An orbit is the curved path an object follows around a planet, moon, star, or other body because of gravity. Around Earth, a spacecraft in orbit is not floating because gravity is gone. It is constantly falling toward Earth, but it also has enough sideways speed to keep missing the ground.
This idea, often called falling around the Earth, is one of the key ideas behind astronautics.
Gravity provides the inward pull that bends the spacecraft path into a curve instead of a straight line. If the spacecraft moves too slowly, it falls back into the atmosphere or hits the surface. If it moves fast enough, its path curves at the same rate Earth curves away beneath it.
Engineers use this balance of gravity, speed, altitude, and direction to place satellites, crewed spacecraft, and space stations into useful orbits.
Key Facts
- Orbit means continuous free-fall around a larger body.
- Gravity pulls inward while sideways velocity carries the object forward.
- For a circular orbit, gravity provides centripetal acceleration: a = v^2 / r.
- Orbital speed near Earth depends on distance from Earth center: v = sqrt(GM / r).
- Higher circular orbits have lower orbital speeds but longer orbital periods.
- Orbital period for a circular orbit is T = 2πr / v.
Vocabulary
- Orbit
- An orbit is the curved path of an object moving around a larger body under the influence of gravity.
- Free-fall
- Free-fall is motion caused mainly by gravity, even if the object is moving sideways at the same time.
- Velocity
- Velocity is speed in a specific direction, and orbital motion depends strongly on its sideways direction.
- Centripetal acceleration
- Centripetal acceleration is acceleration directed toward the center of a curved path.
- Orbital period
- Orbital period is the time it takes an object to complete one full trip around its orbit.
Common Mistakes to Avoid
- Thinking astronauts in orbit are weightless because there is no gravity is wrong because Earth gravity is still strong at space station altitude and keeps the station in orbit.
- Drawing an orbit as motion with no acceleration is wrong because the velocity direction constantly changes, so the object is accelerating inward.
- Assuming faster always means a lower chance of escaping Earth is wrong because increasing speed enough can stretch an orbit or send the object away on an escape path.
- Confusing altitude with distance from Earth's center is wrong because orbital formulas use r, the distance from Earth's center, not height above the surface.
Practice Questions
- 1 A spacecraft in a circular orbit has orbital radius r = 6.8 x 10^6 m and speed v = 7.7 x 10^3 m/s. Calculate its centripetal acceleration using a = v^2 / r.
- 2 A satellite completes one circular orbit in 5400 s. How many orbits does it complete in 24 hours?
- 3 Explain why a cannonball fired fast enough sideways from a very high mountain could fall continuously around Earth instead of hitting the ground, assuming there is no air resistance.