Laser Beam Directed at the Lunar Retro-Reflector Array: Observations of the First Returns

Science  03 Oct 1969:
Vol. 166, Issue 3901, pp. 99-102
DOI: 10.1126/science.166.3901.99


On 1 August between 10:15 and 12:50 Universal Time, with the Lick Observatory 120-inch (304-cm) telescope and a laser operating at 6943 angstroms, return signals from an optical retro-reflector array placed on the moon by the Apollo 11 astronauts were successfully detected. After the return signal was first detected it continued to appear with the expected time delay for the remainder of the night. The observed range is in excellent agreement with the predicted ephemeris. Transmitting between 7 and 8 joules per pulse, we found that each return signal averaged more than one photoelectron. This is in good agreement with calculations of the expected signal strength.