In Orbit You Have to Slow Down to Speed Up

"If you watch sci-fi movies, you'd think that flying a spaceship is just like driving a slightly more complicated car (or a Winnebago in Spaceballs). And George Lucas gave us those galactic battles with pilots who look like they're flying fighter jets on Earth. Well, bad news: Space is really, really different. In particular, moving a vehicle in orbit around Earth is way more complicated than that. The maneuvers you might make with a plane sometimes have the opposite effect in orbit."

"There are three big physics ideas we need to understand this kind of motion. First, centripetal acceleration. Recall that acceleration is a measure of how fast the velocity of an object changes. But velocity isn't just speed, it's speed in a particular direction -in other words, it's a vector. If an object is moving in a circle, its direction is constantly changing, which means it's constantly accelerating, even if its speed is constant!"

Spaceflight requires different control intuition than atmospheric flight because orbital motion depends on velocity as a vector and centripetal acceleration. In a circular orbit, constant speed still requires a continuous inward acceleration whose magnitude depends on speed and orbital radius. Gravity supplies the centripetal force, so orbital velocity and altitude are linked. Small changes in velocity alter orbital energy and geometry, producing counterintuitive effects such as raising the opposite-side orbit point or changing orbital period when thrust is applied along the flight path. Successful docking and rendezvous therefore require planning maneuvers that respect orbital dynamics.

Read at WIRED