why do satellites slow down
The Short AnswerSatellites slow down primarily due to atmospheric drag, even in the extremely thin upper atmosphere. Collisions with stray molecules exert a tiny but cumulative force, gradually reducing their orbital velocity. This effect is more pronounced for satellites in lower orbits where atmospheric density is slightly higher.
The Science Behind It
While space is largely a vacuum, satellites in low Earth orbit (LEO) still encounter a sparse atmosphere. This consists of stray molecules of gases like oxygen and nitrogen. As the satellite hurtles through space at thousands of miles per hour, it collides with these molecules. Each collision imparts a tiny bit of momentum to the molecule and takes a tiny bit away from the satellite, acting as a form of friction or drag. Although incredibly small per molecule, the sheer number of collisions over time causes a measurable decrease in the satellite's speed. This means satellites gradually lose altitude and will eventually re-enter Earth's atmosphere if not periodically boosted by their onboard thrusters to maintain their desired orbital path. The effect is more significant at lower altitudes where even this tenuous atmosphere is denser.
Fun Facts
- Even at altitudes of 200 miles, satellites experience enough drag to require periodic boosts to stay in orbit.
- The International Space Station (ISS) needs its orbit adjusted about once a month to counteract this atmospheric drag.