5
$\begingroup$

The question When did Houston figure out where the Eagle had landed? says (in part):

During Apollo 11's stay on the surface, Michael Collins was given seven different locations to look for the Eagle, all of them wrong. Additionally, an attempt to ping the Lunar Ranging Retro-Reflector shortly after deployment was unsuccessful.

but it gives no source for this information, so I thought it best to ask separately to find the best source(s).

Question: How soon after "The Eagle" (Apollo 11) landed did they first attempt to bounce a laser off the Moon? When did it first succeed to optically measure the distance from the Earth to the Moon? (Radar measurements and then unified S Band radio ranging were used then).

$\endgroup$
6
$\begingroup$

Apollo 11 retro reflector was placed on the Moon on 21 July 1969, the first successful use of the reflector was on August 1 and 3 by the Lick Observatory.

Returns were observed on August 20, September 3, September 4, and September 22, 1969, at the Mc­Donald Observatory.

Attempts were made almost immediately but there was only a brief time available before the moon became too low in the sky on that night. Ground instrument difficulties and weather problems caused further delays.

enter image description here

enter image description here

See the paper The Lunar Laser Ranging Experiment by Bender et al. published in Science. 182 (4109): 229–238.

Also the Apollo 11 Preliminary Science Report page 163 to 182.

$\endgroup$
  • $\begingroup$ wow that must have been really exciting! $\endgroup$ – uhoh Feb 1 at 10:47
  • 1
    $\begingroup$ Section “Initial Apollo 11 Observations” - 2nd column The illuminated spot on the Moon is typically 4 to 6 kilometers in diameter, but because each corner reflector sends teh light hitting it back in almost the same direction from which it comes, the return signal at the Earth from the reflector panel is 10 to 100 times larger than the reflected intensity from the lunar surface. It's probably a lot more than 10 to 100 times larger. $\endgroup$ – uhoh Feb 1 at 11:46
3
$\begingroup$

The first use of LIDAR to measure the distance to the moon was in 1962, around seven years before Apollo 11.

https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment#Overview

http://adsabs.harvard.edu/full/2004ESASP.561....3F

The retroreflector arrays left by Apollo 11 and by other lunar missions only made it easier to detect the return pulses.

$\endgroup$
  • $\begingroup$ The retro reflectors enabled to measure the distance to the moon and its variation with incredible accuracy, impossible without the reflectors. $\endgroup$ – Uwe Feb 1 at 22:43
  • 1
    $\begingroup$ That's pretty amazing to read about, thank you! The abstract of the Nature paper linked in both of your sources is only one sentence long: Experiments have been conducted to focus pulsed optical radiation on to the surface of the Moon and to detect the echoes. They claim only to be able to detect the return pulses, not a proper measurement. $\endgroup$ – uhoh Feb 2 at 0:16
  • 1
    $\begingroup$ The "Reminiscenses" article says "The Raytheon laser used for the Moon experiment was capable of radiating 50 J pulses of 0.5 ms at 6943 A: the ruby bar was cooled to liquid nitrogen, and was pumped by 4 flashlamps housed in confocal elliptical cavities; it had no Q switching, implying a poor resolution." So while it seems to be a first demonstration of detection, it's not likely to have generated a useful measurement. $\endgroup$ – uhoh Feb 2 at 0:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.