Orbital Mechanics

Orbital Mechanics 🛰️

The planets circle the Sun, the Moon circles the Earth, and satellites circle both — none of them fly off, and none fall in. How? The answer is one force doing a clever job: **gravity**. Follow how it holds everything in orbit.

Gravity holds the orbit 🌍

A body moving in a straight line would shoot off into space. To follow a **curved orbit** instead, something must keep pulling it towards the centre. That something is **gravity** — the pull of the Sun (or planet) provides the **centripetal** (inward) force that bends the path into an orbit.

Label the orbit

An interactive activity.

What keeps it curving?

A planet follows a curved orbit rather than a straight line. What keeps pulling it round?

• The Sun's gravity, pulling it towards the centre
• A push from the solar wind
• The planet firing engines
• Nothing — it curves on its own

Velocity and direction ➡️

The orbiting body's **velocity** points **tangent** to the orbit — at **90°** to the gravitational force. Gravity does not change how fast it moves; instead it constantly changes its **direction**, bending it round in a circle. A changing direction means the velocity is always changing, even at a steady speed.

Closer means faster ⚡

For a **stable circular orbit** there is one exact speed for each radius: • A **smaller** orbit (closer in) needs a **higher** speed. • A **larger** orbit (further out) needs a **lower** speed.\n\nThat is why **Mercury** whizzes round the Sun far faster than distant **Neptune**.

Inner or outer, faster?

For stable circular orbits, how does an inner planet's orbital speed compare with an outer planet's?

• The inner planet orbits faster — a smaller radius needs a higher speed
• The inner planet orbits slower
• They orbit at the same speed
• Speed has nothing to do with distance

Mission control

An interactive activity.

Match each term to its meaning

Which are artificial?

Pick the TWO artificial (man-made) satellites.

• The International Space Station
• A GPS navigation satellite
• The Moon
• The planet Mars

The orbit rules

A planet stays in orbit because the Sun's _____ provides the inward force. The planet's velocity is at _____ to this force, along a tangent to the orbit. For a stable orbit, a planet closer to the Sun must travel _____.