How Starlink works, and why there are so many satellites
SpaceX's Starlink provides internet from thousands of low satellites. Why it needs so many, how they talk to each other and to the ground, and why new ones look like a train of lights.
Low and many
Traditional satellite internet uses a few geostationary satellites 36,000 km away, so every signal takes a long trip and the connection feels slow. Starlink satellites fly only about 340 to 570 km up, which cuts that delay to something like ground-based internet. The price is coverage: each satellite sees only a small patch of Earth and races past in minutes, so thousands are needed to keep every user connected.
How the connection works
A flat dish on the user's roof steers its beam electronically, with no moving parts, from one passing satellite to the next. Satellites relay traffic to ground stations connected to the internet, and newer satellites also pass data between each other by laser, which lets them serve ships, planes and remote areas far from any ground station.
Why new satellites form a train
A single Falcon 9 releases a stack of a few dozen satellites together in a low orbit. For the first days they fly close behind each other, catching sunlight, so they cross the sky as a bright line of evenly spaced dots. Over the following weeks each satellite raises its orbit with a small electric thruster and the train spreads out and fades. That is why trains are best seen in the first week after a launch.
Concerns
Astronomers worry about streaks across long-exposure images, and SpaceX has added dark coatings and changed how satellites are oriented to dim them. Satellites are placed low enough that failed ones re-enter within a few years, but the sheer number makes collision avoidance a constant task.