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@Heopps' cool answer to the question Any proposed missions to explore the black liquid on Titan? Technical challenges? has an interesting line:

ASRG generator was planned as compact energy source but its development didn't end well enough.

which makes me wonder just how well it ended.

The Wikipedia article for the Advanced Stirling radioisotope generator says:

Despite termination of the ASRG flight development contract in 2013, NASA continues a small investment testing by private companies. Flight-ready Stirling-based units are expected by 2028.

which is still so far in the future that it seems speculative.

The downside of the technology versus thermocouple-based heat-to-electricity conversion includes things like added complexity and moving parts, but I assume the upside is a higher efficiency which means for a given power output either a smaller quantity of a given isotope can be used or a less-effective alternate radioisotope can be selected.

The energy conversion process used by an ASRG is about four times more efficient than in previous radioisotope systems to produce a similar amount of power, and allows it to use about one quarter of the plutonium-238 as other similar generators. (emphasis added)

So I'd like to ask; what is the current status of Stirling engine-based radioisotope generator technology? Has the technology fallen by the wayside in lieu of solar panels, thermocouple-based RTGs and kilopower fission reactors, or is this technology coming along nicely and may indeed be ready in 2028, or even sooner?

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    $\begingroup$ Any NASA technology being worked on usually has reports on NTRS: ntrs.nasa.gov - I’d search there. As far as I know, work on Stirling engines is still ongoing. $\endgroup$
    – hmode
    Commented May 17, 2019 at 1:46
  • $\begingroup$ @hmode thanks, I'll take a look and see how far I can get. For the purposes of this question the work would not have be necessarily associated with NASA, substantial progress being made at any agency or company would count. $\endgroup$
    – uhoh
    Commented May 17, 2019 at 1:49
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    $\begingroup$ "1-W Stirling RPS is being developed for notional surface missions" -Nasa, October 2018. With system testing planned in 2020 I would assume the development is coming along nicely, although this is a topic which is properly underfunded. Google Scholar is full of recent papers (2018-2019) about the technology $\endgroup$
    – GittingGud
    Commented May 17, 2019 at 6:14
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    $\begingroup$ You've only been here 45 days but you are really @GittingGud at this! Why not write that up as an answer? ;-) $\endgroup$
    – uhoh
    Commented May 17, 2019 at 6:23
  • $\begingroup$ Wouldn't Stirling engines degrade over time as they wear down? You want your RTG to last, right? $\endgroup$
    – user31448
    Commented May 17, 2019 at 14:15

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According to this presentation from October 2018 held by NASA employee Scott Wilson, NASA is currently working on a radioisotope power system (RPS) using Stirling engines as the generator.

The RPS is supposed to be a low power electricity source of 1-10W DC, with a 1 watt demonstrator/prototype being currently worked on.

The development goals for the RPS is a life time of 20 years with proposed use in small spacecraft missions, especially for notional surface Moon and Mars missions.

The Roadmap currently is:
2018 - Initial Demonstration
2019 - Mature Fidelity
2020 - System Testing

Additionally Google Scholar has several recent publications on this topic, from which a lot are from NASA.

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