4 ms·

Doesn't talk about the downsides, which are mainly that Am-241 has about five times lower power density than Pu-238, and also needs additional radiation shieldi

Doesn't talk about the downsides, which are mainly that Am-241 has about five times lower power density than Pu-238, and also needs additional radiation shielding. https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Fuels https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge... There's no spacecraft-related reason to select this—it's strictly worse—it's just the US/EU are in a decades-long ongoing shortage of Pu-238, and this is the only available alternative. I don't think this is "pioneering" in any positive sense of the word. IMHO, it's embarrassing.

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Retric 2y ago
The half-life of americium-241 is about 432 years, that’s a huge advantage. The Voyager probes at half power and because of exponential decay it’s not easy to simply start with more Pu-238 to compensate. You need to deal with dumping exponentially more waste heat at the beginning when you want to extend their lifespan. Not everyone needs to last that long, but Opportunity died 15 years after launch plenty of time for a significant reduction in power output. When the cost per pound to orbit was higher it was an obvious choice, but these days it’s more questionable.
- deleted 2y ago
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- Dylan16807 2y ago
  Plutonium has a half life of almost 90 years which isn't too shabby. If you want ten more years at full power then you only need to add about 10 percent more. The exponent is not very big. An americium system that starts off bigger is going to be worse for most kinds of mission. Especially Mars landers that are going to wear out one way or another.
  - FranOntanaya 2y ago
    It's still boosting the science profile of the mission to use it as test bed for Americium. There's a lot of science to be done in the Kuiper belt and further out where longevity may matter eventually simply because we wouldn't be sending repeat missions to each dwarf planet out there anytime soon.
    - jjk166 2y ago
      So instead of 10% more of the high power density material, you want to use 500% more of the low power density material?
      - Retric 2y ago
        Weight of a single component isn’t the only consideration. Having less heat output on the launch pad is a real benefit. It may take a 200 year mission for the weight to balance out, but having less variation in output is a benefit on day 0. Further there’s many reasons why someone managing a project may say no to 10% extra plutonium even if the project could benefit from a longer lasting power supply.
  - Retric 2y ago
    You can’t just slap 10% more without consequences. You now need to design a spacecraft to dissipate 10% more heat on all phases of the mission including when it’s well insulated sitting on a rocket ready for launch, for a mission that you want to last 20 years the initial heat load can be quite problematic. And picking a specific end date means giving up on the long tail of overly successful missions. Spirt was sending good data from May 1, 2009 to March 22, 2010 while stuck and NASA kept trying to contact it until May 24, 2011. https://en.wikipedia.org/wiki/Spirit_(rover) https://en.wikipedia.org/wiki/Spirit_(rover)
    - Dylan16807 2y ago
      And you can't just slap in americium without consequences either.
kryptiskt 2y ago
When it comes to an RTG lower power density is the same as more stable output and longer life since that's the other side of the half-life coin. So it's not "strictly worse", it actually has attractive features.
- perihelions 2y ago
  Yes, but not really, because the crossover point is at around 247 years.
  - Dylan16807 2y ago
    The crossover point depends on how the weight is distributed between the radioactive material, the shielding, and the radiators. The first two favor plutonium, the last one favors americium.
    - perihelions 2y ago
      That's an very good point about the radiators—you need some overcapacity with a fast-decaying radioisotope, so you will need more heat dissipation. It's still not a close call, as far as I can tell (I'm not a domain expert). If you plan for even 20 years operating life, then your initial heat dissipation (for Pu-238) would be 1.17 of the nominal power at t+20 years, due it its radioactive decay. The largest RTG, GPHS-RTG, has 13.0 kg of "housing and fins" [0]; if you scale that linearly by 1.17 that would add +2.2 kg. On the flip side, the isotope mass that emits 4,500 Watts of heat goes from 8.3 kg of Pu-238, to 39.5 kg of Am-241—that's +31.2 kg. This is relative to a total mass (GPHS-RTG) of 56.0 kg. (To preempt anyone complaining that this are small quibbles: the total dry mass of New Horzions [1], which contained one of these RTG's, is 401 kg). [0, pdf] https://ntrs.nasa.gov/api/citations/20080003866/downloads/20080003866.pdf https://ntrs.nasa.gov/api/citations/20080003866/downloads/20... (table 1 on page 3, "State-of-the-art RPS. Comparing existing GPHS-RTG with near-term MMRTG and SRG110") [1] https://en.wikipedia.org/wiki/New_Horizons https://en.wikipedia.org/wiki/New_Horizons
    - angiosperm 2y ago
      Strontium-90 has advantages. At density 2.6 vs 12, you can afford to loft more of it to compensate for its 28y half-life, and still come out ahead provided you can dispose of the extra heat at first. Few missions need to run decades.
outworlder 2y ago
Wow, such negativity. > There's no spacecraft-related reason to select this—it's strictly worse—it's just the US/EU are in a decades-long ongoing shortage of Pu-238, and this is the only available alternative. I don't think this is "pioneering" in any positive sense of the word. IMHO, it's embarrassing. No, but there are practical reasons to select this. It doesn't matter _why_ Pu-238 is unavailable, it matters that it is unavailable and this has been a heavy constraint on space missions. New Horizons almost got postponed (with a danger of cancellation) due to this. And even then it launched with 80% fuel. The question is, can they deliver americium RTGs ? That would be great.
- ApolloFortyNine 2y ago
  >It doesn't matter _why_ Pu-238 is unavailable Why not exactly? Because we used to buy it from Russia because we stopped making it ourselves? Anyways they reference here that it's expected to finally increase production to a reasonable amount soon. [1]https://en.wikipedia.org/wiki/Plutonium-238 https://en.wikipedia.org/wiki/Plutonium-238
- deleted 2y ago
  [deleted]
TaylorAlexander 2y ago
There is no spacecraft related reason to use this unless what you want is for your spacecraft to actually fly on schedule. Risk assessment from supply chain issues is part of engineering.
JumpCrisscross 2y ago
> the US/EU are in a decades-long ongoing shortage of Pu-238 Europe is and will continue to be. America is ramping up production [1]. [1] https://www.scientificamerican.com/article/behind-the-scenes-at-a-u-s-factory-building-new-nuclear-bombs/ https://www.scientificamerican.com/article/behind-the-scenes...
- philipkglass 2y ago
  The isotope used in space missions is plutonium-238. That Scientific American article is about weapons pits made of plutonium-239. The US has a large surplus of stockpiled Pu-239 for weapons: https://www.energy.gov/nnsa/articles/nnsa-issues-final-surplus-plutonium-disposition-program-environmental-impact https://www.energy.gov/nnsa/articles/nnsa-issues-final-surpl... Sadly, it is of no use for space missions.

deleted 2y ago

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Retric 2y ago

The half-life of americium-241 is about 432 years, that’s a huge advantage. The Voyager probes at half power and because of exponential decay it’s not easy to simply start with more Pu-238 to compensate. You need to deal with dumping exponentially more waste heat at the beginning when you want to extend their lifespan. Not everyone needs to last that long, but Opportunity died 15 years after launch plenty of time for a significant reduction in power output. When the cost per pound to orbit was higher it was an obvious choice, but these days it’s more questionable.

deleted 2y ago
[deleted]
Dylan16807 2y ago
Plutonium has a half life of almost 90 years which isn't too shabby. If you want ten more years at full power then you only need to add about 10 percent more. The exponent is not very big. An americium system that starts off bigger is going to be worse for most kinds of mission. Especially Mars landers that are going to wear out one way or another.
- FranOntanaya 2y ago
  It's still boosting the science profile of the mission to use it as test bed for Americium. There's a lot of science to be done in the Kuiper belt and further out where longevity may matter eventually simply because we wouldn't be sending repeat missions to each dwarf planet out there anytime soon.
  - jjk166 2y ago
    So instead of 10% more of the high power density material, you want to use 500% more of the low power density material?
    - Retric 2y ago
      Weight of a single component isn’t the only consideration. Having less heat output on the launch pad is a real benefit. It may take a 200 year mission for the weight to balance out, but having less variation in output is a benefit on day 0. Further there’s many reasons why someone managing a project may say no to 10% extra plutonium even if the project could benefit from a longer lasting power supply.
- Retric 2y ago
  You can’t just slap 10% more without consequences. You now need to design a spacecraft to dissipate 10% more heat on all phases of the mission including when it’s well insulated sitting on a rocket ready for launch, for a mission that you want to last 20 years the initial heat load can be quite problematic. And picking a specific end date means giving up on the long tail of overly successful missions. Spirt was sending good data from May 1, 2009 to March 22, 2010 while stuck and NASA kept trying to contact it until May 24, 2011. https://en.wikipedia.org/wiki/Spirit_(rover) https://en.wikipedia.org/wiki/Spirit_(rover)
  - Dylan16807 2y ago
    And you can't just slap in americium without consequences either.

kryptiskt 2y ago

When it comes to an RTG lower power density is the same as more stable output and longer life since that's the other side of the half-life coin. So it's not "strictly worse", it actually has attractive features.

perihelions 2y ago
Yes, but not really, because the crossover point is at around 247 years.
- Dylan16807 2y ago
  The crossover point depends on how the weight is distributed between the radioactive material, the shielding, and the radiators. The first two favor plutonium, the last one favors americium.
  - perihelions 2y ago
    That's an very good point about the radiators—you need some overcapacity with a fast-decaying radioisotope, so you will need more heat dissipation. It's still not a close call, as far as I can tell (I'm not a domain expert). If you plan for even 20 years operating life, then your initial heat dissipation (for Pu-238) would be 1.17 of the nominal power at t+20 years, due it its radioactive decay. The largest RTG, GPHS-RTG, has 13.0 kg of "housing and fins" [0]; if you scale that linearly by 1.17 that would add +2.2 kg. On the flip side, the isotope mass that emits 4,500 Watts of heat goes from 8.3 kg of Pu-238, to 39.5 kg of Am-241—that's +31.2 kg. This is relative to a total mass (GPHS-RTG) of 56.0 kg. (To preempt anyone complaining that this are small quibbles: the total dry mass of New Horzions [1], which contained one of these RTG's, is 401 kg). [0, pdf] https://ntrs.nasa.gov/api/citations/20080003866/downloads/20080003866.pdf https://ntrs.nasa.gov/api/citations/20080003866/downloads/20... (table 1 on page 3, "State-of-the-art RPS. Comparing existing GPHS-RTG with near-term MMRTG and SRG110") [1] https://en.wikipedia.org/wiki/New_Horizons https://en.wikipedia.org/wiki/New_Horizons
  - angiosperm 2y ago
    Strontium-90 has advantages. At density 2.6 vs 12, you can afford to loft more of it to compensate for its 28y half-life, and still come out ahead provided you can dispose of the extra heat at first. Few missions need to run decades.

outworlder 2y ago

Wow, such negativity. > There's no spacecraft-related reason to select this—it's strictly worse—it's just the US/EU are in a decades-long ongoing shortage of Pu-238, and this is the only available alternative. I don't think this is "pioneering" in any positive sense of the word. IMHO, it's embarrassing. No, but there are practical reasons to select this. It doesn't matter _why_ Pu-238 is unavailable, it matters that it is unavailable and this has been a heavy constraint on space missions. New Horizons almost got postponed (with a danger of cancellation) due to this. And even then it launched with 80% fuel. The question is, can they deliver americium RTGs ? That would be great.

ApolloFortyNine 2y ago
>It doesn't matter _why_ Pu-238 is unavailable Why not exactly? Because we used to buy it from Russia because we stopped making it ourselves? Anyways they reference here that it's expected to finally increase production to a reasonable amount soon. [1]https://en.wikipedia.org/wiki/Plutonium-238 https://en.wikipedia.org/wiki/Plutonium-238
deleted 2y ago
[deleted]

TaylorAlexander 2y ago

There is no spacecraft related reason to use this unless what you want is for your spacecraft to actually fly on schedule. Risk assessment from supply chain issues is part of engineering.

JumpCrisscross 2y ago

> the US/EU are in a decades-long ongoing shortage of Pu-238 Europe is and will continue to be. America is ramping up production [1]. [1] https://www.scientificamerican.com/article/behind-the-scenes-at-a-u-s-factory-building-new-nuclear-bombs/ https://www.scientificamerican.com/article/behind-the-scenes...

philipkglass 2y ago
The isotope used in space missions is plutonium-238. That Scientific American article is about weapons pits made of plutonium-239. The US has a large surplus of stockpiled Pu-239 for weapons: https://www.energy.gov/nnsa/articles/nnsa-issues-final-surplus-plutonium-disposition-program-environmental-impact https://www.energy.gov/nnsa/articles/nnsa-issues-final-surpl... Sadly, it is of no use for space missions.