2.2 - 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.

Orbital speed and the size of the orbit

• For a circular orbit, the gravitational force acts at right angles to the object's motion.
• This changes the object's direction without changing its speed.

Orbital speed depends on the radius

• Gravitational pull is stronger the closer an object is to the body it orbits.
• The stronger the pull, the faster the object must travel to maintain a stable orbit.
• If the speed of an orbiting object changes, its radius must change too:
  • A faster object orbits with a smaller radius.
  • A slower object orbits with a larger radius.
• This is why planets closer to the Sun travel faster and have shorter orbital periods than planets further out.

Satellite orbits, uses and geostationary satellites

• A satellite is an object that orbits a larger object in space.
• The Moon is a natural satellite; satellites launched from Earth are artificial satellites.

A satellite is a projectile in free fall

• A satellite is launched with a high horizontal velocity, with gravity (weight) as the only force acting on it in space.
• The satellite is in constant free fall towards the Earth.
• Its horizontal velocity curves its path to match the curvature of the Earth, so it keeps falling around the Earth without getting closer to the surface.
• If it travels too slowly it falls back to the ground, and if it travels too quickly it flies off into space, so only the correct horizontal velocity gives a stable orbit.

Orbital period and altitude

• The orbital period is the time taken for one full orbit.
• A satellite in a higher orbit has a longer orbital period.

Geostationary satellites

• A geostationary satellite orbits above the 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 is ideal for communications, as a ground receiver dish can stay fixed in one position.

Uses of satellites

• GPS / Navigation - Determining positions for vehicles and phones
• Weather forecasting - Tracking weather patterns and storms
• Communications - Transmitting television, phone, and internet signals
• Earth observation - Monitoring climate change, land use, and pollution

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

Satellite communication with microwaves and radio waves

Communication to and from satellites uses microwaves (and high-frequency radio waves) because they pass straight through the Earth's atmosphere without being reflected or absorbed.

How satellite communication works

1. A signal is transmitted from Earth up into space.
2. A dish on the satellite receives the signal.
3. The satellite amplifies and transmits the signal back down to a different location on Earth.
4. The signal is picked up by a ground satellite dish.

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.

Orbital altitude and orbital period

• The orbital period (the time taken for one full orbit) depends on the satellite's altitude.
• A satellite at a higher altitude experiences a weaker gravitational pull, travels at a lower orbital speed, and has a longer orbital period.
• A geostationary satellite orbits at about 36,000 km and has a period of 24 hours.
• A low Earth orbit satellite orbits a few hundred kilometres up and completes an orbit in roughly 90 to 120 minutes.