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The BBC News article Aeolus: Wind-mapping space laser is losing power says that

Europe's Aeolus satellite was launched last year to gather data to improve weather forecasts, and its observations have unquestionably proved their worth.

However, the laser is now degrading and has already lost half its power.

Engineers plan to switch Aeolus to its back-up light source in June to see what difference this could make.

If the same issues arise, the UK-assembled spacecraft may not be able to complete the minimum three years expected of the mission.

"We're losing strength in the outgoing power of about 1 millijoule per week," said Dr Josef Aschbacher, the director of Earth observation at the European Space Agency (Esa).

"We don't know why. We have some speculations but that's all.

"That's the bad news; the good news is that despite the degrading laser, the quality of the wind data is fantastic." (emphasis added)

What is the most likely cause, or at least what is the speculation? My answer to Do things get dirty in space? describes the problems of UV-induced degradation of optical surfaces by that has plagued Aeolus for a decade:

The BBC article Aeolus: Wind satellite weathers technical storm summarizes a drama of over a decade where the Earth wind observing spacecraft project was delayed because the ultraviolet optics kept getting dirty even in a simulated space vacuum.

Outgassing (mentioned in @called2voyage's answer to Do things get dirty in space? as well) often contains carbon-based molecules, and when these are combined with ultraviolet light (as mentioned in @Tristan's comment) they can crack and become more permanently attached to surfaces. In this case these were extremely important and numerous optical surfaces.

Is it most likely that they never fixed this problem, or is it more likely to be something new, like atomic oxygen perhaps? Aeolus (2018-066A, 43600) is only at about 320 km.


Source "Engineers had to find a way to stop the laser damaging its own optics"

https://www.bbc.com/news/science-environment-44415752 "Engineers had to find a way to stop the laser damaging its own optics"

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    $\begingroup$ alternate and highly apostrophized title: Why is ESA's Aeolus' laser's power's decline so rapid? $\endgroup$
    – uhoh
    May 15, 2019 at 5:54
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    $\begingroup$ Notably, this question might remain unanswered until the operational team of Aeolus publish their final reports on this case. $\endgroup$
    – Mefitico
    May 16, 2019 at 14:01
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    $\begingroup$ Your dually-upvoted* answer ;)? Thanks for sending me down that rabbit-hole of googling stuff about UV degredation. $\endgroup$ May 20, 2019 at 21:31
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    $\begingroup$ @uhoh the link credited to me in this question (space.stackexchange.com/a/17198/12102) doesn't appear to actually be by me. $\endgroup$ Oct 23, 2019 at 14:23
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    $\begingroup$ @OrganicMarble fixed... finally $\endgroup$
    – uhoh
    Jan 18, 2022 at 20:05

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Short version: I don't know. But I can say that space-based lasers have a history of flight anomalies. ICESat-1 being a prime example.

In that case, the failure review board concluded (link below):

...that the most likely cause of failure of GLAS Laser 1 was an unexpected failure mechanism in a pump diode array that resulted in excessive power degradation and catastrophic failure. Manufacturing of the laser diode arrays introduced excessive indium solder that resulted in a metallurgical reaction that progressively eroded the gold conductors through the formation of a non-conducting gold-indium intermetallic, gold indide, at a rate dependent on temperature.

The second necessary factor contributing to the catastrophic failure was the development of a current shunt in a diode array bar that combined with the relatively high current requirements (compared to MOLA) in the gold wire bonds. The current shunt forced the redistribution of current, which accelerated gold indide formation and wire fatigue through increased thermal stresses and ultimately contributed to open bond wires. At some point the remaining wires were unable to handle the stress and fused open. An arc then formed a short to ground that overstressed other diodes, and fused them open as well.

Note that this is a very specific cause, not something generic, and I doubt very much that Aeolus' anomaly's cause, whatever it turns out to be, will be the same.

ICESat-2 flew a laser too, and as far as I know, it has performed well in flight so far. However, the laser development had serious development issues, as described in the second link.

The ICESat lasers are near-infrared instruments, while Aeolus is ultraviolet. Generating that frequency is another technological step. And ultraviolet optics are exquisitely sensitive to contamination. Having issues like this are of course unfortunate, and I'm sure the team did everything they could to ensure success. But given the track record of space-based lasers, I can't say it's a big surprise to me.

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    $\begingroup$ "I don't know." & "I doubt very much that Aeolus' anomaly's cause, whatever it turns out to be, will be the same." So, not an answer to the question that was asked? $\endgroup$ Jan 18, 2022 at 20:53
  • $\begingroup$ If the question was about electronics failures this would be an interesting partial answer. But this question is about a specific failure of a specific laser on a specific spacecraft, and the source of the slow degradation in optical performance seems to be... optical and related to outgassing, rather than a bad solder joint. Also, what is "gold indide"? $\endgroup$
    – uhoh
    Jan 18, 2022 at 21:07
  • $\begingroup$ Organic Marble: That's right, I don't know. I thought it would be helpful to give some background to problems with similar systems. $\endgroup$ Jan 20, 2022 at 0:19
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    $\begingroup$ uhoh: The question quoted as follows: ""We're losing strength in the outgoing power of about 1 millijoule per week," said Dr Josef Aschbacher, the director of Earth observation at the European Space Agency (Esa). We don't know why. We have some speculations but that's all." Outgasing is one possibility, and certainly it is a strong one. But it's by no means the only one. I was trying to give some background as helpful information, not claim I know how their failure investigation will turn out. As for gold indide, sounds like it is a compound of gold and indium. They call it an intermetallic. $\endgroup$ Jan 20, 2022 at 0:23

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