Two satellites in identical orbits help measure the Earth with a unique plaque for future Earthlings.

Imagine you need to measure the distance between two points on the earth to less than an inch for every thousand miles between the two points. Or you want to track the shift of tectonic plates on the earth.

The LAGEOS (LAser GEOdynamic Satellite) are the most accurate way to make these measurements. They’re two satellites in orbit around the earth 3700 miles (5900 kilometers) above us. They carry no sensors or instruments. Instead, they have a reflective surface with indents like shiny golf balls. Their only purpose is to be a stable target to bounce signals off then measure distance based on the time the signal returns.

The satellites are covered with 426 cube corner reflectors. All but four of the reflectors are made with fused silica glass to bounce laser beams off. Four of the reflectors are made of germanium, a shiny grey metal, for infrared reflection studies to measure reflectivity and orientation of the satellite. The satellites are shiny balls with reflective prisms.

The LAGEOS-1 satellite was launched by NASA on May 4, 1976. The LAGEOS-2 satellite was launched by the Italian Space Agency on October 22, 1992.

In addition to measuring distances between earth locations and the satellites, changes in their orbit were used to develop models of Earth’s gravitational field, as well as how sunlight heating small objects like near-Earth asteroids affects their orbital paths.

The LAGEOS-1 satellite also includes an unusual plaque designed by the astronomer Carl Sagan. The plaque includes images of our current continents and the continents as they should appear 8.4 million years from now when the satellite falls back to earth. When the satellite falls into a future earthling’s backyard millions of years from now, Sagan wanted the plaque to tell them when the satellite was launched.

Learn More


Now 40, NASA’s LAGEOS Set the Bar for Studies of Earth


Associated Press LAGEOS Video (5/2/1976)


  • Tim Slavin

    Tim is an award-winning writer and technologist who enjoys teaching tech to non-technical people. He has many years experience with web sites and applications in business, technical, and creative roles. He and his wife have two kids, now teenagers, who are mad about video games.

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