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Space & Astronomy25 Essential Exam Concepts
Why Do Satellites Stay in Orbit? Orbital Velocity & Gravity Physics
In astrophysics, classical mechanics, and aerospace engineering, a widespread misconception persists that artificial satellites remain suspended in space because "there is no gravity in space." Scientifically, this assumption is completely incorrect. At the altitude of the International Space Station—approximately four hundred kilometers above Earth's surface—the gravitational pull of the Earth remains exceptionally powerful, retaining approximately eighty-nine to ninety percent of its sea-level strength (gapprox8.7extm/s2). Satellites do not defy gravity; rather, they stay in stable orbit because they are in a state of perpetual, continuous Freefall toward Earth, but possess an immense horizontal (tangential) velocity that prevents them from ever hitting the surface.
The conceptual foundation of orbital flight was first illustrated by Sir Isaac Newton in his 1687 masterpiece, "Philosophiae Naturalis Principia Mathematica," through his famous Cannonball Thought Experiment. Newton imagined placing a powerful cannon atop a colossal mountain above Earth's atmosphere. If the cannon fires a ball with low horizontal velocity, gravity pulls it down in a parabolic arc until it strikes the ground. As the propellant charge and horizontal velocity increase, the projectile travels farther across the globe. Eventually, at a critical velocity, the downward curve of the projectile's trajectory precisely matches the natural downward curvature of the spherical Earth: for every eight kilometers the projectile travels horizontally, the Earth's surface curves downward by approximately five meters. The projectile falls continuously "around" the planet without ever colliding with it, establishing an orbit.
Mathematically, a stable circular orbit is achieved when Earth's gravitational attraction provides the exact necessary Centripetal Force required to keep the satellite moving in a curved path: Fg=Fc, yielding the orbital velocity formula vo=sqrtGM/r, where G is the gravitational constant, M is Earth's mass, and r is the orbital radius. In Low Earth Orbit (LEO, 200–2,000 kilometers altitude), satellites must maintain a blistering velocity of approximately 7.8 kilometers per second (twenty-eight thousand kilometers per hour) to avoid falling back, completing one full orbit every ninety minutes. At Geostationary Orbit (GEO, at an altitude of 35,786 kilometers), the orbital velocity drops to 3.07 kilometers per second, matching Earth's rotational period of twenty-three hours, fifty-six minutes, and four seconds, allowing communication satellites to hover permanently over a single equatorial geographic longitude.