How much speed do you need to never fall back to Earth?
We often think of orbits as perfect circles, but the reality is far more dynamic. From Newton’s cannonball to Einstein’s curved spacetime, understanding how objects move through the void requires balancing gravity, velocity, and the very fabric of the universe.
For centuries, humanity viewed the heavens through the lens of 'celestial spheres'—perfect, fixed rings described by Hellenistic astronomers like Aristotle and Eudoxus. This geocentric model, later refined by Ptolemy with complex 'epicycles' to account for observed planetary shifts, was eventually simplified by Copernicus, who placed the Sun at the center. However, it was Johannes Kepler who truly revolutionized our understanding. He discovered that orbits are actually elliptical, not circular, and that planets do not move at constant speeds, but rather accelerate as they approach the Sun.
The mechanics of these paths can be understood through a famous thought experiment: Newton's cannonball. Imagine a cannon atop a high mountain. If fired at low speeds, the ball falls to Earth. At higher speeds, the ball's path curves along with the planet's surface. At a specific 'orbital injection' speed, the ball falls toward the Earth at the same rate the ground curves away, resulting in a continuous, non-interrupted orbit. If the velocity exceeds 'escape velocity,' the trajectory becomes an open parabola or hyperbola, allowing the object to break free from the planet's gravity.
While Newtonian mechanics—which treats gravity as an inverse-square force—is remarkably accurate for most short-term calculations, it is not the final word. Albert Einstein’s 1916 general theory of relativity revealed that gravity is actually the curvature of spacetime, with orbits following 'geodesics.' While Newton's laws are still used for their simplicity and sufficient accuracy, Einstein's model is essential for explaining phenomena like the precession of Mercury's perihelion and the precise movements of objects in intense gravitational fields.
Source: Orbit
