This is LAGEOS. Since 1976, this perfect disco sphere has been orbiting Earth carrying 426 cube corner reflectors made of fused silica glass - except four, made of germanium. It also contains a coded message, designed by Dr Carl Sagan:
The message was duplicated in two 4x7-inch stainless steel plates and contains several codes telling things about Earth. The first message are the numbers one to 10 in binary notation, located in the upper centre. To the right there's a drawing of Earth orbiting the sun, with an arrow pointing to the right. The arrow means one year, the time of it takes the Earth to rotate around the sun.
The rest of the plaque is covered by three maps: The first map shows Earth and its fused continent Pangea, 268 million years ago, indicated by an arrow to the left and the 268 million in binary notation. The second map shows Earth at the time of launch. That's time zero. Finally, the third map shows Earth 8.4 million years from now, which is the total life estimated by NASA for this 23-inch satellite.
According to NASA, this map shows "many important changes in the Earth's surface [...]including the drift of California out into the Pacific Ocean." In the future, if any alien civilisation arrives to Earth, they would be able to instantly tell the history of Earth's geography and know where in time we lived.
But telling alien civilisations when we lived and how Earth was in our time is not the main objective of this 406kg metal sphere. Floating 5860km above Earth, LAGEOS-1 is targeted with lasers from ground stations on a daily basis. It's "the first spacecraft dedicated exclusively to high-precision laser ranging and provided the first opportunity to acquire laser-ranging data that were not degraded by errors originating in the satellite orbit or satellite array".
In 1992, Italy built another of these spheres, called LAGEOS-2. In the future, USA, France, Germany, Italy, Spain and the UK will launch LAGEOS-3 which will measure "for the first time, a quasi-stationary property of the Earth - its gravitational magnetic dipole moment as predicted by Einstein's theory of general relativity."