Satellites stay in orbit because gravity pulls them toward Earth while their sideways motion keeps them from falling straight down. They are constantly falling, but Earth curves away beneath them at the same time. This balance makes an orbit a special kind of free fall.
Understanding orbits helps explain GPS, weather forecasting, communications, Earth imaging, and space science missions.
The speed a satellite needs depends mainly on its distance from Earth's center. Closer satellites must move faster because gravity is stronger and the circular path is smaller. Higher satellites move more slowly but take longer to complete one orbit.
Engineers use Newton's laws and gravitational equations to choose an orbit that matches a satellite's job.
Key Facts
- Gravitational force provides the centripetal force that bends a satellite's path around Earth.
- Newton's law of gravitation: F = Gm1m2/r^2.
- Circular orbit speed: v = sqrt(GM/r), where M is Earth's mass and r is distance from Earth's center.
- Orbital period for a circular orbit: T = 2πr/v.
- Low Earth orbit satellites typically travel about 7.8 km/s.
- A geostationary satellite orbits once every 24 hours above Earth's equator, so it appears to stay over one location.
Vocabulary
- Orbit
- An orbit is the curved path an object follows around a planet, moon, star, or other body due to gravity.
- Centripetal force
- Centripetal force is the inward force needed to keep an object moving in a curved path.
- Orbital speed
- Orbital speed is the sideways speed an object needs to keep following a stable path around another body.
- Low Earth orbit
- Low Earth orbit is a region a few hundred to about 2,000 kilometers above Earth's surface where many satellites and space stations travel.
- Geostationary orbit
- A geostationary orbit is a circular orbit above Earth's equator with a 24 hour period, making the satellite appear fixed in the sky.
Common Mistakes to Avoid
- Thinking there is no gravity in orbit is wrong because gravity is what keeps the satellite moving in a curved path around Earth.
- Using Earth's radius as the orbital radius without adding altitude is wrong because r must be measured from Earth's center, not from the surface.
- Assuming higher satellites move faster is wrong because circular orbital speed decreases as orbital radius increases.
- Confusing orbital speed with escape speed is wrong because orbital speed keeps an object circling Earth, while escape speed lets it leave Earth's gravity well without further propulsion.
Practice Questions
- 1 A satellite is in a circular orbit 400 km above Earth's surface. Use Earth's radius 6.37 x 10^6 m and GM = 3.99 x 10^14 m^3/s^2 to estimate its orbital speed.
- 2 A satellite orbits at a radius of 4.22 x 10^7 m from Earth's center and moves at 3.07 x 10^3 m/s. Use T = 2πr/v to find its orbital period in hours.
- 3 Explain why a satellite in a stable circular orbit does not need engines firing continuously, even though gravity is always pulling on it.