Physics
Orbital Mechanics
Quick fact
A satellite in low Earth orbit travels at about 7.8 km/s—fast enough to circle the entire planet in roughly 90 minutes.
Why this is interesting
You throw a ball forward—it curves down and hits the ground. Now imagine throwing it so fast that the Earth curves away beneath it just as fast as it falls. Why does it never hit the ground?
Read the full explanation
Understanding Orbital Mechanics
Orbital mechanics is the study of how objects move through space under the influence of gravity. The key insight is that an orbit is a balance between two things: the object's forward momentum (inertia) and the gravitational pull of the central body. Imagine a cannonball fired horizontally from a high mountain. If you fire it slowly, it falls to Earth in a short arc. As you increase the speed, the arc gets longer. At a certain speed—orbital velocity—the cannonball falls exactly as fast as the Earth curves away, so it never hits the ground. It is in free fall around the Earth. This is exactly what happens to the International Space Station: it is constantly falling toward Earth, but moving forward so quickly that it keeps missing.
A deeper explanation
The underlying mechanism is described by Newton's law of universal gravitation: every object with mass attracts every other object with a force proportional to their masses and inversely proportional to the square of the distance between them. For an orbiting body, this gravitational force provides the necessary centripetal force to keep it moving in a curved path. Mathematically, for a circular orbit, the gravitational force equals the centripetal force: GMm/r² = mv²/r, leading to orbital velocity v = √(GM/r). Kepler's laws generalize this: orbits are ellipses with the central body at one focus; planets sweep equal areas in equal times; and the square of the orbital period is proportional to the cube of the semi-major axis. Orbital mechanics matters because it allows us to send spacecraft to other planets, maintain GPS and communication satellites, and understand the dynamics of star systems and galaxies. The concept of escape velocity, derived from conservation of energy, defines the minimum speed needed to leave an orbit permanently.