Science: Talk Between Planets

A space traveler who happened to be standing on the dark side of the moon last week, in the mountains southeast of the crater Albategnius, would have been startled to see 13 brief red flashes flame up on the dark side of the distant earth. The unexpected spurts of light marked the position of Lincoln Laboratory near Lexington, Mass. They came from a ruby laser—a source of pure light of a single frequency—fitted into a 12-in. telescope.

Since laser light can be concentrated into a thin beam that barely spreads out at all, Professor Louis Smullin and Dr. Giorgio Fiocco, the M.I.T....

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