The Earth's Changing Shape

LAGEOS is a 903 pound satellite which was launched into an orbit 4,500 miles above the earth's surface in 1976. It carries no instrumentation or power supply, but merely acts as a target for laser beams from the earth which can measure its distance to an incredible accuracy of one millimeter (about the thickness of the wire in a paperclip).

Although it is comparatively "dumb," as satellites go, LAGEOS is providing scientists with new knowledge on how the earth's shape is changing with time.

In grammar school, most of us learned that the earth was not a perfect sphere, but something called an "oblate spheroid," having a greater diameter at the equator due to the earth's spin and bulging somewhat in the southern hemisphere in a manner reminiscent of a middle age spread. In all, this pear-shaped earth did not differ in equatorial and polar radius by more than 15 miles, but this in itself was important to know. Now it is being found that the earth is gradually "rounding itself out" again.

Because the orbital altitudes of LAGEOS can be calculated so precisely, it is possible to determine variations in the earth's gravitational field, and thereby to determine its present shape.

Since the retreat of the glaciers some 20,000 years ago, the areas previously covered are now "rebounding." More of the planet's mass is moving back toward the poles, and school children a million years from now may not have to memorize that their previously imperfect earth was once an "oblate spheroid."

According to a recent issue of Science News, another surprising finding regarding LAGEOS is that its orbit is decaying much faster than anticipated. It was expected that the satellite would continue to circle the earth for hundreds of millions of years, but at the present rate of the loss of about one millimeter per day, it's now not expected to last more than another few million years. Scientists blame collisions with charged particles in space and pressure exerted by the earth's reflected light.