Planetary exploration’s radioactive decay
by Jeff Foust
|After 50 years of service, including powering missions that have explored the far corners of the solar system, the future for radioactive power sources may be dimming.|
In late June several hundred people gathered at the National Air and Space Museum in Washington, DC, to celebrate a lesser-known, but still noteworthy, 50th anniversary: the first flight of a spacecraft carrying a radioisotope power source, or RPS. On June 29, 1961, a Thor-Able rocket launched Transit 4A, a US Navy navigation satellite—an ancestor of today’s Global Positioning System. The small satellite carried a Systems for Nuclear Auxiliary Power, or SNAP-3B power source. The device, weighing 2.1 kilograms, used the decay of plutonium-238 to generate 2.7 watts of electrical power. That wasn’t much power—the satellite’s primary power source was its solar cells—but it demonstrated the potential for such systems to power future missions.
The 50th anniversary got little attention, and what it did get focused more on the event itself, and the propriety of NASA spending a reported $37,500 on the event. While a valid concern in an era of tight budgets, it’s at best a peccadillo compared to the bigger worry facing NASA and, in particular, the planetary sciences community: that after 50 years of service, including powering missions that have explored the far corners of the solar system, the future for radioactive power sources may be dimming.
The best-known radioactive power sources for space missions, radioisotope thermoelectric generators (RTGs), are relatively simple devices when compared to full-fledged nuclear reactors. RTGs use thermocouples to convert the heat created by the natural decay of an isotope like plutonium-238 into electricity. While the efficiency of RTGs is low—the best convert only about six percent of heat from radioactive decay into electricity—they are, in the words of a 2009 National Academies report on the subject, “compact, rugged, and extraordinarily reliable.”
Since the launch of Transit 4A 50 years ago over two dozen other missions have flown RTGs. These includes all the Apollo missions after Apollo 11, the two Viking Mars landers, Pioneers 10 and 11, Voyagers 1 and 2, Galileo, Ulysses, and Cassini. The most recent mission to use RTGs is New Horizons, launched in 2006 and en route for a 2015 flyby of Pluto. NASA’s Mars Science Laboratory (aka Curiosity), scheduled for launch in late November, will also get its power from an RTG.
RTGs have demonstrated their ability to generate electricity in regions of the solar system where solar power isn’t feasible. This includes the outer solar system, where the Sun is too faint to generate power, as well as locations in the inner solar system where solar energy is attenuated or interrupted for extended periods: the two-week-long lunar night, the surface of cloud-shrouded Venus, and even the Martian surface, where dust storms can block the sun and deposit dust on solar panels, degrading their efficiency (although the Mars Exploration Rovers demonstrated that dust is less of a concern than originally thought for power generation.)
“Without RTGs, we wouldn’t have been able to go beyond Mars or the asteroid belt,” said NASA associate administrator Chris Scolese at the 50th anniversary event in June. Noting that several RTG-powered missions are heading out of the solar system, he added, “Humanity’s touch is now outside the solar system, travelling into interstellar space, and all of it is enabled by RPS.”
There’s no shortage of future mission concepts that could make use of RTGs or other radioactive power systems. One of the speakers at the June event was planetary scientist Steve Squyres, who admitted that he was “kind of an odd choice” since the mission he’s best known for, the twin Mars Exploration Rovers, Spirit and Opportunity, used solar power and not RTGs. However, he noted that even those spacecraft benefited from a different kind of radioactive power system: small devices called radioisotope heater units (RHUs) that use the decay of plutonium-238 to provide heat to keep spacecraft warm. “These are kind of the unsung heroes of the plutonium space program,” he said. The Mars rovers use six RHUs each “and they are absolutely fundamental to keeping our rovers safe, warm, and alive during the cold Martian nights.”
|“Without a restart of plutonium-238 production, it will be impossible for the United States, or any other country, to conduct certain important types of planetary missions after this decade,” the decadal report warned.|
Squyres’s main role at the event, though, was to talk about the potential future of planetary exploration and how they rely on RPS, leveraging his role as the head of the most recent planetary decadal survey (see “Tough decisions ahead for planetary exploration”, The Space Review, April 4, 2011). That study identified a number of high-priority missions that can make use of such power systems, including various missions to the outer solar system as well as the Lunar Geophysical Network, a set of landers that would use RPS to operate through the lunar night. Beyond the scope the decadal survey there are even more ambitious missions that require the use of RPS, including concepts for a submersible to explore Europa’s subsurface ocean that particularly piqued Squyres’s interest. “If I could figure out how to do submarines on Europa,” he said, “I wouldn’t be messing around with rovers on Mars.”
That bright future for planetary exploration, already clouded to some degree by general budget constraints on major missions, faces another obstacle: the diminishing supply of plutonium-238. Domestic production of the isotope ended in the US in the late 1980s when facilities for producing nuclear weapons were closed. (Plutonium-238 is not itself used in nuclear weapons; a different isotope, plutonium-239, is used instead.) In recent years the US has purchased supplies of plutonium-238 from Russia as a stopgap until a new domestic production line could be started. However, that production hasn’t started and even Russia’s stockpiles of the isotope are now running low, with no other significant supplies of plutonium-238 known to exist worldwide.
The exact amount of plutonium-238 available isn’t publicly known, although the planetary sciences decadal report issued this spring estimated the stockpile in the US at 16.8 kilograms. A 2010 letter from NASA to the Department of Energy (DOE) estimated the space agency’s needs for plutonium-238 for various projected missions through 2027 at 37.4 kilograms. (A similar letter from NASA to the DOE in 2008 projected the need for more than 100 kilograms of plutonium-238 through 2008; that letter included 56 kilograms for lunar rovers that were part of the now-canceled Constellation program.) Meeting those needs requires the purchase of an additional 10 kilograms of the isotope from Russia as well as restarting domestic production around mid-decade at the rate of 1.5 kilograms a year.
The decadal survey warned that without restarting plutonium-238 production, many of the missions it envisioned for the upcoming decade and beyond simply can’t be flown. “Without a restart of plutonium-238 production, it will be impossible for the United States, or any other country, to conduct certain important types of planetary missions after this decade.”
The simple solution, then, would be to restart domestic production of plutonium-238. Yet, despite several years of efforts by the scientific community, Congress has failed to provide the funding to do so, less out of budget worries than out of a debate regarding who should pay for it.
In the fiscal year 2012 budget proposal submitted by the Obama Administration, NASA and DOE each requested $10 million to restart plutonium-238 production using DOE facilities. That alone would not be sufficient to start producing the isotope: the 2009 National Academies report estimated the total cost of restarting production at $150 million or more.
Efforts to win funding for the program have been mixed this year. House and Senate appropriators have included the $10 million NASA requested; with the House in particular urging NASA “to work expeditiously with the Department of Energy to bring Pu-238 production back online as quickly as possible” while also developing technologies to make more efficient use of the isotope.
However, the same appropriations committees declined to provide the same funding to the DOE. The Senate, in its report, offered no explanation for the decision not to fund the DOE’s share of the restart work. House appropriators, though, offered their rationale for denying funding for the project in their report: “The Committee remains concerned that the Administration continues to request equal funding from NASA and the Department of Energy for a project that primarily benefits NASA. The Committee provides no funds for this project, and encourages the Administration to devise a plan for this project that more closely aligns the costs paid by federal agencies with the benefits they receive.”
|“We did not allow our prioritization to be driven by the presence or absence of plutonium-238,” Squyres said. “What we did instead was to turn the problem and say, look, these are the missions that ought to be flown. Now here is the plutonium-238 need profile that results from that.”|
At first glance, that appears to make sense: since NASA benefits the most from restarting plutonium-238 production, shouldn’t it pay the bulk of the cost of doing so? That logic, though, runs afoul of how government agencies have interpreted the law, dating back to the Atomic Energy Act of 1954. That law gives what is now the DOE the sole ability to own space nuclear power systems; DOE gives “custody” of them to NASA for use on the space agency’s missions. That approach has been supported by various national space policies, including the one released by the Obama Administration last year, which gives the Secretary of Energy the responsibility to “[m]aintain the capability and infrastructure to develop and furnish nuclear power systems for use in United States Government space systems.”
That responsibility has been interpreted to mean that DOE should pay at least half the cost of restarting plutonium-238 production, hence the 50-50 split between NASA and DOE in the 2012 budget request, and similar divisions of funding in previous budget requests. “Because of the DOE’s statutory responsibilities, it is also appropriate for the funding of these facilities [for producing plutonium-238] to be included in the DOE budget rather than passing these funds through NASA’s budget,” the National Academies report noted, adding that these facilities could be closed once they process the last remaining plutonium-238 if production isn’t restarted.
Unless that DOE funding is restored later in the budget process—neither the full House nor the full Senate has taken up their respective appropriations bills for either the DOE or NASA—it’s quite likely the restart of plutonium-238 production could be pushed back further, which could threaten the ambitions of planetary scientists who already have to cope with budget concerns for some of their high-priority science missions. In a worst-case scenario, missions, particularly to the outer solar system, may have to be deferred regardless of their importance or available funding because of a lack of plutonium-238 needed to power them.
For now, scientists are assuming that RTGs will be there, in one form or another, to power their key missions. “We did not allow our prioritization to be driven by the presence or absence of plutonium-238,” Squyres, referring to the decadal survey, said at a forum on planetary exploration organized by The Planetary Society on Capitol Hill earlier this month. “What we did instead was to turn the problem and say, look, these are the missions that ought to be flown. Now here is the plutonium-238 need profile that results from that.”
He added that one thing that the decadal survey also did was to recommend development of a new RPS technology called the Advanced Stirling Radioisotope Generators (ASRG). The ASRG replaced the thermocouples used on conventional RTGs with a Stirling engine that is much more efficient at converting heat into electricity. The ASRG, as currently proposed, could generate slightly more electricity than the Multi-Mission RTG (MMRTG) being flown on Mars Science Laboratory—140–150 watts versus 125 watts—but require a little under one kilogram of plutonium-238, versus the 3.5 kilograms used on the MMRTG.
|Ultimately the supply of plutonium-238 in both the US and Russia will be exhausted, and unless the federal government restarts production, the lack of that isotope will effectively close off many options for exploration of the solar system.|
Squyres noted that the decadal survey recommended that NASA pursue at “the highest possible priority” the development of the ASRG, “basically elevating it to the same level of attention as a planetary flight project.” Because ASRGs require far less plutonium-238 than conventional RTGs, the report also recommended that one high-priority flagship mission, the Jupiter Europa Orbiter, switch to ASRGs. “That dramatically decreased the projected future need” for plutonium-238, he said.
In rare cases, it may be possible to avoid using nuclear power altogether. NASA’s Juno spacecraft, launched in early August, will be the first spacecraft to go to Jupiter using solar power. While an RTG might have been preferable, none were available when the Juno team submitted their proposal. With the use of high-efficiency solar cells, the spacecraft’s three large solar panels will generate just enough power while in orbit around Jupiter. However, the mission requires careful planning to keep the spacecraft in view of the Sun as it orbits Jupiter, something that wouldn’t be necessary if the spacecraft had an RTG.
Such measures, though, are only stopgaps, and not long-term solutions, to the problem of finding power sources for planetary missions that can’t effectively use solar power. Ultimately the supply of plutonium-238 in both the US and Russia will be exhausted, and unless the federal government restarts production, the lack of that isotope will effectively close off many options for exploration of the solar system. “A decision to wait to a ‘better time’ to fund activities required to restart domestic plutonium production is just a different way of ending the program, eliminating future science missions dependent on this technology for implementation,” the planetary sciences decadal survey report concluded. In other words, there’s no time like the present to restart production, lest the options for future planetary missions, like radioactive isotopes, decay.