The simplest test is the tail: a comet is made of ice, dust and rock, and grows a glowing tail as its ice vaporises near the Sun, while an asteroid is rocky and has no tail. Think of Halley's Comet streaking across the sky versus an asteroid quietly orbiting in the belt between Mars and Jupiter.
The core difference
A comet is icy and grows a tail near the Sun; an asteroid is rocky and does not.
- comet — an icy, dusty body that develops a glowing tail near the Sun: Halley's Comet is visible from Earth roughly every 76 years.
- asteroid — a rocky body smaller than a planet, mostly orbiting between Mars and Jupiter: Scientists are tracking an asteroid that will pass close to Earth next year.
How to tell them apart
The memory hook is composition: a comet is basically a dirty snowball, so heat from the Sun turns its ice to gas and creates the tail everyone recognises. An asteroid is solid rock (or metal), so it has nothing to vaporise and stays tail-less no matter how close it gets to the Sun.
The mix-up usually shows up in news reports about objects passing near Earth, where headline writers sometimes use the words loosely. Checking for a tail in any photo settles it: no tail, it is an asteroid.
