An object in orbit is in continuous freefall around a larger body — gravity provides the centripetal force, pulling the object inward while its tangential velocity carries it forward, so it curves around rather than falling straight down. The Moon orbits the Earth this way; satellites orbit the Earth at heights chosen to match their purpose.
Why does an orbiting object not fall to Earth?
A common misconception is that orbiting satellites are "above gravity". In fact, gravity acts on them continuously — it is precisely what keeps them in orbit rather than flying off in a straight line.
The key idea is the combination of two motions:
- Downward (inward): gravity pulls the satellite towards Earth's centre.
- Forward (tangential): the satellite's velocity carries it horizontally.
If the forward speed is correct for the orbital height, the curvature of the satellite's path exactly matches the curvature of the Earth's surface. The satellite falls towards Earth, but Earth's surface curves away at the same rate — so the satellite stays at a constant height. This is often described as "falling around" the Earth.
What provides the centripetal force for orbital motion?
Centripetal force is the inward force needed to keep any object moving in a circle. For satellites, this is provided entirely by gravitational attraction:
$$F = \frac{mv^2}{r}$$
where F is the centripetal force (N), m is the satellite's mass (kg), v is the orbital speed (m/s), and r is the orbital radius (m).
Because gravity provides this force, and gravity decreases with distance (following an inverse square law), satellites in higher orbits need less centripetal force. A lower centripetal force at a larger radius requires a lower orbital speed. This is why satellites in high orbits travel more slowly than those in low orbits.
What is the relationship between orbital height and orbital period?
Higher orbits have:
- Larger circumference (greater distance to travel).
- Lower orbital speed (gravity weaker at greater distance).
Both effects make the orbital period (time for one complete orbit) longer.
| Orbit type | Typical altitude | Orbital period | Example |
|---|---|---|---|
| Low Earth orbit (LEO) | 200–2,000 km | ~90 minutes | International Space Station; spy satellites |
| Medium Earth orbit (MEO) | ~20,200 km | ~12 hours | GPS satellites |
| Geostationary orbit (GEO) | 35,786 km | 24 hours exactly | Communications and weather satellites |
| Moon's orbit | ~384,400 km | ~27.3 days | The Moon |
What is a geostationary orbit?
A geostationary orbit is a special circular orbit at an altitude of approximately 35,786 km directly above the equator. At this height, the orbital period is exactly 24 hours — matching Earth's rotation period.
Key properties:
- The satellite remains stationary relative to a fixed point on Earth's surface.
- It always covers the same geographical region.
- Three geostationary satellites spaced equally around the equator can cover most of Earth's surface (excluding polar regions).
Uses of geostationary satellites:
- Television broadcasting (dish antennae point to a fixed point in the sky).
- Weather monitoring (the same region viewed continuously).
- Telecommunications (routing phone and data signals across continents).
Limitation: very high altitude means a slight signal delay (~0.25 s round trip), which can cause noticeable lag in telephone calls and real-time video links.
What is a polar orbit?
A polar orbit passes over (or near) both the Earth's poles at a relatively low altitude (typically 200–1,000 km).
Key properties:
- The satellite orbits from pole to pole while Earth rotates beneath it.
- Over successive orbits, the satellite sweeps a different strip of the Earth's surface.
- After approximately 14 orbits (one day), the satellite has covered the entire Earth's surface.
Uses of polar orbit satellites:
- Earth observation and mapping (entire surface surveyed regularly).
- Environmental monitoring (ice caps, deforestation, ocean temperatures).
- Military reconnaissance.
- Some weather satellites (providing global coverage).
Comparison of geostationary and polar orbits:
| Feature | Geostationary | Polar |
|---|---|---|
| Altitude | ~35,786 km | ~200–1,000 km |
| Period | 24 hours | ~90–100 minutes |
| Coverage | Fixed region (equatorial) | Whole Earth (over time) |
| Signal delay | ~0.25 s | Negligible |
| Uses | TV, comms, weather (fixed) | Mapping, observation, spying |
How do you calculate orbital speed?
Worked example:
The International Space Station orbits at an altitude of 408 km above Earth's surface. Earth's radius = 6,371 km. Orbital period = 92.65 minutes.
Step 1: Find the orbital radius (from Earth's centre): r = 6,371 + 408 = 6,779 km = 6.779 × 10⁶ m
Step 2: Find the orbital circumference: C = 2πr = 2 × π × 6.779 × 10⁶ = 4.259 × 10⁷ m
Step 3: Convert period to seconds: T = 92.65 × 60 = 5,559 s
Step 4: Calculate orbital speed: v = C / T = 4.259 × 10⁷ / 5,559 = 7,661 m/s ≈ 7.7 km/s
Frequently asked questions
Why do satellites need to travel so fast?
A satellite in low Earth orbit must travel at approximately 7.7 km/s (about 28,000 km/h) to maintain its orbit. If it travelled slower, gravity would pull it downward faster than its forward motion curves the trajectory — it would spiral inward and re-enter the atmosphere. If it travelled faster, its trajectory would curve less sharply than the Earth's surface — it would spiral outward and escape into a higher orbit. Orbital speed is the precise balance at which the satellite continuously "falls" at the same rate that the Earth curves beneath it.
What happens if a satellite loses speed?
If a satellite in low Earth orbit experiences drag (from the very thin atmosphere) and loses speed, its orbit gradually decreases in altitude — it spirals inward. Eventually it enters denser atmosphere, experiences greater drag, heats up due to friction and re-enters. Most small satellites burn up completely; larger objects may survive and impact the surface. The International Space Station periodically uses onboard thrusters to "reboost" its orbit because atmospheric drag at 408 km altitude, though very slight, gradually lowers the orbit over months.
Why must a geostationary satellite orbit directly above the equator?
For a satellite to appear stationary relative to the ground, its orbital plane must be aligned with Earth's rotation — which occurs only in the equatorial plane. A satellite in any other orbit (inclined to the equator) follows a figure-of-eight path relative to the ground as Earth rotates, appearing to oscillate north and south. Geosynchronous satellites (same period but inclined orbit) do this; true geostationary satellites stay fixed above one equatorial point. This is why all geostationary satellite dishes point southward in the UK.
How is orbital motion related to Newton's law of gravitation?
Newton's law of universal gravitation states that every two masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. For a satellite, this gravitational force provides the centripetal force for circular orbital motion. Setting the gravitational force equal to the centripetal force and solving for orbital speed gives a relationship where orbital speed depends only on the mass of the central body (Earth) and the orbital radius — not on the satellite's own mass. This explains why astronauts experience weightlessness: they and their spacecraft fall at exactly the same rate, so there is no normal force between them.
For Socratic GCSE physics with Professor Newton — predicting orbital motion from gravitational first principles through to satellite engineering choices — visit aitutors.me.