Orbits and Satellites

What is an orbit?

An orbit is the curved path that one object takes around another, held in place by gravity. The Moon orbits the Earth. The Earth and the other planets orbit the Sun. And thousands of human-made satellites orbit the Earth right now, giving us GPS, weather forecasts, phone signals and television.

But here is the puzzle that confused people for centuries: if gravity is always pulling a satellite down toward the Earth, why doesn't it just fall and crash? The answer is one of the most elegant ideas in physics — and it was first explained by Isaac Newton over 300 years ago. You have met gravity as a force in gravity explained; now we will see how that same force creates orbits.

The secret: falling around the Earth

Throw a ball horizontally and it travels forward while gravity pulls it down, so it curves to the ground a short distance away. Throw it harder and it goes further before landing. The faster you throw, the further the curve carries it.

Newton imagined a cannon on a very tall mountain, firing a ball horizontally — an idea now called Newton's cannonball. Fire it gently and it lands nearby. Fire it faster and it lands further away. But fire it fast enough, and something magical happens: the ball falls toward the Earth at exactly the same rate that the round Earth curves away beneath it. The ball keeps falling but never gets any closer to the ground. It has gone into orbit.

The key insight: a satellite is always falling toward the Earth. It just moves sideways so fast that it keeps missing. The Earth's surface curves away as quickly as the satellite drops.

So an orbiting satellite is not "escaping" gravity — gravity is essential. Gravity provides the constant inward pull that bends the straight-line motion into a closed curve. Take gravity away and the satellite would fly off in a straight line into space. This same balance between forward motion and an inward pull is explored further in circular motion and orbits.

Speed and distance

How fast does a satellite need to travel? It depends on how high it orbits, and the rule is the opposite of what many people guess:

• Closer to Earth, gravity is stronger, so the satellite must move faster to fall around the planet without spiralling in. The International Space Station orbits about 400 km up and races around the Earth at roughly 7.7 km/s — about 28,000 km/h — completing a full orbit every 90 minutes. Astronauts see a sunrise every 45 minutes!
• Further from Earth, gravity is weaker, so the satellite moves more slowly and takes longer to complete an orbit. The Moon, about 384,000 km away, ambles along at just 1 km/s and takes about 27 days to orbit once.

This is a real pattern across the whole solar system: the inner planets whip around the Sun quickly, while distant planets crawl. Neptune takes 165 Earth-years to orbit the Sun once.

Types of orbit

Not all orbits are the same. Engineers choose an orbit to suit the satellite's job:

Orbit	Height	Time per orbit	Used for
Low Earth Orbit (LEO)	~200–2,000 km	~90 min	Space station, imaging, Starlink
Medium Earth Orbit	~20,000 km	~12 hours	GPS / navigation
Geostationary (GEO)	~36,000 km	24 hours	TV, weather, communications

Geostationary orbit is especially clever. At about 36,000 km above the equator, a satellite takes exactly 24 hours to go around — the same time the Earth takes to spin once. So the satellite stays parked above the same point on the ground, appearing to hover motionless in the sky. That is why a satellite TV dish can be bolted to a wall pointing at one fixed spot and never needs to move.

Natural and artificial satellites

A satellite is simply anything that orbits a larger body. There are two kinds:

• Natural satellites — like the Moon orbiting Earth, or the dozens of moons around Jupiter and Saturn. These formed naturally.
• Artificial satellites — machines we launched into orbit. There are now thousands of them.

What satellites do for us

Satellites quietly run much of modern life:

• GPS navigation. Your phone works out where you are by timing signals from several navigation satellites at once. It is doing geometry with space — and the timing must be so precise that engineers even correct for Einstein's relativity.
• Weather forecasting. Satellites photograph clouds and storms from above, letting forecasters track hurricanes days in advance.
• Communication and TV. Geostationary satellites bounce phone calls, internet and television signals across the planet.
• Science. Telescopes like Hubble orbit above the blurring atmosphere to get crystal-clear views of the universe.

Why "weightlessness" happens

A common myth is that astronauts float because "there is no gravity in space." Not true! At the space station's height, Earth's gravity is still about 90% as strong as on the ground. Astronauts float because they are in free fall — they and the station are both falling around the Earth together at the same rate, so there is nothing pressing them against the floor. It is exactly the brief floating feeling you get at the top of a roller-coaster drop, except it never stops.

Try it yourself! 🧪

Demo — orbit in a bucket. This shows how an inward pull turns motion into a circle.

1. Half-fill a small, sturdy bucket with water (do this outside).
2. Hold the handle and swing the bucket in a fast vertical circle, over the top of your head and back down.
3. If you swing fast enough, the water stays in the bucket even when it is upside-down above you!

Why it works: the water "wants" to fly off in a straight line, but the bucket keeps pulling it inward, bending its path into a circle — just as gravity keeps pulling a satellite inward and bends its motion into an orbit. The water is, in a sense, "falling" toward the centre of the circle but moving sideways fast enough to keep going around. Swing too slowly and the inward pull is not enough, so the water falls out — exactly like a satellite that is moving too slowly and spirals back to Earth.

Demo 2 — Newton's cannonball, on paper. Draw a large circle for the Earth and a little cannon at the top. Sketch several paths: a slow ball curving down to land nearby, a faster one landing further around, and a fast one whose curve matches the Earth's curve so it loops all the way around. This simple sketch captures the whole secret of orbits: enough sideways speed turns "falling down" into "falling around."