Lunar Ranging experiment on the Big Bang Theory

Factual Questions
1

On an episode of The Big Bang Theory, the characters run a lunar laser ranging experiment on the roof of their building - clip.

Is the depiction of the experiment and its apparatus accurate, besides the obviously bad special effects? Is it feasible to successfully bounce a laser off the Apollo retroreflector and detect its reflection using only portable equipment?

Are there any documented instances of amateurs or students successfully doing the experiment themselves? Is it even remotely possible to do with readily available equipment? I know it’s usually done at places like the McDonald Laser Ranging Station, and I suspect it’d be impossible to obtain the necessary power and accuracy otherwise, but googling about I couldn’t find anything definitive.

2

What’s bad about the special effects? The pulsing was a lot slower than it should have been, but millisecond-pulsing wouldn’t have shown up on TV. If you’re referring to the visible beam, yes, any laser powerful enough for this experiment would show a visible beam from atmospheric scattering.

Accuracy wouldn’t be a problem, since the beam is going to spread out anyway, so you only really need to get your aim as good as the beam’s spread. Power would be the big question, and on that, I don’t know if student equipment would be enough.

3

Maybe it’s not so bad - discount that comment then, it was just an aside.

This blog post suggests accuracy is a major problem: “it’s like a pigeon pooping directly into my coffee cup while flying at 35 km/hr at 32,000 feet”.

ETA: To clarify, I think that statement in the blog post is referring to the accuracy required to aim, not the accuracy of the range measurement. I’m not really interested in whether it’s possible to measure the distance accurately, just whether or not it’s possible to detect reflected photons.

4

Although, come to think of it, they weren’t doing it to try to get an actual precise measurement that they would draw some physics conclusions from; they were doing it for the sake of doing it. They could be using longer integration times to compensate for a less powerful and less-well-collimated laser, at the expense of precision of the distance measurement, and that could also explain the slow pulses.

5

It’s just a TV show… Don’t confuse it with reality. Sorry.

6

He calculates a beam about 10 km wide for a 1 m diameter beam. If their beam was instead eight inches in diameter (e.g using an 8 inch scope backwards), that’s about 1/5 the size, and the beam would be closer to 50 km wide at the Moon, or about 1.5 percent of the Moon’s diameter. Hard but not outlandish, I would think. (If it was easy, everyone would do it!)

It wouldn’t surprise me if there was ephemeris data for the lunar retroreflectors. Maybe they just punch in the numbers in their telescope drive.

7

What precisely is the point of this post? The o.p. clearly knows that the show is fictional; his question is whether the event and instrumentation portrayed therein are remotely workable. Given that the show does employ technical consultants from both Caltech and JPL, it is entirely reasonable to speculate on how much influence they have over how science and technology is portrayed on the show.

Not only would it be possible for a team of knowledgable amateurs to perform this experiment, but there is actually a listserv dedicated to free space laser rangefinding. I recall reading about an SPIE program to build an amateur laser rangefinder (although I don’t know if I would really call professional members of SPIE “amateurs” in any sense of the word except that they’re not funded by government or industry). Although the equipment portrayed on the show sounds too small to be workable, and a residential power supply is probably not adequate to provide sufficient power for the laser, it is not globally infeasible. Maybe they shouldn’t have shorted across the building transformer…but more importantly, did they get a charge? cue up a Bryan Adams song and a pool party in the orchestra pit with students from the Wanda Trussler School of Beauty :wink:

Stranger

8

nm

9

If I understand correctly, the fascinating thing about the lunar ranging experiments is that we can measure the distance to the reflector to within about one centimeter and we can use those measurements to tell us the speed the moon is receding from the Earth to within about one millimeter per year.

10

Thanks Stranger. I get a kick from knowing that people are experimenting with this stuff in back yards and garages.

That list appears to be about laser communications in general, not rangefinding specifically, but poking through the archives I found this thread, which looks to describe an unsuccessful attempt to bounce off the lunar reflector.

The page here has more details and some pictures of the equipment involved.

11

Mythbusters went to the Apache Point Observatory to see their laser ranging operation.
They said that the laser is a 1 gigawatt laser. They sent about 200 quadrillion photons at the target at got about 1-3 back.

12

Amaters do bounce radio signals off the moon

Brian

13

Your link doesn’t work, but bouncing ham radio signals off the Moon is different from lunar ranging with a laser off of the retroreflectors left on the Moon by the Apollo and Luna programs.

Stranger

14

Ah, yes, I’d forgotten about that.

Wiki says the first LLR tests were by MIT in 1962 - though obviously that wasn’t with the Apollo retroreflector. I don’t know how powerful their laser was, but I’ve found a couple of mentions of lasers being in the 10 watt range in the early 60s, and kilowatts a few years later. Suffice to say that it can be done with a smaller laser than the one shown on Mythbusters.

15

Shooting the Moon.

A good article describing the work required to bounce light off the ALSEP reflectors. A laser beam fired from earth is about 1 mile across when it gets to the moon, and the reflected signal is about 10 miles across when it gets back to the Earth.

The team doing laser rangefinding using the Apollo reflectors has a 3.5 meter telescope for retrieving the return signal. That’s a BIG telescope. You can see one here. Even so, they describe retrieving actual photons from the reflectors as ‘winning the lottery’.

So no, a bunch of guys on a rooftop will not be able to do this. But in the world of TV, you have to give them plenty of credit for at least getting the basic facts right about using lasers to bounce off of reflectors left by Apollo to measure the distance to the Moon. That alone is impressive attention to detail.

16

I do not know if an amateur could pull this off. Technically I suppose they could but I think the equipment is out of reach of most people.

Mythbusters (which was debunking the myth that the moon landings were a hoax) went to an observatory to have them shine a laser on the moon and get the return from the reflector.

According to the show (jump to about 1:30 to get to the meat of it) the laser used is a 1 gigawatt laser and shines around 200 quadrillion photons per pulse at the moon. They get a return from that of about 1-3 photons per pulse.

So, do amateurs have access and power sufficient to run a 1 gigawatt laser? Do they have access to detectors capable of picking out 1-3 photons of returned light?

I have no idea but seems a reach for me to think an average schmo could have this (although there have been wealthy amateur astronomers with pretty amazing equipment).

17

on one hand, aren’t these guys grad students or post docs or something? things can be “borrowed” from the science dept.

on the other hand, the stuff they need seems too large to be lifted. also, they’re on an apt. rooftop in the middle of the city. the light pollution is probably too much for them to overcome for an experiment like this, with the equipment they have access to.

18

As above, this experiment first done in the 1960s, when lasers were nowhere near the gigawatt range. This PDF mentions pulses of 3 and 7 joules used for some of those early experiments.

(I do think you’re right though - nobody’s going to do this with stuff bought at a hardware store, and even with a bunch of lab equipment it’s probably out of reach)

19

The lasers used in the McDonald Observatory experiments from 1969 onwards may not have quite exceeded the one gigawatt threshold initially, but they were certainly close, even back then.

With pulsed-laser ranging, it’s crucial to send out as many photons as possible (to increase one’s chances of detecting the “return” signal) in as short a time as possible (since fine distance measurement relies on short laser pulsewidths). A high repetition rate is also a bonus, in order to get good signal-averaging.

According to the above-linked pdf (5th page), the ruby laser’s Q-switch dumped the first (i.e. oscillator) stage in around 4ns. This was typically amplified in the three following stages up to 3 Joules per pulse.

Let’s say that the resulting output maintained the 4ns pulsewidth (they don’t show the temporal pulse shape in the paper, but the following page says: “The statistical fluctuation in the range measurement for a single pulse is roughly ± 2 nsec because of the laser pulse length and some jitter in the photomultiplier”, which supports a ~4ns output pulsewidth).

Then, 3 Joules of laser energy in 4ns gives a peak power – which is what counts in this type of experiment – for the 1969-era laser of 0.75gigawatts.

Now, the average power of that ruby laser would only have been around 1W (a 3 Joule pulse every 3 seconds), but it’s the peak power that made the experiment feasible. It just took them a long time to accumulate good data with the laser only firing once every 3 seconds…

By comparison, if one considers the laser in the Mythbusters clip (the Apache Point Observatory Lunar Laser-ranging Operation, or APOLLO): the setup starting a few years ago is described in this PDF (page 4, bottom). The energy per pulse is 115mJ (cf. 3J in 1969), but the pulsewidth is now 120ps (cf. 4ns in 1969), which gives just under 1 gigawatt peak power – not that much more than the 1969 experiment! The laser’s pulse repetition rate at APOLLO is now 20Hz (cf. 0.33Hz in 1969), so it takes a lot less time to get good data.

*[And the guy in the video comparing the 1mW of a laser pointer with the 1GW of the pulsed APOLLO laser? Average vs peak, apples vs oranges. The average power of the APOLLO laser is around 2.3W, which doesn’t sound nearly as impressive… since the laser pointer is a continuous-wave device as opposed to pulsed, its peak and average have the same value.]
*

20

The power is tricky, but detecting the 1-3 photons isn’t all that hard, if you have a way of distinguishing them from the background. That’s one advantage of using a laser: It’s a very narrow frequency line.