5.2 - 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.

Orbits can be circular or elliptical

• Most planets and moons have slightly elliptical (stretched circular) orbits.
• Comets have highly elliptical orbits, with the Sun near one end of the ellipse.
• A comet's orbital period is much longer than a planet's because it travels far into the outer solar system.
• A comet travels much faster near the Sun than far away, because the gravitational pull is stronger the closer it gets.