Starlink has many downsides. The megaconstellation interferes with astronomy, pollutes the atmosphere with metallic re-entry debris, and has pushed Low Earth Orbit to the brink of the Kessler Syndrome. On the other hand, it makes a terrific barometer. Earth is absolutely surrounded by Starlink satellites. More than 11,000 of them are circling our planet right now. Without exception, they all skim the outermost layers of our atmosphere, and so they experience aerodynamic drag. Drag pulls them down, and they have to fight back with thrusters. This makes them ad hoc detectors of air pressure. Every day, we read those detectors and boil the result down to one number: the sink rate. What the number means The sink rate is the answer to a simple question. If a typical Starlink at 480 km switched off its thrusters today, how many meters of altitude would it lose in a day? A value of, say, 30 m/day means it would sink 30 meters. Why 480 km? Because that's where most Starlinks are.…