30 years' worth of moon beams

Sun Journal

Mission: From a West Texas mountaintop, scientists have been bouncing laser light off the moon since Apollo 11 left, adding to lunar lore.

July 20, 1999|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Thirty years after the first men landed on the moon, the Apollo mission continues on a lonely mountaintop in West Texas.

Five or six times a month, scientists in an old trailer at the McDonald Observatory near Fort Davis point a 30-inch telescope at one of three old Apollo landing sites on the moon, and fire a burst of laser light.

If they've aimed well, their green needle of light strikes one of three briefcase-sized reflectors the moon-walkers left behind. It bounces back to Texas and down the throat of the telescope

By precisely timing the light's round trip -- about 2 1/2 seconds -- scientists can measure, to within 1 1/2 inches, the distance between the telescope and the moon.

The three reflectors are the only Apollo instruments on the moon that still work. And after 30 years, they are still teaching researchers new things about the moon, the Earth, their linked histories and their fate. For example:

* The moon is drifting away from Earth at a steady 3.82 centimeters (about 1 1/2 inches) every year. It's now about 3 feet 9 inches farther away than when Neil Armstrong set foot there July 20, 1969. Its slow-motion escape -- caused by Earth's ocean tides and slowing rotation -- was known before Apollo, but never so precisely.

In a few billion years, the tides will cease, and the moon will hover over just one side of our planet. Eventually it will just drift away.

* The moon's spin is wobbly, jostled by gravitational tugs from the sun and every planet in the solar system. By measuring those tiny effects, scientists have been able to fine-tune the navigation of interplanetary spacecraft.

* By sorting out the subtle patterns in its changing spin and orbit, scientists can map the moon's interior structure. In March, a NASA team reported strong evidence that the moon still has a liquid core, probably of molten iron and sulfur.

"That couldn't have been said 30 years ago, or three years ago," says planetary scientist Jim Williams, of NASA's Jet Propulsion Laboratory.

* Laser ranging has proved that Galileo and Einstein were right about gravity theory -- all objects really do fall at the same rate of acceleration, regardless of their composition or mass.

The Apollo laser reflectors on the moon have "proven to be one of the best things NASA has ever done," says James Faller. He is a research scientist at JILA, a Boulder, Colo., research institute jointly operated by the University of Colorado and the National Institute for Standards and Technology, in Gaithersburg.

The relectors were Faller's idea. Back in the early 1960s, he says, "we knew where the moon was, sort of, plus or minus many tens of kilometers."

As a graduate student at Princeton, Faller calculated that the best lasers of the day might be just strong enough to be bounced off a reflector on the moon. The light's travel time could reveal the distance to the moon with an accuracy measured in centimeters, not kilometers -- a 100,000-fold improvement.

"It was just on the edge of being doable," Faller says. All scientists had to do was get a "corner cube reflector" to the moon -- a faceted mirror similar to a bike reflector. They're designed to receive light from any angle and bounce it directly back to its source.

NASA liked the idea. The moon landings were risky and expensive, and planners wanted "the simplest possible science they could find," Faller says. "And what could be simpler than to carry a suitcase out, set it down and kick it around until it's pointing more or less toward Earth."

As soon as the Apollo 11 astronauts left the moon, Faller and his team at the Lick Observatory in California began firing their laser. For nearly two weeks they got nothing back.

The astronauts "didn't know quite where they were, so we didn't quite know where to point," Faller says. The landing site was too tiny to see. Finally, early one morning in August 1969, detectors at Lick registered the laser light's faint return. "I don't think there has ever been a more happy group of people."

Two more reflectors -- one with 100 corner cubes and one with 300 -- were placed on the moon by the Apollo 14 and 15 missions. Unmanned Russian landers carried French-built reflectors to two other lunar sites.

Together, they're like a safe-cracker's fingers -- a sensitive five-point platform for tracking tiny changes in the moon's rotation, orbit and even its shape.

Scientists are less interested in how far the moon is at a given moment -- that averages about 239,000 miles -- than in the changes in that distance over time, Faller says. "If you can obtain a regular and continuous data set over weeks and months and years, you can separate out all the various motions of the moon."

It's those motions -- the unexpected twitches and subtle rhythms in orbit and rotation -- that have revealed the tug of distant planets, the tidal pull of the Earth on the lunar rock and the irregular densities inside the moon.

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