What Is A Comet?

For thousands of years, the sudden appearance of a blazing, hairy star streaking across the night sky struck terror into the hearts of ancient civilizations. Rulers viewed them as ominous messengers warning of plagues, wars, or the fall of empires.

Today, astronomers know comets are not omens of doom. They are something infinitely more precious: pristine, 4.6-billion-year-old cosmic time capsules preserved in the deep freeze of outer space since the very birth of our solar system.

Famed astronomer Fred Whipple famously coined the nickname "dirty snowballs" to describe them. They are essentially city-sized icebergs tumbling through the void, composed of frozen water, dry ice (carbon dioxide), methane, ammonia, and dark organic dust.

The Anatomy of an Icy Wanderer

When a comet is loafing in the sub-zero outer reaches of the solar system beyond Neptune, it is practically invisible. It consists only of its solid core: the nucleus.

A typical comet nucleus measures anywhere from one to ten miles across. Because it is packed with fine silicate dust and dark carbon compounds, its surface is surprisingly dark—blacker than charcoal, reflecting only about 4% of the light that hits it.

However, when a gravitational nudge sends the comet plunging inward toward the Sun, everything changes:

1. Sublimation: In the vacuum of space, ice cannot melt into liquid water. Instead, as the Sun's radiation warms the nucleus, the surface ice instantly vaporizes directly from a solid into a roaring gas—a process called sublimation.
2. The Coma: Geysers of vapor and dust erupt through cracks in the icy crust, enveloping the nucleus in a giant glowing cloud of gas called the coma. The coma can swell to hundreds of thousands of miles across—often growing larger than the planet Jupiter!

The Tale of Two Tails

As the comet nears the inner solar system, the Sun's intense light and charged particle stream begin pushing against the coma, carving out not one, but two distinct glowing tails that can stretch for tens of millions of miles:

• The White Dust Tail: Made of microscopic silicate dust grains blown backward by the gentle, continuous pressure of sunlight itself (solar radiation pressure). Because dust particles are relatively heavy, this tail curves gently along the comet’s orbital trajectory.
• The Blue Ion Tail: Made of electrically charged gas molecules (ions) stripped by ultraviolet sunlight. The powerful solar wind sweeps these super-light gas particles straight back into space at hundreds of miles per second, creating a pencil-straight, glowing neon-blue tail.

Here is the most mind-bending fact about comet tails: they do not trail behind the comet like smoke from a car.

Because the solar wind always blows outward away from the Sun, a comet's tails always point directly away from the Sun! When a comet rounds the Sun and heads back toward the cold outer solar system, it actually flies tail-first, like a sailboat being pushed from behind!

The Deep Freeze Reservoirs

Where do these ancient voyagers come from? They originate in two massive cosmic freezers:

1. The Kuiper Belt: A donut-shaped ring of icy debris extending just beyond Neptune (30 to 50 AU from the Sun). This is home to short-period comets—like Halley’s Comet, which completes an orbit every 76 years.
2. The Oort Cloud: A colossal, spherical shell of trillions of icy bodies enveloping our entire solar system nearly a light-year away. Comets nudged out of the Oort Cloud can take millions of years to complete a single journey around the Sun.

Fast Facts About Comets

• Halley Returns in 2061: Halley’s Comet was last seen from Earth in 1986. Mark your calendars: it will return to our skies in the summer of 2061!
• Meteor Showers are Comet Dust: When you watch the Perseid or Geminid meteor showers, you are watching Earth plow through a trail of tiny dust grains shed by a passing comet, burning up as shooting stars in our upper atmosphere.
• Water Bringers: Many planetary scientists believe ancient comet collisions billions of years ago may have delivered a substantial portion of the water that fills Earth’s oceans today!
• Stinky Space Snow: In 2014, the European Space Agency’s Rosetta spacecraft sniffed the gases around Comet 67P/Churyumov–Gerasimenko. It discovered the comet smells like a foul mixture of rotten eggs (hydrogen sulfide), cat urine (ammonia), and bitter almonds (hydrogen cyanide)!