Several Low-Earth-Orbit (LEO) networks were proposed, but only one got off the ground: the Iridium system. The original Iridium proposal called for a "constellation" of 77 satellites, which gave the plan its name: the element iridium has atomic number 77, meaning that an iridium atom has 77 orbiting electrons. Before the satellites were launched, the constellation was scaled back to 66 active satellites, but no one wanted to change the name to Dysprosium.
A NASA astronaut, forced to retire years earlier so he could save his family farm, has never given up his dream of space travel and looks to build his own rocket, despite the government's threats to stop him.
Wang tiles (Hao Wang, 1961) are a class of formal systems. They are modelled visually by square tiles with a color on each side. A set of such tiles is selected, and copies of the tiles are arranged side by side with matching colors, without rotating or reflecting them.
The basic question about a set of Wang tiles is whether it can tile the plane or not, i.e., whether an entire infinite plane can be filled this way. The next question is whether this can be done in a periodic pattern.
In 1966, Wang's student Robert Berger solved the problem in the negative. He proved that no algorithm for the problem can exist, by showing how to translate any Turing machine into a set of Wang tiles that tiles the plane if and only if the Turing machine does not halt. The undecidability of the halting problem then implies the undecidability of Wang's tiling problem.