Laser Geodynamics Satellites (LAGEOS) were a pair of scientific research satellites launched in the late 20th century and designed to provide an orbiting laser ranging benchmark for geodynamical studies. Each was a high density passive laser reflector in a very stable medium Earth orbit (MEO), roughly 5,900 kilometres (3,700 mi) in altitude.
These spacecraft were aluminium-covered brass spheres with diameters of 60cm and weighing approximately 400kg, covered with cube-corner retroreflectors, giving them the appearance of giant golf balls. They had no on-board sensors or electronics, and no attitude control. Measurements were made by transmitting pulsed laser beams from ground stations to the satellites. The laser beams returned to Earth after hitting the reflecting surfaces.
Due to the stability of their orbits, the LAGEOS satellites made it possible to determine positions on the Earth with ultra-high accuracy: better than one inch in thousands of miles. At the time, this made them the most precise location references available. As such, they were used for monitoring the movement of the Earth's tectonic plates, gravitational field, the "wobble" in its axis of rotation, and for determining the exact length of an Earth day.
However, these satellites would fall back to Earth eventually. LAGEOS-1 was predicted to re-enter the atmosphere in 8,400,000 AD. It contained a plaque, designed to allow future descendants of humans to view the arrangement of Earth's continents in the past, present and future.
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