Today is the 40^th anniversary of man’s first walk on the Moon. Thus, it’s also the 40^th birthday of an experiment that keeps on providing crucial data for tests of fundamental physical theories.

During their excursion onto the lunar surface, Apollo 11 astronauts Neil Armstrong and Buzz Aldrin deployed the Laser Ranging Retroreflector (LRRR), an array of corner-cube reflectors inside a metal box angled to face toward Earth. Two weeks after the moonwalk, scientists started firing laser pulses at the reflector box to get a more precise measure of the distance between the two celestial bodies.

As Science reported in January 1970, some six months after the historic space mission, the LRRR project arose from discussions among Robert H. Dicke and other members of the experimental gravitational research group at Princeton University (U.S.A.). Not only did these scientists want to study the basic physics of the Earth-Moon system, but they also wanted to test Einstein’s theory of general relativity.

Three months after Apollo 11, using an electronic receiver with an accuracy of a few nanoseconds, the LRRR team measured the Earth-Moon distance to ± 30 cm from the McDonald Observatory in Texas (U.S.A.). Three years later – after two other Apollo crews had deployed retroreflectors at their lunar landing sites -- scientists had improved the uncertainty to 15 cm and gained new insights into the complexity of the Moon’s mass distribution and rotation.

By 1994, a quarter-century after Apollo 11, and with advances in both laser and detector technology, researchers had improved the Earth-Moon distance accuracy to 2 or 3 cm and had also used lunar laser ranging to measure the Earth’s precession and the Moon’s tidal dissipation. Team members also reported “unprecedented accuracy” in verifying the equivalence principle for massive bodies, one of the tests of relativity. Not bad, considering that the reflected signal received back on Earth represented only 10^–21 of the outgoing beam.

New Scientist recently reviewed the LRRR’s accomplishments over the past four decades. The equivalence principle has held up to one part in 10¹³ and the movement of the Moon can be tracked to within a few millimeters. Thanks to 40 years’ worth of data, researchers have shown that Newton’s gravitational constant changes less than one part in 10¹² per year.

The laser ranging using the three Apollo-era LRRR users continues on clear nights at Apache Point Observatory in New Mexico (U.S.A.). Who knows how much more precision scientists will gain by the year 2019 – or even 2049?

(Incidentally, in honor of the Apollo 11 anniversary, Science opened up the online version of its January 30, 1970, themed issue on lunar science to all with free registration. If you hear someone complaining that no science came out of the first Moon landing, point him or her to this issue.)