The LAGEOS satellites

Its durability and its elegant simplicity are the two most remarkable things about the LAGEOS orbs.
8 June, 2017
When the scientists at NASA in the United States talk about the LAGEOS satellite, they describe it as looking like a giant golf ball. Yet that's something of an understatement for the beauty and precision of the satellites – there are two of them – more often described as being like multifaceted jewels. That's because the satellites, one launched by the U.S. in 1976 and the other by Italy in 1992, are metal spheres, each studded with 426 angular prisms raised above the satellite's smooth surface.
The 400-kilogram satellites are still going strong, and some scientists think they may outlast humanity with a projected stable-orbit lifespan of 8.4 million years, assuming no surprises. That's partly because they have no complicated moving parts: there are no technical instruments, batteries, or fuel aboard.
Image: ILRS
What they do have is an elegantly simple construction, with a brass core at the center and aluminium shell covering the exterior surface. Most of the 426 cubed-cut prisms embedded within the aluminium surface are made of fused silica, a special kind of glass. There are four other prisms made of germanium, an element similar to tin and silicon.

The fact that it was made with durable materials and a commitment to design simplicity contributes to the longevity of LAGEOS, but that leads to another question: if the satellites don't take measurements or relay signals or house any instruments at all, then what are they doing in orbit? In effect, they serve as giant mirrors that refract light beams back to earth. The LAGEOS was the first NASA satellite dedicated to "laser ranging," a technique for taking precise measurements on the basis of laser light travel times. It was able to reduce the margin of error for location data from about one meter to just one centimeter.
Over the years, LAGEOS satellites have communicated with 183 laser stations around the world, with a few dozen still doing so. A laser pulse is beamed up to the station and part of that light returns to the laser ranging station that sent it; the measurement determines the exact distance from the satellite.

Once those baseline numbers are known, the satellite – and its later-generation cousins – contribute to a geoscientific understanding of earth. These include small changes in mass at the earth's core, shifts in the tectonic plates, and small irregularities in the rotation of the planet or polar motion. While most people think of the earth as a perfect sphere, the geoid is actually "malformed" and lumpy in terms of how its mass is distributed, and what it would look like without the effects of tides and atmosphere. The LAGEOS identifies the precise center point of this invisible earth's shape for critical navigation purposes.
Image: ILRS
The LAGEOS also helped to build human knowledge about Earth's gravitational field and how the sun heats objects. "It set a new standard for laser ranging and has provided 40 years of continuity for these measurements," said Stephen Merkowitz, manager of the NASA project department at Goddard Space Flight Center, to mark the LAGEOS anniversary.

It also does one more thing. The original LAGEOS carries a three-frame plaque depicting earth, placed by the late Carl Sagan, in case it's ever found by other intelligent life.
Banner image: La Cronica