The managers of NASA's planetary exploration are caught between that proverbial rock and a hard place. Given their situation, I admire them for making a tough call to cancel an advanced technology program that could have enabled a new generation of small missions to several exciting destinations in the solar system. The good news is that in the end, the cancellation may make little difference in the missions that actually will fly. That's the bad news, too.
The cancellation was for NASA's Advanced Stirling Radioisotope Generator (ASRG) that would have provided a new generation of plutonium-238 (Pu-238) based electrical power systems. ASRGs could have stretched NASA's limited supply of plutonium to potentially enable missions to the perpetually-shadowed polar craters on our moon, to flyby Uranus, or to float for months on a Titan lake.
NASA has an alternative, flight-proven generator technology, the Multi-Mission Radioisotope Thermal Generators (MMRTGs). Because MMRTGs use approximately four times the Pu-238 as ASRGs for a similar power output, NASA now will need to hoard its Pu-238 supply to power its largest expected missions of the 2020s. That would enable the planned 2020 Mars rover and the hoped for Europa Clipper multi-flyby missions. Supplies for follow-on missions would not be available until late in the 2020s for a small mission and the early 2030s for a larger mission.
Comparison of the MMRTG and ASRG power systems
In my post today, I want to give the background for NASA's decision (that rock and a hard place) and to talk about the types of missions that can still be flown and those that would be lost.
The "rock" is NASA's present tight budget for planetary exploration, which has been cut in each of the last several years. NASA recently learned that bringing ASRGs to flight ready status would cost $100M more than originally expected. By canceling the ASRG program, NASA saves $170M over the next three years.
NASA has not publicly identified where the savings would go. They could use the funds for other NASA programs or Congress could lower NASA’s budget by that amount. However, if the planetary program is allowed to keep the savings, $170M could go a long way to stretching thinned budgets for missions or research. (Coincidentally, $170M would be almost enough for the Cassini spacecraft, which may otherwise be turned off in 2015, to fly until its fuel is exhausted in 2017.)
The "hard place" is NASA projected budget for planetary exploration. While Congress appropriates funds for each current year, the President's Office of Budget and Management provides estimates for future budgets. NASA's managers can only start a new mission when those projected budgets show room in the budget for a new mission. Right now, NASA's planetary science projected budget shows no room to start a new mission that would launch before the end of the decade and more likely in the early 2020s. At the rate of mission flights suggested by the projections, there would be little demand for plutonium-powered missions beyond the current Pu-238 supply.
Examples of funding peaks for NASA’s current approved smaller planetary missions along with projected funding for the next generation of missions
OSIRIS-REx is a New Frontiers program mission and the Mars MAVEN and InSight missions are Discovery program-class missions.
NASA's missions in development follow a roller coaster funding profile with development requiring high peak funding for a year or two. As one mission rolls off its peak funding, funds become available to start the next mission. Projected funding (see chart above) suggests that budgets won't support the funding of a new mission until the end of this decade, with a launch then or in the early 2020s. As a result, the new missions that might require plutonium power supplies are projected to be – unfortunately – too few and far between. (Part of NASA's future mission challenge is that late in this decade most of its new mission budget will go to the $1.5B to 1.7B 2020 Mars rover.)
The U.S. has approximately 30 kilograms of Pu-238, or enough for five MMRTGs. A recently released presentation slide from NASA shows one of those MMRTGs reserved for the 2020 Mars rover, which will be a close copy of the MMRTG-powered Curiosity rover now on Mars. The other four are held in reserve for a mid-2020’s mission or missions. While not stated in the presentation, this could be the proposed Europa Clipper multi-flyby mission that would require the equivalent of four MMRTG’s electrical power.
NASA’s expected supply and use of plutonium-238 for the next two decades
The U.S. has just approved plans to produce new Pu-238 for the first time in decades. The amounts will be small, around one kilogram a year. At that rate, approximately two MMRTGs could be fueled in a decade with new Pu-238. However, the U.S. has a stockpile of degraded Pu-238 (that presumably consists of material old enough that a significant proportion has become useless because of radioactive decay). NASA plans to mix its new Pu-238 with reworked older material to produce enough usable material to power several MMRTGs in the 2020s.
Given NASA’s new dependence on MMRTGs, what types of missions can it still fly and which become impossible or unlikely?
Answering that question requires understanding when a radioisotope power supply either is absolutely necessary to fly a mission or would substantially enhance it. The alternative is solar power from solar photovoltaic panels. Too far from the sun, and sunlight is too feeble to power a spacecraft. Until a few years ago, that line of demarcation fell somewhere in the asteroid belt. With improvements in low light and low temperature (it's cold far from the sun) solar cells, solar powered missions at Jupiter are feasible. One (NASA's Juno) has launched, a second (Europe's JUICE mission) will launch in 2022, and the proposed Europa Clipper mission could use solar power.
Several studies have looked at using solar power for missions to Saturn. The low light, low temperature solar cells should work there. The major problem is that at Saturn, a set of solar panels like those on the Juno spacecraft that produce ~440 W at Jupiter would provide only ~110 W at Saturn. For the Juno spacecraft, approximately half of its 440 watts of power will go to powering the spacecraft systems and instruments and half will go to running heaters to keep the spacecraft warm. Keeping warm is even a bigger problem at the more distant Saturn, and after running heaters, little electrical power might be available for anything else. More solar panels could be added at the cost of additional weight and fuel to maneuver with that weight. As the appendix at the end of this post states, solar power at Saturn is technically possible, but the trade offs are significant.
Beyond Saturn, the size and weight of solar panels would become prohibitive. Realistic spacecraft for Uranus, Neptune, and destinations beyond require radioisotope power supplies (or a future generation of solar panels).
Concept design for a solar powered outer planets spacecraft from the Trojan asteroid Decadal Survey mission study
Other missions requiring Pu-238 missions are those where solar power is intermittent or unavailable. This would include long-term landers or rovers for the perpetually shadowed craters at the poles of the moon or the surface of Titan. Long-lived lunar stations would benefit from radioisotope power since they must survive nights lasting 14 terrestrial days. Mars rovers also benefit from Pu-238 because bright sunlight is available only for part of the day, winter brings dimmed solar light, and dust storms can make noon as dark as twilight. Another challenge for these types of missions (and those in the outer solar system) is keeping the spacecraft warm. Pu-238 power systems have lots of excess heat that can be used without diverting electrical power to heaters.
NASA / GSFC / University of Maryland
Comet hopper mission concept
The ASRG unit is the rectangular box with cooling fins on top of the spacecraft.
Some inner solar system missions can't be done with large solar panels. One clever idea was a comet hopper (called CHOPPER) that would land in multiple places on the surface of a comet. Landing repeatedly on a rough surface with large solar panels would be impractical, making a Pu-238 power system an enabling technology.
The AVIATR airplane concept at Titan depended on the low mass of the ASRGs to keep it light enough to fly. With only MMRTGs available, that concept is unfeasible. However, hot air balloons for Titan couldn't stay aloft with the lower waste heat available from ASRGs to heat the air for their balloons. This type of mission requires the heat from the additional Pu-238 in an MMRTG.
NASA has a list of missions it would like to fly in its Flagship ($1.5B to $2B) and New Frontiers ($750M to $1B) programs. The following table summarizes whether solar power could be used, and if MMRTGs would be a benefit, how many would be useful. These data are from mission studies done for the last Decadal Survey effort to plan NASA’s planetary science program. The studies assumed ASRGs, but MMRTGs provide similar levels of power, so if a mission requires two ASRGs, it is likely two MMRTGs would be required. (Many of the studies looked at multiple configurations with different numbers of ASRGs, and I've shown the minimum number for a credible mission. In addition, ASRGs, unlike MMRTGs, have moving parts. For redundancy against mechanical failure, all studies assumed at least two ASRGs. It is possible that some of the missions could be done with a single MMRTG.)
Possible with solar power?
Number of ASRGs/MMRGs
Mars 2020 rover
Comet sample return
Lunar sample return
Trojan astroid orbiter
*MMRTG is likely to minimize design changes to the Curiosity rover design on which it will be based
Radioisotope power requirements of NASA's candidate Flagship and New Frontiers planetary missions. From the Decadal Survey mission studies.
Based on this table, a reasonable question might be whether the cancellation of the ASRG program will impact the missions that fly. From the list of candidates for Flagship and New Frontiers missions, the answer may be no. There's sufficient Pu-238 for the 2020 rover and Europa Clipper (which could switch to solar power anyway). Under current budget forecasts, a Uranus orbiter wouldn't launch before the new production of Pu-238 becomes available. All the New Frontiers missions on the list could be done with solar power although this would like incur design challenges for the Saturn probe and lunar network.
Where we may see a loss is in the lowest cost class of missions, the Discovery program ($425M to $500M). The expectation had been that NASA would make at least one pair of ASRGs available for a Discovery mission. Engineers and scientists came up with clever ideas for ASRG-based missions – the comet hopper, a Titan lake lander, an orbiter to revisit Titan and Enceladus, a Uranus flyby, and others. With MMRTGs now the only option, NASA needs to hoard its supply of Pu-238. It also has lost the motivation to test a new technology – ASRGs – on a relatively low cost mission. MMRTG technology is already proven.
So NASA's managers made the tough call, and if I were in their shoes, I'd have done the same. I do see two glimmers of hope to resurrect those clever Discovery missions though, or to give engineers the flexibility to use MMRTGs for New Frontiers missions. The first is that Congress for the 2013 budget made it clear that it wanted higher funding for future missions. If this desire becomes policy (and the President's budget office would have to reflect this in their budget projections), then there may be more demand for Pu-238 than NASA is currently envisioning. The second is that if the Europa Clipper design team decides to go with solar power instead of MMRTGs, NASA will have more Pu-238 than missions planned to use it. In this case, they might make MMRTGs available for New Frontiers or even Discovery missions.
As part of the 2012 Decadal Survey to create a roadmap for NASA's planetary missions, studies of a number of potential missions were conducted. Two looked at smaller spacecraft to carry and atmospheric probe to Saturn and relay its data back to Earth and to conduct multiple flybys of Enceladus. Both studies concluded that solar cells were possible, but radioisotope power systems were preferable. The following excerpt from the Saturn probe mission study report discusses the challenges of solar power at Saturn.
"Regarding power systems, although it might be possible to use solar arrays for the carrier-relay spacecraft's primary electric power system, operating at 10 AU would push the very limit of current solar cell technology, requiring large margins and an expensive parts selection program for the solar cells. For the mission time period studied, a radioisotope power source (RPS) is less expensive and lower risk for this mission than a solar array system, and would perform well in all mission phases.
"Unexpectedly, the solar vs. nuclear trade study concluded that the nuclear option, specifically the use of ASRGs, would provide significant cost savings and risk reduction relative to the solar option. There are multiple reasons. Despite using no RPS, the solar option would nonetheless require radioisotope heater units (RHUs), some in the proposed probe and some in the proposed carrier-relay spacecraft. Thus it would incur some costs associated with nuclear payloads anyway, nullifying one potential cost-saving advantage of the solar option. Solar cells from a production process are not all exactly the same, and differences that are small under normal illumination conditions could be greatly magnified under the low intensity, low-temperature (LILT) conditions in the outer solar system. For a mission to Saturn, selecting acceptable solar cells from production batches would require a significant program of testing and screening, increasing the cost per cell. Because such testing does not always guarantee expected performance, the solar arrays would need to be designed with somewhat larger margins, increasing the size, cost and risk of producing and flying already-large arrays. These large arrays would have masses far greater than the mass of ASRGs producing similar power. Solar array size and mass influenced the proposed launch vehicle selection and subsequent spacecraft operations: The dimensions and mass of the arrays would require a larger launch vehicle and significant operational constraints, contributing significantly to the total cost and risk difference."