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Comment by Retric | Hacker News Reader

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.

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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.

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.

So instead of 10% more of the high power density material, you want to use 500% more of the low power density material?

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.

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)

And you can't just slap in americium without consequences either.

Any energy source needs consideration initially. However it is a real advantage be able to modify one parameter without needing to adjust several others.

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