Comets, Asteroids and Meteors

The solar system contains many smaller objects in addition to the eight planets.

Asteroids

• Rocky or metallic bodies, mostly found in the asteroid belt between Mars and Jupiter.
• Range in size from a few metres to hundreds of kilometres.
• Irregularly shaped (not spherical; insufficient gravity to pull them into a sphere).
• Orbit the Sun in roughly circular orbits.
• Leftover material from the formation of the solar system that never formed a planet (Jupiter’s gravity prevented it).

Comets

• Made of ice, dust and rocky material (“dirty snowballs”).
• Orbit the Sun in highly elliptical (elongated) orbits.
• Spend most of their time far from the Sun, moving slowly.
• When close to the Sun, the ice vaporises and solar radiation creates a visible tail pointing away from the Sun.
• The tail always points away from the Sun, regardless of the direction of motion.
• Examples: Halley’s Comet (period: 76 years).

Meteors and meteorites

• Meteoroid: a small piece of rock or debris in space.
• Meteor: a meteoroid that enters Earth’s atmosphere and burns up due to friction, creating a streak of light (“shooting star”).
• Meteorite: a meteor that survives the journey through the atmosphere and hits the Earth’s surface.

Key differences

Feature	Asteroid	Comet	Meteor
Composition	Rock/metal	Ice, dust, rock	Rock/metal (small)
Orbit	Nearly circular	Highly elliptical	N/A (fragments)
Location	Mostly asteroid belt	Outer solar system / inner visits	Earth’s atmosphere

Common errors and how to correct them

• Confusing meteors with comets. A meteor is a streak of light in Earth’s atmosphere; a comet is a body orbiting the Sun.
• Saying the comet’s tail trails behind it. The tail always points AWAY FROM the Sun, so it can actually be in front of the comet as it moves away from the Sun.

How to apply this in an exam

Describe the composition and orbit shape. Explain the comet tail direction. Distinguish between meteoroids, meteors and meteorites.