On May 4, 1976, NASA launched LAGEOS (short for Laser Geodynamic Satellite), a 900-pound orbiter dedicated to a precision measurement technique called laser ranging. The still-functioning satellite has no on-board sensors or electronics, no moving parts, and contains a brass core encased by an aluminum shell dimpled with 426 retroreflectors. NASA describes it as looking like a giant golf ball, but as it was deployed the ‘70s, a different reference point came to our minds: a disco ball.

Laser ranging involves sending a laser pulse from a ground station up to the satellite, where it bounces a signal back. The round-trip time is used to calculate the distance between the earthbound station and the device in orbit, which can then be applied to all sorts of studies. LAGEOS and its sister satellite, LAGEOS-2, have helped scientists examine the Earth’s shape, weight, rotation and gravitational field, measure tectonic plates, and much more.

David E. Smith, who was a LAGEOS project scientis, said of the project: “Today, we see Earth as one system, with the planet’s shape, rotation, atmosphere, gravitational field and the motions of the continents all connected. We take it for granted now, but LAGEOS helped us arrive at that view."

Forty years after its launch, LAGEOS still travels in its original orbit more than 3600 miles above Earth and is expected to stay that way for millions of years. Aboard the satellite is a plaque designed by Carl Sagan that contains three panels depicting the planet at different phases: 268 million years ago, today, and 8.4 million years in the future, when the satellite is predicted to fall to Earth.

For much more on the history of LAGEOS and the work it’s contributed to, check out this retrospective on the NASA website, as well as this 1975 preview of the mission to see the satellite in its groovy glory.