The lunar laser ranging experiment is still going 50 years later | Hacker News Reader

The lunar laser ranging experiment is still going 50 years later

spectrum.ieee.org

101 pointssohkamyung36 comments

I always liked the story of the Lunokhod 1 - the first of the two Soviet Lunar Rovers. Both had reflectors for laser ranging, but Lunokhod 1 was carelessly misplaced in the early 1970s. So no more ranging until 2010 when the NASA reconnaissance orbiter found it again. They didn't lose the other one. :)

I think one of the Lunokhods still holds the record for the furthest ever travelled by an extra terrestrial vehicle.

The Opportunity rover has driven further than Lunokhod [1]. I believe Curiosity has since driven further.

[1] https://www.jpl.nasa.gov/news/news.php?feature=4232

Lunakhod 1 was still responding in 2013 (40 years later) thanks to the NASA LRO (Lunar Reconnaissance Orbiter). French website, as it was a (secret) collaboration between Russians and the CNES who provided the retroreflector: https://sciences-techniques.cnes.fr/fr/web/CNES-fr/10757-st-...

Thanks for that, I'd never heard of the French collaboration before.

You would think the reflectors being on the moon would shut up the “moon landings were faked” crowd.

Alas, that's not how it works. Any video you could show of the laser experiment would also be faked. Even if you were to set them in front of the equipment, that'd be faked too. I often wonder as a thought-experiment what would happen if you bundled them onto a Musk style space tourism rocket and flew them to the moon to see the flag (for a bonus they can look back to see the spherical Earth) and then back, would they even then believe the conspiracy or their lyin' eyes?

(Then I catch myself on and realise it wouldn't even theoretically be worth wasting all that effort and money on someone so determined to be willfully ignorant. We live in a world where schoolkids can send a camera high enough to see the curvature of the Earth, yet we are to believe people who have made the denial of this a core of their identity do not see fit to replicate the experiment?)

I tell them that it’s faked and that NASA hired Stanley Kubrick to film the whole thing. He did such a good job because he filmed it on location.

That Mitchell and Webb has a brilliant sketch on this premise https://www.youtube.com/watch?v=P6MOnehCOUw

<Devil's advocate>No point sending a camera up, the camera manufacturers have tweaked the firmware to create the illusion of curvature.

I _know_ the earth is flat, so I already know that GoPro are in on the conspiracy as well.</Devil's advocate>

Your "throw them on a rocket" idea is the only realistic way to convince them I would imagine. Which gives me an idea, maybe that could be my ticket to the moon

Won't work. The "rocket" is simply an advanced simulator with the ability to simulate zero G with high-def displays. No matter what kind of evidence you provide, they will always provide a (far fetched) counter argument.

You can't even convince me that there is a Moon until after you release me from this simulation.~

Right. And even if they conceded that they were on the moon, they'd argue that the 1969 landing was still fake, with props brought to the moon way later.

I'm considering starting a solipsist meetup group. I can't be sure that there are any like-minded solipsists out there.

Which one of us is me?

It might be able to simulate zero-G, but any linear acceleration in excess of 1G cannot be simulated by a stationary device.

That's what you want me to think but you're part of the conspiracy too...

What if the moon is just really shiny?

Jokes aside, the Moon is rally dark. With an albedo of 0.12, it is one of the darkest major bodies in the Solar system, save for Mercury with an albedo of 0.11.

As if it's not possible to place the reflectors using a robot.

i guess it was just lucky they had robots visit all the apollo sites to set them up, hey....

Maybe they don't know about them. It was an astonishing achievement - skepticism has its place.

It isn't as simple as point laser at moon, see light shining back with your own eyes, it requires a big telescope and computers and delicate sensors to detect because the moon and earth are so far apart.

I mean, if actual pictures of the landing site from LRO don't convince them: https://airandspace.si.edu/multimedia-gallery/628457main1apo... then a few extra photons returning to the earth won't change their mind. (My personal favorite, in an aesthetic sense, is: https://www.nasa.gov/mission_pages/LRO/news/apollo-15.html)

I have long wondered how easy it would be to perform this experiment as an amateur. Powerful lasers are quite cheap now. You could get a telescope (doubling as a beam expander and receiver), and some filters to only allow the laser wavelength to your detector.

Post this to HN when you do it. I would love to follow suit and play around with the idea.

Actually very difficult. They use a 30in telescope for these observations and the beam becomes extremely attenuated. The laser 'spot' projected on the moon is roughly 7km across. Only a tiny fraction of the light is reflected back as the reflector is only a few feet across, and is similarly attenuated on the return journey. The return beam is 20km across by the time it hits the Earth.

Due to the extreme attenuation of the final signal the observation takes several hours using highly sensitive equipment, averaging it out to extract meaningful data.

There's a pretty thorough discussion on Reddit[0].

[0] https://www.reddit.com/r/askscience/comments/qw30x/can_an_am...

The did on The Big Bang Theory show so it's got to be possible :-)

https://movies.stackexchange.com/questions/24686/is-this-las...

YouTube link (for those that like the show): https://www.youtube.com/watch?v=_v52LFgUq-8

I've heard it said that this is the most significant figures ever accurately recorded in any measurement, ever.

Also see the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO)[0]: "APOLLO measures the round-trip travel time of laser pulses bounced off the lunar retroreflectors[1] to a precision of a few picoseconds, corresponding to about one millimeter of precision in range to the moon."

[0]: https://tmurphy.physics.ucsd.edu/apollo/apollo.html

[1]: https://tmurphy.physics.ucsd.edu/apollo/lrrr.html

It also highlights how few impacts the moon gets these days as any impact near this https://en.wikipedia.org/wiki/Retroreflector would kick up moon dust and have a detrimental effect upon such a reflector.

Which is encouraging for any solar installations upon the moon for the one day, eventual moon base.

isn't the lack of impacts near the retroreflector due to the fact that the moon surface around the reflector is always facing earth?

The earth covers a pretty small part of the sky for the moon, so I'm not sure it would have that much effect?

According to NASA [0], you are correct. Furthermore:

"It is likely that each side of the Moon has received equal numbers of impacts, but the resurfacing by lava results in fewer craters visible on the near side than the far side, even though the both sides have received the same number of impacts."

[0] https://lunarscience.nasa.gov/?question=3318

The earth looks huge from the moon. The moon’s diameter is 2,158.8 mi where earth’s is 7,917.5 mi, meaning it looks 3.5x as wide and covers 13.5x the area. So more your fist at arms length than your thumb.

In terms of impact gravity and atmosphere extend that somewhat. Though, that’s still not enough to make a meaningful difference.

If he were still alive today, Stanley Kubrick would be very happy that people are still finding his movie set useful.

Down votes? Really? People, lighten up. Did you not see the smiley?

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