WHAT'S THE DIFFERENCE between a comet and an asteroid? This seems like a simple question. Comets are familiar to most people as celestial objects with a fuzzy head and often one or more long, sweeping tails — features that astronomers collectively describe as ‘activity’. Even those who have never seen a comet with their own eyes have probably seen this activity in glossy photos of comets in magazines such as the one you're reading right now, or in popular media and movies.
In 1950, Fred Whipple described comets as “dirty snowballs,” referring to the mixture of icy and non-icy material that makes them up — which still largely captures how astronomers view comets today. This view naturally and intuitively explains the ‘active’ appearance of comets, which arises when, as the comet emerges from the cold outer Solar System and approaches the Sun, ice in the nucleus (the ‘head’) heats up and transforms into gas. This direct transition from ice to gas, called sublimation, can create geyser-like outflows that drag dust off the nucleus, forming different kinds of tails.
In contrast, asteroids are for the most part decidedly un-comet-like. Being mostly rocky or metallic and primarily found in the main belt between the orbits of Mars and Jupiter, astronomers long thought that asteroids orbit too close to the Sun to carry the ice that powers sublimation-fuelled outbursts. They should therefore be inert — perpetually lifeless rocks or inactive rubble piles.
But the Solar System, it turns out, is not so blackandwhite. We now realise that asteroids can behave like comets and vice versa. What we thought were two kinds of bodies are in fact part of a single, sprawling family, their properties not always
