The Apollo Laser Ranger Retroreflector Experiment

[Ptolemy]

Does anyone know the reference and when the results of this was first published? My apologies, but my scientific literature searching skills are limited to biological science.

I can't help wondering if given the state of the technology of the early 70s whether this might not have been actually a little beyond them - a bit like those early experiments to prove relativity detecting small changes than their instruments were calibrated to read accurately.

I was reading this link

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html
Perhaps it is gilding the lily to impress the rubes but...

"The reflectors are too small to be seen from Earth, so even when the beam is precisely aligned in the telescope, actually hitting a lunar retroreflector array is technically challenging. At the Moon's surface the beam is roughly four miles wide. Scientists liken the task of aiming the beam to using a rifle to hit a moving dime two miles away...
Once the laser beam hits a reflector, scientists at the ranging observatories use extremely sensitive filtering and amplification equipment to detect the return signal, which is far too weak to be seen with the human eye. Even under good atmospheric viewing conditions, only one photon is received every few seconds."

Call me stupid, but if you take the an original beam dispersed to 4 miles wide, of which only about 1 foot squared is available to be reflected back, and that 1 foot square is dispersed on the way back to 4 miles wide, not including any slight imperfections in the retroreflector adding a slight angle. On top of which the 2.3-6 seconds the Earth will be rotating away - so that the beam will be being reflected back to a different position on the Earth's surface from where it was sent.

In sum, I am a little doubtful if in the early 70s they have the equipment that could detect and amplify this one photo every few seconds.

It would be interesting to read the original publication - if they actually published in a peer reviewed journal.

 

[russ_watters]

The moon is about 238,000 miles from earth, so the round-trip is 2.5 seconds. With a radius of about 3960 miles, and a rotational period of 24 hours, in 2.5 seconds, the Earth rotates about 3/4 of a mile at the equator. By all means, check my math on that.

Regarding peer review - this is one of the more important experiments performed and I'm sure there were dozens of papers published on it in the 70s. Perhaps someone else can help you find them...

 

[pervect]

Here's one to get you started

http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:gr-qc/0507083

many other articles are in the bibliography of this paper. The lead reasearchers are
Drs. James G. Williams, Dale Boggs, J. Todd Ratcliff and Jean O. Dickey, and the expriments are done at JPL.

 

[Ptolemy]

Russ - yes I meant to write 2.3 -2.6 seconds. In effect if the reflected beam has been reflected back over an area of 4 miles square as the Nasa article was suggesting the rotation perhaps isn't important. It sounds a little bit like detecting someone flashing a very dim pen light on the moon.

pervect - I am guessing this is probably the earliest and most accessible publication
H. H., Poultney, S. K., Silverberg, E. C., Wilkinson, D. T., Williams, J. G., and Alley, C. O., "The Lunar Laser Ranging Experiment," Science 182, 229-237 (1973).

There are a couple of earlier ones but less available sources.
Oh and
"Laser beam directed at the lunar retro-reflector arraybservations of the first returns," Science 166, 99-102 (1969).

Further it seems a bizarre and unnecessarily complex way to solve the problem, wouldn't putting a small solar powered radio beacon on the moon be a far more effective and efficient way of getting the same data?

 

[pervect]

I really don't understand your hostile attitude towards this experiment.

Offhand, I would expect a powered radio transmitter to suffer from numerous, obvious disadvantages: Size of the component required to be sent to the moon, limited lifespan, and lower accuracy due to the longer wavelength.

The corner reflector is passive, no power supply needed lightweight, small, and very rugged - it's still giving us results to this day.

This is not based on any particular detailed knowledge of the experiment, it's just common sense.

 

[Creator]

Further it seems a bizarre and unnecessarily complex way to solve the problem, wouldn't putting a small solar powered radio beacon on the moon be a far more effective and efficient way of getting the same data?

What data?
How can a one way signal from the moon get you ranging data? You'd have to know the exact time in which the signal was sent in order to determine distance, something which can't be known precisely in a one way transmission originating from the lunar surface.

Creator

 

[Ptolemy]

"How can a one way signal from the moon get you ranging data? You'd have to know the exact time in which the signal was sent in order to determine distance, something which can't be known precisely in a one way transmission originating from the lunar surface. "

Plenty of work arounds. A syncronised clock before departure or a receiver/transmitter.

I looked up the earliest reference and the dispersal of the laser beam said it made a spot on the moon 3.2 kilometers in diameter. I am just not convinced you can pick up the return through the atmosphere against the background of the lunar day.

It wouldn't be the first time that results were announced using instrumentation that was simply not sensitive enough. cf the 1919 solar eclipse and the measurement of gravitational effects on light.

 

[russ_watters]

Plenty of work arounds. A syncronised clock before departure or a receiver/transmitter.

No. Because of all of the relativity corrections - not to mention 30 years of clock error - this would be far inferior to the current method.

It wouldn't be the first time that results were announced using instrumentation that was simply not sensitive enough.

Since more than one experiment has been performed using this setup, that logic doesn't work here.

 

[pervect]

Browsing the web, I found a useful but terse description of a setup used at the university of Texas at Austin. Many labs apparently use this retroreflector, JPL (the first source I ran across) isn't the only one by any means.

The web page is here:

http://www.csr.utexas.edu/mlrs/dda.html

A non-technical description would be something like this, but see the original article for more detail.

The best data is gotten when the reflector is in the dark (this depends on the phase of the moon). Very sharp frequency filters are used to screen out any light other than the laser frequency. Spatial filters are used to make sure the light comes from the area of the moon where the retroreflector is located. A guide telescope tracks something visible to keep the telescope used for the ranging measurement pointed at the right place even when that place is not visible.

The approximate distance to the moon is known, so returned pulses that don't occur at the right time are also thrown out / not recorded. Single photon detectors are used. Various types of single-photon detectors are possible, there's a little more info in the article though not a lot. When you pay more, you can get a more sensitive detector.

U of T gets a measurement every few minutes this way.

Many different laboratories do ranging. (I presume they use different laser frequencies, this wasn't mentioned). The French have one of the better systems.

As technology has improved, the accuracy of the system has improved - because all the active elements of the ranging system are on Earth, this is easy to do.

 

[Garth]

There are many factors to be taken into account, so the simpler the experiment the better. You cannot get much simpler than timing an 'echo'.

The duration of this 'echo' does change with time.

The Moon's orbit is elliptical, it suffers perturbation and tidal interaction with the Earth, Sun and other planets, and GR effects. Furthermore there may be some secular evolution due to non-GR effects, a change in G for example.

Therefore this experiment is of fundamental importance; however analysis even of this simple experiment is model dependent. Separating out all the different factors will keep theorists happy for years to come!

Garth

 

[Chronos]

Are the error bars outside predicted ranges? I really haven't kept up with this project. I don't see any model dependent problems unless the speed of light is variable. In that case, it is very important.