8.3 - Orbits

Gravity provides the force that creates orbits

• Planets orbit the Sun, and moons and artificial satellites orbit planets, in almost circular orbits.
• An object moving in a circle is constantly changing direction, so even at a constant speed its velocity is constantly changing.
• A changing velocity means the object is accelerating, which requires a resultant force.
• This force acts towards the centre of the circle.

Gravity provides the inward force

• For an orbiting object, this inward force is provided by gravity:
  • The Sun's gravitational pull keeps planets and comets in orbit.
  • A planet's gravitational pull keeps moons and artificial satellites in orbit.
• Because the object is already moving, the gravitational pull doesn't pull it straight in - it constantly bends its path into a circle.

Gravity keeps planets and satellites in circular orbits

• Planets orbit the Sun in almost circular orbits, and moons and artificial satellites orbit planets the same way.
• The force that keeps them in orbit is the gravitational force, which acts towards the centre of the orbit.

Why velocity changes while speed stays constant

• The gravitational force acts at right angles to the object's instantaneous velocity.
• This changes the object's direction of motion without changing its speed.
• Because direction changes constantly, the velocity is constantly changing and the object is accelerating towards the centre of the orbit.

Orbital speed and radius

• Gravitational force is stronger closer to a star or planet.
• To maintain a stable orbit at a smaller radius, an object must travel faster so it doesn't get pulled inwards.
• If an object's orbital speed changes, its radius must change too:
  • A faster object orbits at a smaller radius.
  • A slower object orbits at a larger radius.

Natural and artificial satellites and their uses

• A satellite is an object that orbits a larger object in space.
• The Moon is a natural satellite of the Earth; satellites placed into orbit by humans are artificial satellites.

Uses of artificial satellites

• Communications - Transmitting television broadcasts, phone calls, and internet data
• Navigation - Providing GPS signals for vehicles and smartphones
• Weather forecasting - Monitoring clouds, temperatures, and storms
• Earth observation - Tracking climate change, agricultural land use, and pollution
• Astronomy - Housing telescopes (like the Hubble Space Telescope) above the atmosphere to avoid atmospheric distortion
• Military - Surveillance and reconnaissance

Geostationary and polar satellites

• A geostationary satellite orbits above the Earth's equator at an altitude of about 36,000 km with an orbital period of 24 hours.
• Because its period matches the Earth's rotation, it stays above the same point on the surface.
• This makes geostationary satellites ideal for communications, as ground receiver dishes can point at a fixed spot in the sky.

Low polar orbits

• A satellite in a polar orbit travels over the North and South Poles at a much lower altitude, with an orbital period of around 2 hours.
• As the Earth rotates beneath the satellite, it covers a different strip of the Earth on each orbit, eventually imaging the entire surface.
• This makes polar orbits ideal for weather monitoring, mapping, and Earth observation.