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Lasers today have become extremely sensible, they can detect sub-millimeter movements of a surface from a distance. Could this feature be exploited to create a lunar seismometer working from a probe orbiting the moon instead of placing an instrument on the surface? The main problem is that the lunar surface is not as reflective as it seems. But on the other hand there is no atmosphere, no attenuation, no distortion.

The advantage would be the possibility to monitor many places at once allowing an extensive coverage. The other advantage would be the reduced cost and reduced mission risk because there would be no landing and no need for landing equipment.

BTW The same principle could be applicable to many other space objects, but the moon seems the natural testbed for the idea.

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  • $\begingroup$ laser vibrometry is really cool; the more I think about this the more I think that various challenges are solvable, though I'm definitely curious about what the right way to do it is. It seems like you'd want to track surface features from the satellite instead of just pushbrooming along. $\endgroup$
    – Erin Anne
    Commented May 26, 2023 at 18:05
  • $\begingroup$ @ErinAnne I was thinking about an orbit that is not too fast. Choose some points at regular intervals and monitor them while the satellite is around the vertical position. Another way would be to choose some landmarks that for their shape are likely to vibrate more and monitor each of them for a fixed interval of time. I don't think that scanning the entire surface would return useful data in this scenario. $\endgroup$
    – FluidCode
    Commented May 26, 2023 at 18:19
  • $\begingroup$ @FluidCode the slower you want an orbit to be relative to the ground track, the further away the object will be from the ground $\endgroup$ Commented May 27, 2023 at 4:46
  • $\begingroup$ One of the benefits of a network of land based seismometers on Earth is there are numerous seismometers that can begin recording seismic motion as it happens & for large events results can be compared with other seismometers & magnitudes ascertained accordingly. If satellite based seismometry, using lasers, were possible numerous satellites would be required, possibly in "geosynchronous" type orbits, if all seismic data needed to be captured. If only one satellite was used, a quake on the other side of the celestial body might be missed or only partially captured. $\endgroup$
    – Fred
    Commented May 28, 2023 at 5:28
  • $\begingroup$ @Fred If they are tidal moonquakes and the probe monitored many spots for an interval of time on the long term some kind of pattern could emerge. But the probe in order to be stable would have to be far from the surface, more than 1000 Km or even 2000 Km. That distance would also allow to keep a laser close to the vertical for a longer period of time. But I do not know If ranging from big distances would work. $\endgroup$
    – FluidCode
    Commented May 28, 2023 at 7:53

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The biggest issue is probably that earthquakes, at least on earth, are short-term events. The longest lasting ones are for mere seconds, while the longest recorded one was around 10 minutes. That does not include the quakes tapering off at the end or aftershocks. There is a lot of information on earthquakes, obviously.

From what I can find, due to the dramatic differences between the earth and the moon (molten vs. solid core, etc.,) "moonquakes" last quite a bit longer; large, shallow ones can hit a 5.5 on the Richter scale, and 10 minutes duration at maximum movement has been seen. Then they taper off slowly, up to being over several hours. There is relatively little data and a lot of unknowns on them.

So even though they aren't quite the same thing, these are still relatively brief events compared to say a high altitude satellite orbital period. Unfortunately, most satellites are always "moving" in their orbits. Well, I guess there must be a luna-synchronous (?)(!) orbit, but again, you would be locked into a limited observational area.

For a probe/satellite, even in a very high, slow orbit, it might only be able to face one third of the surface of the moon at most; at lower orbits this would be a smaller fraction of the surface. You would only be able to observe vertical movement; quakes are multidimensional.

Plus the peripheral observable surface would be at a very low angle instead of from above, making detection of any motion more difficult; the distance from the probe would not change in the case of vertical motion on the surface anywhere near the edges, and the angle would make both vertical and horizontal measurements less precise. In addition, the reflectivity of the surface at such angles would probably be far less.

So what we end up with is only periodic observation of the desired areas, yet we're looking for brief, finite events. The probability of taking a measurement coinciding with the quake event would be fairly low with a single satellite, at least for localized events.

This could be solved, however by using more than one satellite. A minimum of three in a very high equatorial orbit could start to approach full coverage at the lower latitudes, but practically at least twice that many would be required. Then you would also want to use different orbits for additional probes to observe the polar regions for which there is currently no data at all.

Also, data from the "dark side" wouldn't be in real time without some data relaying being provided to earth. Although I believe that a system may be in place for that, or in the works, not quite sure.

The four instruments that the Apollo missions placed on the moon worked for between 5 and 8 years each, providing a limited quantity of data already. But of course, that is only for the four specific locations that existed, whereas a satellite could observe many, many points in very large areas. But only some of the time.

The big advantage of being on the ground is that you can observe all motion directions at that location; and with the moon being tidally locked to the earth, direct communication is possible on the side facing the earth. You would still need data relaying on the other side.

The concept and use of technology is good, but perhaps the implementation would be more difficult than using localized measurement.

References: Comparative information on earthquakes. https://www.osti.gov/servlets/purl/67453/ NASA lunar seismometer and moonquake information. https://science.nasa.gov/science-news/science-at-nasa/2006/15mar_moonquakes

P.S. I'm a total newbie here, so any feedback on whether this Answer is appropriate, acceptable or proper would be appreciated.

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Wikimedia's "File:How-the-Richter-Magnitude-Scale-is-determined.jpg" https://commons.wikimedia.org/wiki/File:How-the-Richter-Magnitude-Scale-is-determined.jpg

Source: Wikimedia's File:How-the-Richter-Magnitude-Scale-is-determined.jpg

Assuming you are sitting on top of an Earthquake (there's no corrections for distance included in this graphic) a magnitude 6 earthquake would cause a displacement of something like +/- 1 meter, and for magnitude 2.5 the displacement might be something like 1 mm.

At some nominal frequency, say 1 Hz or $2 \pi$ rad/sec, those would correspond to velocities of the order of 6 m/s and 6 mm/s, making them potentially amenable to laser Doppler measurements - even on rough ground.



Laser distance measurements

This is done with time-of-flight measurements for the photons, and can be accurate enough to map your face!

Since the Moon is mostly rough with craters and rocks and you need to laser-measure line-of-sight displacements on the order of say millimeters or centimeters for "normal" moonquakes if you are pointed at the epicenter (unless you only want to detect the biggest moonquakes (M 5.5)), I think the only hope of getting good distributed readings is either

  1. sprinkle corner cube retro-reflectors over the Moon so that there are always several to choose from from orbit (I'm thinking somewhere between a dozen and thousand of them) and
  2. aiming for a flat area for some kind of averaging, and not letting your beam deviate from its location during the measurement

Laser doppler measurements (like a police "radar gun")

This works better because you're working in frequency space, and it might work on the Moon from a satellite as well.

If you have a narrow beam spot at the surface of the Moon (which you'd better!) then all the boulders and craters in your field of view will be moving at roughly the same speed relative to your satellite. So you'll get a strong Doppler peak which will be constantly changing due to geometry but pretty narrow.

Assuming you use a sufficiently narrow band (long "coherence length") and low-frequency-noise laser, you'll see any ground motion in your line-of-sight only as a random wiggling of that peak position on the scale of say 0.1 to 10 Hz, superimposed on the super-duper smooth drift in peak position from the geometry of the satellite's smooth orbital motion and the Moon's smooth rotation.

While probably being easier on a lunar surface sprinkled with retro-reflectors, I don't think it will be necessary so

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