Comets: How Icy Visitors Build Their Tails, and When You Can See One

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A comet is a snowball the size of a town that grows a tail longer than Earth's orbit on the way to meet the sun. Here is the physics of the two-tailed transformation, where these visitors come from, and how to plan a night around one.

The Dirty Snowball That Was Right Enough

For decades the standard description of a comet was Fred Whipple's "dirty snowball" — and spacecraft eventually proved it understated the dirt. A comet's nucleus is a rubble-and-ice body, typically ten to twenty kilometres across, made of water ice mixed with dust, rock, frozen carbon dioxide, carbon monoxide, methane and ammonia. When Deep Impact threw an impactor at comet Tempel 1 in 2005, and when Rosetta orbited comet 67P for two years and dropped a lander onto its duck-shaped nucleus, the picture that emerged was a porous, dry, dark body — a skeleton of dust with ice hidden beneath a crust, less snowball than dirty sponge with a frozen heart. The important point for observers is what happens when that heart gets warm.

How a Nothing-Bright-As-Coal Object Grows Two Tails

Far from the sun, a comet nucleus is an invisible black rock — darker than asphalt, reflecting only a few per cent of light. As it nears the sun, sunlit patches of subsurface ice begin to sublimate: turning straight from solid to gas without becoming liquid, jetting out of the crust in geysers that carry dust with them. Suddenly the comet has an atmosphere, the coma — a cloud of gas and dust hundreds of thousands of kilometres wide around a ten-kilometre nucleus, which is the entire reason a comet can outshine a planet despite being a pebble in comparison.

The tails come next, and there are always two, made by different forces. Solar radiation pressure — the physical push of sunlight itself — drives dust particles away from the sun; because those grains are also orbiting, the dust tail curves gently along the comet's path and looks broad and yellowish-white, reflecting sunlight. The gas, ionised by ultraviolet light into plasma, gets caught in the solar wind's magnetic field and blown directly anti-sunward at tremendous speed, forming a narrow, straight, blue ion tail that can point in a different direction from the dust tail entirely. A long ion tail means one thing to an observer: strong solar wind, which is why aurora forecasts and great comet tails tend to share a space-weather headline. Both tails always point away from the sun, which produces the delightful paradox of a comet approaching the sun with its tail in front of it.

Where They Come From, and How Long They Stay

Short-period comets — returning every few decades or less, like Halley on its 76-year circuit — originate in the Kuiper belt, the ring of icy bodies beyond Neptune, perturbed inward by Neptune's gravity. Long-period comets fall from the Oort cloud, a spherical swarm of trillions of frozen objects reaching perhaps a light-year out — the solar system's true, distant outer edge — disturbed into inward falls by passing stars and galactic tides over millions of years. Some, like 'Oumuamua and Borisov in recent memory, are not from our system at all: genuine interstellar visitors caught briefly in the sun's gravity well. Every return spends a little of the nucleus: jets erode crust, and close passes split or evaporate bodies outright. Comet families fragment on orbital timescales; the Shoemaker–Levy 9 comet that crashed into Jupiter in 1994 had been torn apart by Jupiter's gravity itself, its train of impact scars visible for months.

Watching One — Including the Next Big Chance

Naked-eye comets appear on average only about once a decade, and predicting brightness is notoriously humbling: a comet can disappoint badly (predicted for the skies, found by binoculars) or explode in brightness weeks before perihelion (predicted for binoculars, bright enough to cast a shadow). The practical method is to follow the orbit calculations and accept the error bars, then look where the geometry says: a fresh morning comet swings low in the east before dawn, an evening one rides the west after sunset, and near perihelion it can be lost in the sun's glare for a week or two. Use binoculars rather than a telescope first — a comet's beauty is in the sweep of its tail across a wide field, which no magnification can reproduce. The century's most anticipated sight is the return of Halley's Comet in 2061 — a once-in-a-lifetime event for anyone reading this now; the constellation guide to where it will rise and the catalogue of fainter fuzzies at tukuk.org/messier are the way to spend the intervening nights preparing for it.

See also

• Why We Have Seasons — It Is Not Because Earth Gets Closer to the Sun
• Why the Sea Rises and Falls: How Tides Work
• Why Some Holidays Move Around the Calendar Every Year
• What Is a Black Hole? A Plain-Language Guide With No Equations
• How to Track the ISS and Catch It Crossing Your Night Sky
• The Doppler Effect: How Motion Changes Everything You Hear — and See