutonium-239, it was originally created in the reactors that made material for nuclear bombs, but US production halted when those facilities were shut down in 1988. To fill the gap, the US purchased plutonium-238 from Russia until 2009, when a contract dispute ended the supply. With Russia now also running short, it is doubtful that any new deal will be struck.
So the US government must decide whether or not to resume production. According to a 2009 report by the US National Research Council, NASA has access to about 5 kilograms of the stuff, which could last it until the end of the decade (see diagram). Officials at the DoE say that if they get the go-ahead now, 2 kilograms could be made annually by 2018 – just in time to restock NASA’s cupboards. But funding is proving hard to come by. NASA has agreed to share the burden and released about $14 million for studies to work out the costs of restarting the production line – which would most likely be at Oak Ridge National Laboratory in Tennessee. However, costs could eventually spiral to $150 million, suggest some at the research council, and Congress seems loath to provide any funding directly to the DoE.
Clearly, making plutonium-238 is an expensive business. The conventional way to produce it involves placing a batch of neptunium-237 inside a powerful nuclear reactor and irradiating it with neutrons for up to a year (see diagram). The sample must then be put through a series of purification steps to separate plutonium-238 from the other fission products that also form.
At the NASA Innovative Advanced Concepts (NIAC) symposium in March, however, Steven Howe of CSNR proposed what could be a simpler and cheaper way to make it. The trick is to use a mechanical feed line, a coiled pipe surrounding the reactor core. Small capsules containing just a few grams of neptunium-237 are pushed continuously along this pipe, each one spending just days in the reactor. As they pop out the other end, the plutonium-238 is extracted and the remaining neptunium-237 is sent through the line again. About 0.01 per cent of the neptunium is converted on each pass, so this cycle would need to be repeated thousands of times to create the kilos of material required by NASA.
This technique brings some significant advantages, including shorter irradiation times causing far fewer fission products to be generated. This simplifies the subsequent chemical separation steps and reduces the amount of radioactive waste. In addition, it can work in small reactors that are far cheaper to run than the powerful national lab facilities that are required for the batch processing of old. Howe even envisions operating along commercial lines, so NASA and the DoE would just purchase the final product, rather than footing the bill for the entire production process.
The CSNR team working on this concept is already funded by a $100,000 NIAC grant and has submitted a proposal to build a prototype feed line and to demonstrate that they can mechanically push the capsules through it, as well as perform the subsequent separation steps. Howe believes they can have their process up and running in just three years, at a cost of about $50 million – half the proposed cost of restarting conventional production – and could create about 1.5 kilograms of plutonium-238 each year.
Though the team still has to determine the optimum irradiation time, operating the process continuously instead of converting several kilograms in batches twice a year should help keep costs and the facility size down. And if they charge $6 million per kilogram – less than the latest Russian asking price – this process would be cost-effective for private industry, Howe says. “Like commercial space travel, we’re doing commercialised plutonium production,” he says.
Whether or not Howe’s technique saves money, or even makes it, breathing fresh life into plutonium production is not popular with everyone. Plutonium-238 is highly toxic, and an accident during or after launch could release it into the atmosphere. In 1964, for example, a US navy navigation satellite re-entered the atmosphere and broke up, dispersing 1 kilogram of plutonium-238 around the planet, roughly double the amount released into the atmosphere by weapons testing. Though the plutonium’s containers have been redesigned to survive re-entry intact, the Cassini probe’s near-Earth fly-by in 2006 triggered widespread public protests. Restarting plutonium production is “a very frightening possibility”, says Bruce Gagnon of the Global Network Against Weapons and Nuclear Power in Space based in Brunswick, Maine. “It obviously indicates that the nuclear industry views space as a new market,” he says. “It’s like playing Russian roulette.” Gagnon is also worried by the prospects of a commercialised production line. “When you introduce the profit incentive, you start cutting corners,” he says.
Then there are concerns over proliferation and political capital. While plutonium-238 cannot be used to make a nuclear weapon, it is a different story with neptunium-237. This is weapons-grade material: bombarded by fast neutrons, it is capable of sustaining a chain reaction without unstable heat decay. Edwin Lyman at the Union of Concerned Scientists based in Cambridge, Massachusetts, believes that given these safety and security issues, non-nuclear power generation systems should be a priority for space applications. “Alternatives need to be explored fully,” he says. “If the US proceeds with the restart, it will be more difficult for us to dissuade other countries from doing the same, should they decide they need to produce their own plutonium-238 supply.”
Can sunlight help fill the gap? The intensity of light drops with distance from the sun, following an inverse square law, so sending solar-powered spacecraft to the outer planets looks like a non-starter. In Pluto’s orbit, for example, it would take a solar array of 2000 square metres to generate the same amount of power as a 1-square-metre array in Earth’s orbit. Nevertheless, in August 2011, NASA launched Juno, the first mission to Jupiter using solar energy instead of plutonium. Juno relies on three 10-metre-long solar panels to gather the power it needs to operate. And according to a 2007 NASA report, solar-powered missions beyond Jupiter are not out of the question.
What’s needed, says James Fincannon of NASA’s Glenn Research Center, are new solar cells that can cope with the extreme conditions in the outer solar system. Great strides are being made in developing lightweight, high-efficiency solar cells, he says. If the cost and mass of these arrays can be reduced further, and if a spacecraft’s power needs can be reduced to less than 300 watts – about half that of the Galileo probe – Fincannon suggests that a craft powered by a 250-square-metre solar array could operate as far away as Uranus. Gagnon agrees. “For years we’ve said that solar would work even in deep space,” he says.
There are even plutonium-free ways to power craft exploring the darker reaches of the solar system where Fincannon’s arrays would not work. At the NIAC symposium where Howe discussed his plutonium production process, Michael Paul of Pennsylvania State University’s Applied Research Laboratory described a novel engine that could power craft on the surface of cloud-wrapped worlds where little sunlight penetrates.
Take Venus. Paul proposes combining lithium fuel with carbon dioxide from the greenhouse-planet’s atmosphere, and burning it to provide heat for a Stirling engine – a heat pump that uses a temperature difference to drive a piston linked to a generator (see “Cloud power”). The system would not need nuclear launch approval, could operate at very high power levels and could be modified to work on Titan, Mars or even in the permanent dark of the moon’s south pole, he says. With further development Paul believes the technology could be ready to launch by 2020. “I see this power system as a way to enable a whole new set of opportunities that are closed off because we just don’t have enough plutonium,” he says.
Paul admits that his lithium-powered landers would last just a fraction of the decades-long lifetime achievable using plutonium. “Fifty years of work has shown that there are applications where there are no alternatives – period,” says Ralph McNutt of the Johns Hopkins University Applied Physics Laboratory. But, he adds, “to the extent that there are alternatives to radioactive power sources, we should take them”. Fincannon agrees: “It is always a good time to come up with alternative power sources,” he says.
Besides, spending money developing lightweight solar cells or more efficient Stirling engines could offer benefits on, as well as off, Earth. Engineers are already exploring ways to turn metal powder into fuel for vehicle engines, and Paul suggests his technology could help expand underwater exploration missions, too. The same can no longer be said for plutonium-238. Once used to power cardiac pacemakers, security and health concerns mean that the material is no longer welcome.
So which way will NASA jump? Howe remains determined to fight in plutonium’s corner and recently presented his case to the agency. As far as space is concerned, this power struggle isn’t over yet.
When this article was first posted, it contained an incorrect reference to Isaac Newton
Solar power is not an option for landers heading through thick clouds like those surrounding Venus. That’s where the generator conceived by Michael Paul and his team at Pennsylvania State University comes in. Paul’s team suggest powering a Venus lander by burning lithium with carbon dioxide sucked in from the planet’s atmosphere – eliminating the need to carry along an oxidiser with the fuel.
The heat from combustion would drive a small turbine or Stirling engine, which would power the lander’s electronics. But on boiling-hot Venus, the biggest challenge is keeping the lander’s electronics cool. Paul predicts that four-fifths of the engine’s power output will go towards pumping heat away from the craft’s electronics. Previous missions to Venus have lasted no more than 2 hours beyond touchdown before their batteries petered out. Paul calculates that 200 kilograms of lithium will be enough to keep sensors running for a week.
He believes that adding the planet’s carbon dioxide to lithium is the only way to pack enough punch to power a Venus lander. To provide electrical power and cooling for a week-long mission would otherwise require 850 kilograms of batteries, for instance, or 50 plutonium-powered generators. Even NASA’s colossal Cassini probe – “the flagship of all flagship missions” – doesn’t have that many, says Paul.