Orbital velocity and terminal velocity are two distinct concepts related to the motion of objects in different contexts.
Orbital Velocity: Orbital velocity is the minimum velocity required for an object to maintain a stable orbit around a celestial body, such as the Earth. It is the speed at which the object must travel horizontally to counterbalance the gravitational pull and continuously fall towards the Earth while constantly missing it due to its forward motion. The exact orbital velocity depends on the altitude and the mass of the celestial body being orbited. For example, in low Earth orbit (LEO), the average orbital velocity for a satellite is approximately 28,000 kilometers per hour (17,500 miles per hour).
Terminal Velocity: Terminal velocity is the maximum constant velocity that a falling object can attain when the force of gravity pulling it downwards is balanced by the opposing force of air resistance. Initially, when an object falls, it accelerates due to gravity. However, as it gains speed, the force of air resistance increases until it equals the force of gravity. At this point, the net force on the object becomes zero, resulting in a constant velocity known as terminal velocity. The magnitude of terminal velocity depends on factors such as the object's mass, size, shape, and the density of the medium it is falling through (usually air).
When satellites reenter the Earth's atmosphere, they are subjected to atmospheric drag, which causes their velocity to decrease. As the satellite descends through the atmosphere, it encounters increasing air density, which leads to an increase in air resistance. This resistance opposes the satellite's motion and gradually slows it down. As a result, the satellite's speed decreases, and it eventually reaches a point where its velocity matches the terminal velocity for its specific shape and size.
Satellites are designed to withstand the reentry process and the associated heat generated due to atmospheric friction. They typically have heat shields and other protective measures to prevent them from burning up during reentry. Additionally, their trajectories are carefully calculated to ensure a controlled descent and to avoid crashing into inhabited areas or causing harm. By considering various factors such as the angle of reentry, the timing, and the location of the splashdown, engineers can ensure a safe and controlled return of satellites to Earth.