What if a comet passed close to Earth?

Sometimes a comet shows up in the night sky. It looks like a fuzzy star with a long tail.

What is a comet? And what if one passed close by Earth?

Every so often, a comet appears in the night sky as a fuzzy patch of light, sometimes with a long, glowing tail. NASA explains that people long ago were awed by comets, which seemed to appear without warning.

Today, scientists know what comets are and how they move. So what would we see if a comet passed close to Earth? What would it leave behind? And how do astronomers know where comets are going? This is a thought experiment built on what scientists know. Choose a path and explore.

Comets are, in NASA's words, leftovers from the dawn of our solar system. Short-period comets, with orbits under 200 years, often return predictably; long-period comets, many from the distant Oort Cloud, can take as long as 30 million years per orbit.

This thought experiment follows a comet passing near Earth: how its coma and tails form, what its debris does, and how astronomers compute and follow comet orbits. Facts come from NASA; anything about a particular imagined comet is labeled as an idea, and the lab makes no predictions about real ones.

Far from the Sun, a comet is a frozen lump of ice, gas and dust. It has no tail at all.

It travels on a long path around the Sun.

Far from the Sun, a comet is just its frozen center, called the nucleus. NASA says a nucleus is often a few miles across and is made of icy chunks and frozen gases with bits of dust mixed in.

Many comets come from an icy belt beyond Neptune, where Pluto orbits, or from a far more distant region called the Oort Cloud. Every so often, gravity nudges one onto a path that brings it into the inner solar system, closer to the Sun.

A comet's nucleus, often a few miles across, holds icy chunks and frozen gases with embedded dust (NASA). Short-period comets come from the Kuiper Belt beyond Neptune, a disc of icy bodies theorized by Gerard Kuiper in 1951; long-period comets, many from the Oort Cloud about 100,000 astronomical units out, can take up to 30 million years to orbit.

Gravity occasionally pushes Kuiper Belt objects onto orbits that bring them closer to the Sun, and they become short-period comets; many of these return predictably because they have passed by before.

As the comet gets near the Sun, it warms up. Some of its ice turns into gas.

The gas and dust make a glowing cloud and a long tail. What a sight!

As a comet nears the Sun, NASA explains, the Sun's heat turns some of its ice into gas, building a fuzzy atmosphere called the coma that can stretch hundreds of thousands of kilometers. Sunlight and a stream of fast particles from the Sun, the solar wind, push the dust and gas away from the Sun, sometimes making a long, bright tail.

Comets actually have two tails, one of dust and one of gas. If a comet passed close to Earth, it might look big and bright in our sky for a while, before heading back out into space.

Solar heating sublimates the nucleus's ices, so the coma grows; NASA notes it may extend hundreds of thousands of kilometers. Radiation pressure and the solar wind blow coma dust and gas away from the Sun, forming a dust tail and an ion (gas) tail, both blown away from the Sun.

How a passing comet would look from Earth would depend on its size, its activity and its distance, which is why the lab's view of a close comet is an idea. Some comets, called sungrazers, fly so close to the Sun that they break up and evaporate.

A comet drops bits of dust along its path, like crumbs.

When Earth passes through the crumbs, they zip into our air and glow. That's a meteor shower!

As a comet travels, it sheds dust and rock that spread out along its path. NASA explains that when Earth passes through one of these dusty trails, the bits rush into our atmosphere and burn up as meteors, or shooting stars. Lots of them at once make a meteor shower.

Some meteor showers happen every year, as Earth passes through the same dusty trails again. The Perseids, which peak around August 12, are tiny pieces of Comet Swift-Tuttle.

Meteoroids are pieces of larger bodies; some come from comets. NASA explains that meteor showers occur annually or at regular intervals as Earth passes through the trail of dusty debris left by a comet (and, in a few cases, asteroids). Every Perseid meteor is a piece of Comet Swift-Tuttle, which passes the Sun every 135 years; the Leonids come from Comet Tempel-Tuttle; and Halley's debris gives the Eta Aquarids in May and the Orionids in October.

Whether a newly passing comet would produce a shower later depends on whether Earth's orbit ever crosses its debris; for an imagined comet, that is an idea, not a prediction.

Long ago, a scientist named Edmond Halley worked out when a comet would come back. He was right!

Today, scientists at NASA keep watch on comets and work out their paths.

In 1705, astronomer Edmond Halley used Isaac Newton's ideas about gravity to work out the paths of comets. He noticed that bright comets seen in 1531, 1607 and 1682 had very similar paths, and suggested they were one comet coming back again and again. He predicted it would return in 1758, and it did. It was named Halley's Comet, and NASA says it will return in 2061.

Today, NASA's Planetary Defense Coordination Office, set up in 2016, finds and tracks asteroids and comets that come near Earth.

Halley's 1705 orbit calculations, based on Newton's gravitation, linked the comets of 1531, 1607 and 1682 into one returning object and correctly predicted its 1758 return: the first such prediction, NASA notes. Halley's Comet was last seen from Earth in 1986, when an international fleet of spacecraft met it, and will return in 2061 on its roughly 76-year orbit.

In 2016 NASA established the Planetary Defense Coordination Office to find, track and better understand asteroids and comets that could pose an impact hazard to Earth. Its planned NEO Surveyor telescope is designed to help find most of the potentially hazardous asteroids and comets that come within 30 million miles of Earth.