Launched in 1976, LAGEOS-1 (LAser GEOdynamic Satellite) is unusual in that it contains no instrumentation, electronics, power supply or means of propulsion. It is spherical, weighs just under 407kg and looks a bit like a disco ball. It may not be accurate to say that it does nothing, but unlike most satellites its function is completely passive. It is also one of the oldest scientific satellites still in service. The lens-shaped objects covering the surface of LAGEOS-1 are corner cube retroreflectors, which have the ingenious effect of always reflecting incident light directly back to its source. Ground stations fire short laser pulses and measure the time it takes for the light to return, a form of time-of-flight measurement. Because LAGEOS-1’s orbit is very stable, it provides a reliable reference point for measuring even small changes in the Earth itself. As a result, the size, shape, rotation, and more of our planet can be measured. LAGEOS data (LAGEOS-2 was launched in 1992) have also been used in tests of general relativity. Its orbit and construction were deliberately chosen so that atmospheric drag and other disturbances were minimal. The simple, maintenance-free design combined with an extraordinarily stable orbit means that LAGEOS is expected to circle our world for millions of years. LAGEOS-1 also contains a message for the future in the form of two identical plaques prepared by Dr. Carl Sagan in case there is someone nearby who can find it one day. Watch the short 1975 video from NASA, embedded just below.
