Half a kilo of cosmic fuel reignites NASA's deep space dreams
- Reference: 1700658913
- News link: https://www.theregister.co.uk/2023/11/22/nasa_plutonium238_shipment/
- Source link:
Plutonium-238 (Pu-238) is essential for NASA missions using Radioisotope Power Systems (RPS). RPS makes use of the natural decay of Pu-238 to provide heat and electricity through systems such as the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). As such, it's essential for missions where solar power is not an option.
NASA's recent Mars rovers use the fuel, and Perseverance uses an MMRTG to provide the robot with continuous heat and approximately 110 watts of electricity. Other missions, such as Cassini, also used the isotope, and Pu-238 power has kept the Voyager probes running decades after the launch.
[1]
[2]According to NASA : "Three dozen missions have explored space for decades using the reliable electricity and heat provided by RPS."
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However, supplies have been limited. Pu-238 is a byproduct of the process used to make nuclear weapons. As the need for new nuclear weapons has ebbed over the years, stockpiles of the fuel have dwindled. In 2015, [5]Popular Science estimated that approximately 35 kg remained available to NASA, and of that, only 17 kg was good enough to use.
[6]FAA stays grounded in reality as SpaceX preps for takeoff
[7]SpaceX celebrates Starship launch as a success – even with the explosion
[8]Double Moon crater riddle solved? Spent Chinese rocket booster carrying mystery payload crash landed
[9]NASA's Psyche spacecraft beams back a 'Hello' from 10 million miles away
Hence the need to restart production. In 2010, the US Department of Energy submitted a [10]report [PDF] to Congress with the goal of achieving an average production rate of 1.5 kg per year by 2015. According to NASA, that target won't be hit until at least 2026.
That said, small quantities of the fuel have been delivered in recent years. Some was used in the Perseverance Mars rover.
RTGs are the only option for deep space missions as far as NASA is concerned. Two technologies that could have made more efficient use of Pu-238 – the Advanced Stirling Radioisotope Generator (ASRG) and the Enhanced Multi-Mission Radioisotope Thermoelectric Generator (eMMRTG) – [11]were axed by the agency due to technical, schedule, and cost issues.
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NASA is having another crack with its [13]Next Generation RTGs , which it claims will produce more than twice the power of an MMRTG with a significantly lower power degradation over time. ®
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[2] https://www.nasa.gov/centers-and-facilities/glenn/nasa-one-step-closer-to-fueling-space-missions-with-plutonium-238/
[3] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44ZV4zttp4kxZLn6etJVb9UAAAAEQ&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0
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[5] https://www.popsci.com/nasa-can-make-3-more-nuclear-batteries-and-thats-it/
[6] https://www.theregister.com/2023/11/21/elon_musk_next_starship_ready/
[7] https://www.theregister.com/2023/11/20/spacex_celebrates_starship_explosion/
[8] https://www.theregister.com/2023/11/17/chinese_rocket_moon_crash/
[9] https://www.theregister.com/2023/11/17/first_light_achieved_by_nasa/
[10] https://www.energy.gov/sites/default/files/2015/09/f26/Final_Startup_Plan_for_Plutonium238.pdf
[11] https://www.theregister.com/2023/03/21/nasas_space_nuclear_power_program/
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[13] https://rps.nasa.gov/resources/171/next-generation-radioisotope-thermoelectric-generators/
[14] https://whitepapers.theregister.com/
Re: significantly lower power degradation over time
RTGs basically use thermocouples to convert the heat produced by radioactive decay to electricity (and plutonium generates a *lot* of heat - I lump of plutonium will happily glow red hot from decay heat), however this process is very inefficient - the MRTGs installed on the Curiosity and Perseverance rovers generate about 110W of energy from 2,000W of heat - an thermal conversion efficiency of a tad over 5% (the waste heat is simply radiated away). I'm guessing that NASA are trying to find ways of improving the thermal conversion efficiency - this would obviously mean that they could use smaller RTGs to generate the same power (a big weight saving since RTGs are heavy) but it would also make the problem of dumping the waste heat that much simpler.
Re: significantly lower power degradation over time
The terms RPS (Radioisotope Power System) and RTG (Radioisotope Thermoelectric Generator) were both used in the article. I was half wondering if that implied there might be some form of RPS that isn't an RTG, i.e. uses some more efficient method of converting the heat, but from a quick Google, NASA only seems to be talking about RTGs. I wonder if there's a reason is for this particular instance of acronym proliferation.
Re: significantly lower power degradation over time
I was wondering exactly the same. The NASA document linked to near the end of the article seems to imply that RPS is a generic term and RTG is a subset - a particular type of RPS.
The NASA document goes on to mention several different types of RTG, such as MMRTG (in the Reg article), GPHS-RTG (General Purpose Heat Source RTG) and "Next Gen" RTG, which is an enhancement of an existing GPHS-RTG.
Other than that, I'm none the wiser and not bothered looking harder. If anyone else does please post here.
Re: significantly lower power degradation over time
The approximately 88-year half life of Plutonium 238 is a certainly major limiting factor on the useful life of an RTG, but not the only factor. Helium gas is produced by the fuel decay (when a Plutonium 238 atom becomes Uranium 234 by emitting an alpha particle - a Helium nucleus). This tends to reduce the electric output because Helium gas has a relatively high thermal conductivity, which (slightly) reduces the temperature difference between the hot and cold ends of the thermoelectric conversion elements within the RTG. The lower the temperature difference, the less electricity is generated via the Seebeck effect. See: https://en.wikipedia.org/wiki/Thermoelectric_effect#Seebeck_effect
To counteract the buildup of Helium, some RTGs (in particular the SNAP-19 Viking RTG) had a reservoir of mostly Argon gas atop the RTG housing that would very slowly permeate through a Viton O ring into the RTG housing. Argon's lower thermal conductivity helped to maintain a higher thermal gradient across the thermoelectric couples.
Another life-limiting factor is degradation of the thermoelectric elements due to continuous exposure to high heat. The materials tend to very gradually sublime, which reduces their mass and thus the resulting magnitude of the Seebeck effect.
Re: significantly lower power degradation over time
I did a quick google, [1]Wikipedia states that in 1977 the RTGs were generating a total of 470 watts. As you say, Pu-238 has a half-life of 88 ish years, so would lose 0.79% of their power output each year. If that reduction was consistent then today they’ would be knocking out 299ish watts.
The actual figure is lower because the thermocouples that convert heat to electricity also degrade over time, in 2011 for instance, the Voyagers were [2]generating 270 watts , rather than 343 watts.
As of Feb this year, Voyager 1 with it's 4 working instruments only has [3]8.3 watts to spare . Voyager 2 with it's 5 instruments only has 3.6 watts to spare. To keep comms up NASA will have to either turn another instrument off on each, or come up with clever ideas in typical NASA fashion (along the lines of how they kept the rovers going) to somehow ration the power between all working devices, outside of the original design.
Maybe if a more efficient RTG design had been available in the 70s, the Voyagers could have been kept in service say an extra decade, before reaching the point we are at now where NASA are having to power instruments down. Hopefully in the last 47 years, we've made some progress in power conversion efficiency that can be applied to future devices.
[1] https://en.wikipedia.org/wiki/Voyager_1
[2] https://www.universetoday.com/142802/nasa-has-figured-out-how-to-extend-the-lives-of-the-voyagers-even-longer/#:~:text=Each%20Voyager%20launched%20generating%20470,power%20they%20started%20out%20with.
[3] https://www.inverse.com/science/heres-how-nasa-plans-to-keep-voyager-1-2-running-until-2035
So how much is 1.5 kg?
So you don’t have to look it up, Perseverance uses 4.8 kg.
ASRG
The “advanced Stirling radioisotope generator” implies a working fluid, does it not?
I can’t see that working reliably for 8-10 decades continuous without service.
And yet Apple still can't make a phone battery that lasts more than a day.
significantly lower power degradation over time
Aren't they rather constrained by the 88 year half-life of the isotope? Or is it that they're looking for improvements in the conversion of (radiation generated) heat to electricity? What's the limiting factor in current systems?