LAGEOS

LAGEOS, or Laser Geodynamics Satellite, refers to two geodetic satellites launched to support precise measurements of Earth’s shape, gravity field, and dynamics through satellite laser ranging (SLR). LAGEOS-1 was launched in 1976 and LAGEOS-2 in 1992. Both are passive, unpowered spacecraft covered with cube corner retroreflectors that reflect laser pulses sent from ground stations back to their origin, enabling highly accurate range measurements.

Each satellite is a dense, nearly spherical metal body about 60 centimeters in diameter and weighing roughly 400 kilograms, designed to have very low drag and stable attitude characteristics. They carry no propulsion or scientific payload beyond the retroreflectors, and their orientation is governed by spin and solar radiation pressure, which contributes to their long-term stability.

Orbits are high and nearly circular. LAGEOS-1 follows a retrograde orbit with an inclination close to 109.8 degrees, while LAGEOS-2 is prograde with an inclination around 52.6 degrees. Their altitudes place them roughly in the 5,800–6,000 kilometer range above Earth’s surface, allowing extended lifetimes and minimizing atmospheric drag. The global SLR network continuously tracks both satellites, yielding precise measurements of the Earth’s gravity field and its temporal variations, crustal motion, and rotation parameters.

Scientific significance includes providing a stable reference frame for geodesy, refining models of Earth’s gravity field, monitoring tectonic processes, and contributing to tests of general relativity, particularly frame-dragging (Lense–Thirring). The LAGEOS mission remains active, with ongoing data usage in geodesy and gravitational physics; LARES (launched later) complemented their measurements to improve relativistic tests. The program is managed by NASA’s Goddard Space Flight Center with support from the Italian Space Agency.