What Neil and Buzz Left on the Moon

Apollo 11 lunar laser ranging retroreflector array
Apollo 11 lunar laser ranging retroreflector array. NASA

The most famous thing Neil Armstrong left on the Moon when he visited years ago is his footprint, a boot-shaped depression in the gray surface dust. Millions of people have seen pictures of it, and one day, years from now, lunar tourists will flock to the Sea of Tranquility to see it in person. Peering over the rails somebody will ask, "Hey, Mom, is that the first one?"

Will anyone notice, 100 feet away, something else Armstrong left behind?

If they pay attention, they'll see not only a piece of lunar history, but a working science experiment. 

Ringed by footprints in the dust lies a two-foot-wide panel studded with a hundred mirrors pointing at Earth. It's the Lunar Laser Ranging Retroreflector Array. Apollo 11 astronauts Buzz Aldrin and Neil Armstrong put it there on July 21, 1969, about an hour before the end of their final moon walk. All these years later, it's the only Apollo science experiment still running, helping scientists understand the Moon's motions in space.

Using these mirrors, scientists can 'ping' the moon with laser pulses and measure the Earth-Moon distance very precisely. It also helps them to chart the moon's orbit and to test theories of gravity.

How It Works

The experiment is deceptively simple. A laser pulse shoots out of a telescope on Earth, crosses the Earth-Moon divide, and hits the array. Because the mirrors are "corner-cube reflectors," they send the pulse straight back where it came from, to detectors on Earth.

Telescopes intercept the returning pulse — which can be just a single returning photon of light. 

The round-trip travel time pinpoints the Moon's distance with staggering precision: better than a few centimeters out of 385,000 km, typically. The information gathered by this "ping" yields near-instant measurements of distance and motion, which adds much to our store of knowledge about the Moon.

 

Targeting the mirrors and catching their faint reflections is a challenge, but astronomers have been doing it ever since the reflectors were set up. A key observing site is at the McDonald Observatory in Texas, where a 0.7-meter telescope regularly pings reflectors in the Sea of Tranquility (Apollo 11), at Fra Mauro (Apollo 14) and Hadley Rille (Apollo 15), and, sometimes, in the Sea of Serenity. There's a set of mirrors there onboard the parked Soviet Lunokhod 2 moon rover — maybe the coolest-looking robot ever built.

Details about What we Learn

For decades, researchers have carefully traced the Moon's orbit, and learned some remarkable things:

  1. The moon is spiraling away from Earth at a rate of 3.8 cm per year. Why? Earth's ocean tides are responsible.
  2. The moon probably has a liquid core.
  3. The universal force of gravity is very stable. Newton's gravitational constant G has changed less than 1 part in 100-billion since the laser experiments began.

NASA and the National Science Foundation funded the Apache Point Observatory Lunar Laser-ranging Operation (in New Mexico), called "APOLLO" for short. Using a 3.5-meter telescope with good atmospheric "seeing," researchers there can examine the moon's orbit with millimeter precision, 10 times better than before.

This experiment will continue until something happens to the mirrors or funding is shut off. Its data stream joins the collections of images and mapping data produced by such missions as the Lunar Reconnaissance Orbiter. All the data will be important as mission scientists plan the next trips to the Moon for both robotic probes and (eventually) people. The system is still working well: lunar mirrors require no power source. They haven't been covered with moon dust or pelted by meteoroids, so their future is good. Perhaps future lunar visitors will see it in action when they make their own "first steps" on the lunar surface as part of a museum tour or school field trip. 

Edited by Carolyn Collins Petersen.