Lasers to the Moon

The pictures taken by Surveyor 7's camera on the moon last week appeared to show only a crescent-shaped earth glowing in the lunar sky. But closer inspection showed two seemingly insignificant starlike dots of light on the night portion of the earth. They were historic dots. Each represented the light from an argon-ion laser beam aimed from Tucson, Ariz., and Wrightwood, Calif., at Surveyor's location near the lunar crater Tycho, some 240,000 miles away.

In an engineering test that was a preliminary to actual laser experiments during the Apollo moon mission, a group headed by University of Maryland Physicists Carroll Alley and Douglas Currie set up the lasers in four East Coast locations in addition to the two in the West. Each was projected backwards through a telescope--into the viewing end--toward Surveyor's lunar site. The telescopes were used not only to aim the beams precisely but also to further confine the beam of the coherent laser light, which diverges very little even without telescopic aid. Alley estimates that both beams had diverged to a diameter of only two or three miles at the surface of the moon.

Although diffused sunlight may have obscured the beams from East Coast stations, the two Western beams were recorded by Surveyor's camera. To make certain that the dots were indeed Alley's lasers and not transmission static, scientists turned the beams on and off, while Surveyor snapped away. On the sequence of Surveyor shots that resulted, the dots obediently appeared and disappeared.

When Apollo astronauts finally set down on the moon, one of their tasks will be to set up an array of reflectors. Scientists will bounce more powerful ruby laser pulses off the reflectors and will measure the time it takes for the pulses to return to earth. This data will enable them to determine the distance from earth to a fixed point on the moon with an accuracy of 6 in., measurements that should enable scientists to learn the precise size of the moon, to analyze its motions, to confirm continental drift on the earth, and perhaps even to learn if the universe's gravitational constant is changing.

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