A Space Platform for Laser Ranging

Thirty-five years ago today — May 4, 1976 — the LAGEOS-1 satellite launched on a Delta rocket from Vandenberg AFB.

(LAGEOS. NASA image.)

The Laser Geodynamics Satellite (LAGEOS) (also known as the Laser Geodetic Satellite) “was the first spacecraft dedicated exclusively to high-precision laser ranging and provided the first opportunity to acquire laser-ranging data that were not degraded by errors originating in the target satellite.”

The spacecraft itself was simple: a sphere covered with 426 “cube corner reflectors” or retroreflectors which return light directly to its source no matter the incident angle. According to this page, LAGEOS-1 also carried a small plaque designed by Carl Sagan:

The plaque is 4 inches by 7 inches (10 cm by 18 cm) stainless steel plate. The spacecraft carries two identical copies included in its interior. In its upper center it displays the simplest counting scheme, binary arithmetic. The numbers one to ten in binary notation are shown. At upper right is a schematic drawing of the Earth in orbit around the Sun, and an arrow indicating direction of motion. The arrowhead points to the right, the convention adopted for indicating the future. All arrows accompanying numbers are “arrows of time”. Under the Earth’s orbit is the binary number one, denoting the period of time used on the plaque — one revolution of the Earth, or one year. The remainder of the LAGEOS plaque consists of three maps of the Earth’s surface. The first map denotes the Earth 268 million years in the past. All the continents are shown together in one mass. The close fit of South America into West Africa was one of the first hints that continental drift actually occurs. The middle map represents the zero point in time for the other two maps. It displays the present configuration of the planets. The final map shows the Earth’s surface 8.4 million years from now — very roughly the estimated lifetime of the LAGEOS. Many important changes in the Earth’s surface are shown, including the drift of California out into the Pacific Ocean. Whoever comes upon the LAGEOS plaque needs only compare a current map of the Earth’s geography with that in the lower two maps to calculate roughly the difference between his time and ours.

Put it on your calendar: let’s meet up with LAGEOS-1 8 million years from now and see how accurate Sagan’s continental drift picture is.

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