How do satellites stay in orbit around the Earth?

In one line

The rule

A satellite stays in orbit when its forward speed is just right so that Earth's gravity pulls it into a curved path around the planet, not straight down.

The main formula for orbital speed is: v = √(GM/r) where:

• v = orbital speed (how fast the satellite moves sideways, in metres per second)
• G = gravitational constant (a fixed number: 6.67 × 10⁻¹¹ N·m²/kg²)
• M = mass of the Earth (about 5.97 × 10²⁴ kg)
• r = distance from the centre of the Earth to the satellite (in metres)

Let's understand it simply

Imagine you are spinning a stone tied to a string above your head. The string pulls the stone inwards, but the stone keeps moving in a circle because it wants to fly away. Here, the string is like Earth's gravity, and the stone is like a satellite. If you let go, the stone flies off in a straight line. But as long as you keep spinning, it goes round and round.

Now, think of a cricket ball thrown fast and straight. If you throw it slowly, it falls to the ground quickly. If you throw it very fast, it goes farther before falling. Imagine you could throw it so fast that as it falls, the ground curves away under it (because the Earth is round). It would keep missing the ground and keep going round and round. This is what a satellite does!

How it works

1. The satellite is launched high above the Earth's surface, usually by a rocket.
2. It is given a very high sideways speed (about 28,000 km/h for low Earth orbit).
3. Earth's gravity pulls the satellite towards itself, just like it pulls down on everything.
4. But because the satellite is moving so fast sideways, as it falls, the ground curves away beneath it.
5. The satellite keeps falling around the Earth, never hitting the ground, making a continuous curved path — this is called an orbit.
6. If the speed is too slow, the satellite falls back to Earth. If it is too fast, it escapes Earth's gravity and flies away into space.

The story behind it

The idea of orbits was first explained by Sir Isaac Newton in the late 1600s. He imagined firing a cannonball from a very high mountain. If it was fired fast enough, it would keep falling around the Earth instead of landing. This was just a thought experiment — rockets and real satellites came much later. The first artificial satellite, Sputnik 1, was launched by the Soviet Union in 1957. It proved Newton's idea was correct.

Where you see it in real life

• TV satellites: The dish on your roof points to a satellite orbiting above the equator. It stays in the same spot in the sky because it orbits once every 24 hours (geostationary orbit).
• Weather satellites: These orbit the Earth and take pictures of clouds, helping us predict the monsoon or cyclones.
• GPS satellites: Help your phone or car know your exact location by sending signals from space.
• Communication satellites: Allow you to make phone calls, use the internet, and watch live cricket matches from anywhere in India.
• The Moon: Our natural satellite, always in orbit around Earth due to gravity.
• Space stations: Like the International Space Station, where astronauts live and work, orbiting Earth every 90 minutes.

Why it matters

• Satellites are vital for communication, weather forecasting, disaster warning, and navigation.
• Understanding orbits helps us launch rockets and send probes to other planets.
• Many competitive exams (like NEET, JEE, UPSC) ask about orbits and satellite motion.
• India's space missions (like ISRO's Chandrayaan and Mangalyaan) depend on this knowledge.

Worked example

Let's say you want to find the speed needed for a satellite to orbit 300 km above the Earth's surface. Earth's radius (R) = 6,371 km = 6,371,000 m Height above surface (h) = 300 km = 300,000 m So, r = R + h = 6,371,000 + 300,000 = 6,671,000 m G = 6.67 × 10⁻¹¹ N·m²/kg² M = 5.97 × 10²⁴ kg

Step 1: Write the formula      v = √(GM/r)
Step 2: Plug in the values     v = √[(6.67 × 10⁻¹¹) × (5.97 × 10²⁴) / 6,671,000]
Step 3: Calculate numerator    (6.67 × 10⁻¹¹) × (5.97 × 10²⁴) = 3.985 × 10¹⁴
Step 4: Divide by r            3.985 × 10¹⁴ / 6,671,000 ≈ 5.97 × 10⁷
Step 5: Take square root       √(5.97 × 10⁷) ≈ 7,730 m/s
Step 6: Convert to km/h        7,730 × 3.6 ≈ 27,828 km/h

So, the satellite must move at about 7.7 km/s (or 28,000 km/h) to stay in orbit at that height.

Common mistakes

• Thinking satellites float because there is no gravity: Wrong. Gravity is strong even in orbit; satellites are always falling, but their sideways speed keeps them from hitting the ground.
• Believing satellites need engines to keep moving: No. Once in orbit, they keep moving due to inertia (an object's habit of staying as it is). Engines are only needed to change orbit or correct path.
• Confusing orbits with escape: If a satellite goes too fast, it escapes Earth's gravity and does not stay in orbit.
• Mixing up geostationary and low Earth orbits: Geostationary satellites stay above one point; low Earth satellites move quickly around the globe.

Remember it

A satellite stays in orbit by moving sideways fast enough that as it falls, the Earth curves away under it — so it never lands!