In Local expansion measured, near zero via Lunar Ranging - what about deep space probes? I used images of the retro-reflector arrays that were left on the moon by Apollo 11, 14, and 15. The Lunar Ranging Retro Reflectors (LRRRs) are there for Lunar Laser Ranging.

The arrays and their supporting structures appear to be significantly different from each other, suggesting design improvements or slight repurposing. Looking closer at the images, I see auxiliary structures on each one. What are all these things and how were they used?

Some appear to be sun-dial-like (for want of a magnetic field), and at least one looks a little like a bubble level with a vertical tab next to it. I'm pretty sure B is not there to blow moon dust off of the array :) - it could maybe be an infrared thermometer, but was there telemetry? See Lunar Laser Ranging Tests of the Equivalence Principle, page 8 for a discussion of the relevance of the widely cycling temperature of the array - for Equivalence Principe (EP) experiments it's necessary to measure throughout the entire 27 day long lunar day-night cycle.

Only one shows the faces of the reflectors directly, and they appear to be corner-cubes, which, individually at least, don't necessarily have to be pointed very accurately at all in every-day use, although I imagine an antireflection coating might have a cone of best performance.

Since the orientation of the arrays is always changing due to Libration - albeit in a very well determined way - if careful orientation was necessary, or at least careful documentation of the orientation, why?

Lunar Libration image from here

Lunar Libration GIF

below: Apollo 11 LRRR annotated detail, original image here (0.7°N 23.5°E)

Apollo 11 retroreflector array detail

below: Apollo 14 LRRR annotated detail, original image here (3.6°S 17.5°W)

Apollo 14 retroreflector array detail

below: Apollo 15 LRRR annotated detail, original image here (26.1°N 3.6°E)

Apollo 15 retroreflector array detail

After reading the answer by @Andy and the linked EASEP Handbook For Apollo ll Flight Crew, I've included Table 4.1-2 here as well.

Apollo 11 EASEP handbook screenshot annotated


1 Answer 1


The Apollo 11 EASEP handbook gives basic information on the first version. Of the LRRR, it has just two pages of text...

There are some (poorly reproduced) images labelling the main parts. It's correct there is a sun "compass" and a level. The large part labelled "B" in the question is a rear support, presumably so the folded device could be stood on the soil during the deployment process.

The part labelled "A" in the question is a handle the crew may grab while aligning and setting up the device.

There wasn't any telemetry by the way, this handbook states the experiment was totally passive.

As for placement accuracy, an Interface Control Specification was written, which covers all of the many considerations which could matter when astronauts are handling the device. Of accuracy, it states only that levelling was to be done within 5 degrees by a bubble, and alignment of the sun compass was to be done within 5 degrees too.

The Apollo 15 version was larger (and has a better manual). Its level indicator has rings for 5, 2 and 1 degrees. enter image description here

(Note nothing I've described here gives the actual accuracy of the reflector prisms, so this is only a half-answer.)

(And a note: various documents mention a "bubble", but I gather at least some lunar equipment in fact used a small ball in a cup. This might have been to avoid having breakables, or just for better visibility. It's not clear what type was used in the LRRR.)

  • $\begingroup$ this is fascinating reading, thank you for finding all of it and reading all the way through yourself first (unless you are recalling from memory!) I know they were quite thorough and systematic, but reading the Interface specification - wow. Table 4.1-2 in the EASEP handbook gives the physical alignment tolerance. Presumably they documented the final values through "notes" and photos. Also good point about possible alternate "bubble" indicators. $\endgroup$
    – uhoh
    Apr 22, 2016 at 11:32
  • $\begingroup$ I'd call it an "answer and a half" rather than a "half-answer"! $\endgroup$
    – uhoh
    Apr 22, 2016 at 11:42
  • $\begingroup$ The interface specification makes scary reading... but it lists "applicable documents" at the beginning, and I haven't read these but I guess those list everything must be considered (including safety) when designing a device. So when a person designs an LRRR, he must account for everything in those when specifying it, how it works, how it's set up etc. $\endgroup$
    – Andy
    Apr 22, 2016 at 11:43
  • 1
    $\begingroup$ This is the kind of stuff that really puts the moon hoaxers allegations into stark perspective. $\endgroup$ Apr 22, 2016 at 12:25
  • 8
    $\begingroup$ @Rikki-Tikki-Tavi - who? Moon hoaxers? Think I've heard people talk about them but I won't believe they exist until I see photographic evidence :) $\endgroup$
    – Andy
    Apr 22, 2016 at 12:30

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