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  Apollo: Laser Ranging Retro-Reflector placement

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Author Topic:   Apollo: Laser Ranging Retro-Reflector placement
LM-12
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posted 04-18-2014 08:55 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The Laser Ranging Retro-Reflector (LRRR) was an ALSEP experiment deployed by the moonwalkers on Apollo 11, 14 and 15.

Did the LRRR have to be set at a specific angle on the lunar surface based on the latitude of its location?

Headshot
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posted 04-18-2014 09:17 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
Yes. Each LLR had the following orientation devices:

A level Indicator
A gnomon
An angle indicator

Not only did the angle of the retro-reflector assembly have to be set during deployment, it had to be aimed towards the Earth via a gnomon device that cast a shadow on a scale.

The March 1970 issue of Scientific American has a nice article about the LLR.

LM-12
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posted 04-18-2014 09:54 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
In the lunar surface photos, the Apollo 11 LRRR seems to be angled west (away from the sun), and the Apollo 14 LRRR seems to be angled east (towards the sun), so I guess the longitude location was also a factor.

The Apollo 15 LRRR seems to be angled south.

Headshot
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posted 04-18-2014 10:04 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
I would guess that, for 14 and 15, the cuff checklists might have the figures for the altitude and azimuth settings for their respective LLRs. Did not Aldrin have a simplified checklist sewn or printed on his sleeve? Otherwise the flight plan may have had the figures for 11's LRR.

garymilgrom
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posted 04-18-2014 10:13 AM     Click Here to See the Profile for garymilgrom   Click Here to Email garymilgrom     Edit/Delete Message   Reply w/Quote
The Apollo 15 device was three times the size of those previously deployed.

Headshot
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posted 04-18-2014 10:18 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
Apollo 11's landing site was 23 degrees to the east of the sub-Earth point, so the LLR was tilted 23 degrees up from the horizontal.

LM-12
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posted 04-18-2014 11:44 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
I see. Sounds like all three reflectors would be parallel, or nearly so.

The Apollo 11 and 14 landing sites were close to the lunar equator. The Apollo 15 landing site was close to the prime meridian.

Glint
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posted 04-18-2014 12:25 PM     Click Here to See the Profile for Glint   Click Here to Email Glint     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by LM-12:
In the lunar surface photos, the Apollo 11 LRRR seems to be angled west (away from the sun), and the Apollo 14 LRRR seems to be angled east (towards the sun), so I guess the longitude location was also a factor.

Although any particular reflector may have been angled toward or away from the sun at the time of the EVA, at countless other times during the ensuing years the sun would have held various other positions in the east or west at odds with the initial orientation. Thus, the sun probably didn't have any significant influence on the configuration of their placement.

However, the Earth would have been "fixed" in the sky, except for the relatively small lunar perturbations. So any angling and orienting of the reflectors was probably due to the altitude and azimuth of the Earth at the site.

Blackarrow
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posted 04-18-2014 12:31 PM     Click Here to See the Profile for Blackarrow     Edit/Delete Message   Reply w/Quote
A better question would be: How much of a tolerance was there in the deployment angle, bearing in mind that the prisms were designed to reflect light back in precisely the same direction? That would suggest that if a LRRR was pointed 5 or 10 degrees away from the Earth, the incoming laser beam would still be reflected straight back to Earth. Or have I missed something?

Headshot
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posted 04-18-2014 12:52 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Headshot:
The March 1970 issue of Scientific American has a nice article about the LLR.
The Scientific American article makes a vague statement about Aldrin aligning the gnomon shadow "within a degree or two" of the desired orientation.

An interesting thing I noticed in the SA illustration of Apollo 11"s LLR is that there were three scales for the gnomon's shadow. I believe there was a dedicated scale for each potential Apollo 11 landing site i.e. Mare Tranquillitatis (ALS-2), Sinus Medii (ALS-3), or Oceanus Procellarum (ALS-5).

I would assume that the tilt angle of the LRR would have been different for each site as well.

LM-12
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posted 04-18-2014 04:27 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
It is mentioned in On The Moon With Apollo 15 on page 25 that there was an LRRR on Apollo 12, but I think that is an error.

Headshot
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posted 04-18-2014 05:19 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
I cannot recall, was a LLR part of Apollo 13's ALSEP?

LM-12
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posted 04-18-2014 05:28 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
No, it was not one of the five ALSEP experiments, according to the Apollo 13 Press Kit.

Headshot
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posted 04-18-2014 05:58 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
I doubt that the original H-4 Apollo 14 would have deployed one at Littrow as the distance between it and Tranquility would have been relatively short.

I wonder which landing sites would have had LLRs deployed had the original Apollo 11-20 missions been flown?

Probably LLRs would have been included in the J-2 Apollo 17 mission to the Marius Hills and the J-4 Apollo 19 mission to Hadley. That would have produced a much larger triangle.

LM-12
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posted 04-18-2014 08:55 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Headshot:
I doubt that the original H-4 Apollo 14 would have deployed one at Littrow
There is a 1970 document on the LPI website that indicates they would have deployed a reflector at the original Apollo 14 Littrow landing site.

LM-12
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posted 04-19-2014 12:19 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
They fired a laser at the Apollo 15 LRRR on one of the "MythBusters" episodes.

Max Q
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posted 04-19-2014 06:21 AM     Click Here to See the Profile for Max Q   Click Here to Email Max Q     Edit/Delete Message   Reply w/Quote
Fascinating thread. I read the PDF file from Bendix Aerospace they allowed five seconds to unstow the UHT from the YO-YO. What pray tell was the YO-YO?

Jeff
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posted 04-19-2014 08:53 AM     Click Here to See the Profile for Jeff   Click Here to Email Jeff     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Max Q:
What pray tell was the YO-YO?
Here's the surface flown Yo-Yo from Apollo 15 that was sold on Bonhams.

Headshot
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posted 04-19-2014 09:25 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
LM-12, nice find on that Bendix document, it is an interesting read.

That March 1970 article in Scientific American, which was probably written before NASA cut Apollo 20 includes the following sentence, "With data from four or more observing stations at well-chosen locations ..."

So apparently before Apollos 15, 19 and 20 were cancelled, scientists hoped to have four or more LRRs deployed on the lunar surface. So the short distance between Tranquility and Littrow would not have been important.

After the cancellations and, the number of LRRs was reduced to the minimum number of three, which would have required the greatest distance reasonable between them.

Headshot
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posted 04-19-2014 06:06 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
It is mea culpa time for me. Either the author of the 5/1970 Scientific America article on Apollo 11 LRR worded a paragraph badly, or I am unable to interpret subtle differences any longer. I won't tell you how my wife voted.

NASA always intended to put only three LRRs on the lunar surface. There was no intention of putting "four or more ..." The Scientific American author was referring to multiple fixed observation stations on Earth so that investigators could remove the effects of Earth's continental drift from the lunar ranging experiment results.

At any rate, a July 17, 1970 NASA/MSC document titled Apollo Experiments Program Plan (MSC-02410, Rev. 1) has tables of experiments to be included on Apollos 11-19. The document can be found on the Lunar and Planetary Institute website under ALSEP Documents.

LRRs were scheduled for the G-1 Apollo 11 mission, the H-3 Apollo 14 mission, and the J-1 Apollo 16 mission. So in 1970 LRRs were to be deployed at Tranquility, Littrow, and Descartes. A small network, but with three LRRs none-the-less.

The actual LRR locations at Tranquility, Fra Mauro, and Hadley resulted in a more spread out network over the lunar nearside that surely produced more exacting ranging results.

LM-12
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posted 04-19-2014 10:42 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The LRRR location is circled in this Lunar Reconnaissance Orbiter image of the Apollo 15 landing site.

LM-12
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posted 04-20-2014 08:41 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Headshot:
LRRs were scheduled for the G-1 Apollo 11 mission, the H-4 Apollo 14 mission, and the J-1 Apollo 16 mission.

The 1970 Program Plan has "H-3" as the original Apollo 14 Littrow mission.

Headshot
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posted 04-20-2014 08:52 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
You are right LM-12. I corrected my previous post. Guess I can't read or type this weekend.

LM-12
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posted 04-21-2014 09:18 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The Program Plan chart mentions bootstrap photos as one of the primary objectives. What are "bootstrap" photos?

Robert Pearlman
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posted 04-21-2014 09:23 AM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
"Bootstrap photography" was the term given to high resolution imagery of potential future Apollo landing sites.

LM-12
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posted 04-21-2014 11:04 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
That Program Plan document is dated July 17, 1970. The Apollo 14 landing site was actually changed from Littrow to Fra Mauro about two months earlier.

The Apollo 15 crew had been announced, but the landing site had not yet been selected.

LM-12
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posted 04-21-2014 02:25 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The Astronomy Photo of the Day website has a photo of a laser beam that was aimed at the Apollo 15 landing site during the total lunar eclipse last week.

LM-12
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posted 04-23-2014 08:26 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Headshot:
So in 1970 LRRs were to be deployed at Tranquility, Littrow, and Descartes.

I am reading the chart differently. To me, it says the original LRRR locations were going to be Apollo 11/Tranquility, Apollo 14/Littrow and Apollo 16/Copernicus.

Copernicus was the original Apollo 16 landing site.

Do I have that right?

Headshot
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posted 04-23-2014 02:41 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
You are not reading anything wrong LM-12, the real problem is that there are too many contradictory lists on the subject of site selection.

I know that, according to Where No Man Has Gone Before, in February 1970 MSC recommended placing Copernicus as the primary site for Apollo 16, but I am not certain that the Apollo Site Selection Board ever acted on this recommendation. Not two paragraphs later there is a footnote that MSC ruled out Copernicus and Censorinus as unsuitable site for landing due to rough terrain. Of course everything went out the window in Sept 1970 when Apollo 15 and 19 were cancelled.

According to Don Wilhelms' To A Rocky Moon, Copernicus remained a backup for Descartes, until May 1971, with Alphonsus and the Kant Plateau as alternates.

LM-12
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posted 04-23-2014 03:05 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
It can get confusing at times. But a landing at Copernicus would have been pretty spectacular.

Here is the central peak of Copernicus Crater in 3-D. http://www.nasa.gov/images/content/450784main_M102286291LE_M102293451LE_3d_lg.jpg

Headshot
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posted 04-23-2014 03:30 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
I agree that a Copernicus landing would have been spectacular. The problem was that there were two Copernicus landing site candidates, near the interior of one of the rims (called the "Wall" landing site) and near the central peaks (called Copernicus Peaks.)

Neither group of supporters would yield to the other and Apollo managers wanted to move on. When Copernicus was dated by findings from the Apollo 12 landing site and interior rebound material was collected at another Apollo site, the scientific value of Copernicus waned and the GLEP and ASSB moved on to other candidate sites.

LM-12
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posted 04-24-2014 08:08 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
Thank you Headshot for those additional details.

I see that landing site selection is also discussed on these other threads:

Headshot
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posted 04-24-2014 09:27 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
Thanks for those threads LM-12.

I find the study of Apollo landing site selection to be both fascinating and daunting. It was such a complex process and being implemented by so many people with sometimes totally different objectives and goals. Someday I hope to generate an annotated, single-page chart detailing the evolution of the site candidates.

This thread also made me realize that the process was far more complex than I thought because of the large variety of experiments that needed to be dispersed at each Apollo landing site.

As another comment to your original post. I recall reading, within the last year, that scientists have detected indications that the Apollo retro-reflectors are beginning to degrade with age. Does any cSer remember seeing anything about this?

LM-12
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posted 04-26-2014 06:29 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The relative position of the Apollo retro-reflectors is described in the Program Summary Report:

The three Apollo reflector sites form an almost equilateral triangle with sides 1250, 1100, and 970 kilometers, and are almost centered on the near side of the moon ...

The accuracy already achieved in lunar laser ranging represents a hundredfold improvement over any previously available knowledge of the distance to points on the lunar surface.

Headshot
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posted 04-26-2014 08:32 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
A paper describing measured degradation of the Apollo LLRs was presented at the Fall 2013 meeting of the American Geophysical Union(Abstract #P51G-1815). The authors have identified the primary candidates for the degradation as dust on the CCRs and/or UV darkening.

These findings will be incorporated into the design of the next generation of LLRs.

LM-12
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posted 04-27-2014 03:24 AM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The subject of lunar dust on the LRRR devices is briefly discussed in comments by David Scott and Eric Jones at 124:19:21 in the Apollo 15 Lunar Surface Journal.

There is reference to a July 2010 article in the science journal Icarus (Volume 208, Issue 1) titled "Long-term degradation of optical devices on the Moon".

Headshot
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posted 04-27-2014 09:32 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
A new issue of Sky & Telescope arrived yesterday included a brief synopsis of a recent article in Icarus about the performance degradation of the LLRs.

The performance degradation is worse around full moon. Researchers now believe that there is a dust layer over the surfaces of at least half of the corner cube reflectors of each LLR. I have not discovered if they have any idea of how thick the dust layer is. This dust: (1) scatters light and, (2) is heated by sunlight (with max heating around full moon), causing optical distortions in the corner cube reflectors.

I also seem to recall that, at one time, it was believed the Teflon spacers holding each CCR in place were degrading as well, which would destroy the critical alignment. But I have not read anything further on that issue.

LM-12
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posted 05-16-2014 01:39 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
The Hadley-Apennine landing site for the Apollo 15 (J-1) mission was selected in September 1970. That was about four months before Apollo 14 was launched.

So they knew where Apollo 15 was going to land. Perhaps that is why the LRRR experiment was added to the Apollo 14 Fra Mauro mission. It was not on the Apollo 13 Fra Mauro mission.

LM-12
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posted 05-21-2014 11:14 PM     Click Here to See the Profile for LM-12     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Headshot:
Did not Aldrin have a simplified checklist sewn or printed on his sleeve?
Actually, it was Armstrong who deployed the LRRR on Apollo 11. Aldrin deployed the PSE.

All times are CT (US)

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