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.
Understanding How Satellites Stay in Orbit
An orbit is not a place where forces disappear. It is a path set by a satellite's position, speed, and direction of travel. At every moment, gravity changes the direction of the satellite's velocity toward Earth's center.
In a circular orbit, this turn happens at the right rate to keep the distance from Earth nearly constant. A small speed change usually produces an elliptical path instead of a circle. The satellite then has a closest point and a farthest point from Earth.
Engineers do not need engines firing all the time. Short engine burns can change a path greatly because they alter the satellite's energy and direction.
Speed changes have effects that can seem backwards at first. When a satellite moves outward along an elliptical orbit, it slows down. When it comes closer to Earth, it speeds up.
This follows from energy conservation. A forward engine burn near the lowest part of an orbit raises the farthest part of the orbit. A backward burn there lowers it.
To lower an orbit, a satellite may first slow down, then later move faster after it reaches the lower path. Learning this sequence helps students understand why spacecraft maneuvers are planned so carefully.
Real orbits are never perfectly smooth. The thin upper atmosphere creates drag on low satellites. Drag removes orbital energy, causing the orbit to shrink over time.
The satellite then reaches denser air and loses energy even faster. Without correction burns, many low satellites eventually reenter the atmosphere. Earth is not a perfectly round, uniform sphere either.
Its slight bulge changes orbital paths slowly. The Moon, the Sun, and sunlight pushing on large panels can make small changes too. Satellites that must remain in an exact position use station keeping burns to correct these disturbances.
The orbit chosen depends on the mission. Earth imaging satellites often use low polar paths so Earth rotates beneath them and they can scan many regions. Some follow sun synchronized paths, which pass each place at nearly the same local sunlight time.
This makes images easier to compare. Communication satellites may use much higher paths to cover a wide area. GPS satellites need carefully arranged orbits and extremely accurate clocks, since tiny timing errors create large errors in position.
When studying orbits, pay close attention to the difference between speed, velocity, energy, altitude, and orbital period. Microgravity is another important idea. Astronauts feel weightless because their spacecraft and everything inside it share the same free fall, not because gravity has vanished.
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.