30 Why Do Comets Have Tails?

Comets are sometimes called dirty snowballs because they’re made of a lot of dust and dirt mixed in with ice. They’re dark, almost as black as charcoal. They’re also extremely old. They’re leftover pieces from the huge cloud of gas and dust that formed our solar system—the extra bits that did not become planets. That makes comets time capsules, because they haven’t changed for billions of years. They allow us to look back to a time before the Earth was even born.

So why are comets usually a surprise when they show up? Because most of the time they’re invisible. We don’t see them unless they have tails, and comets only have tails if they come close to the sun and warm up. Most comets hang out beyond Pluto, where it’s cold and dark and they stay frozen. If they don’t get warm, no tail. And no tail means the comet is too small to be seen in our night sky.

Comets cross the orbits of the planets, which means there is a good chance of a collision. In 1994, astronomers from around the world trained their telescopes to see, for the very first time, a comet running into a planet. And it wasn’t just one collision. The comet, called Shoemaker-Levy 9, started out in one piece, but when it got close to Jupiter, the giant planet’s powerful gravity tore the comet into pieces, forming a long train of comet pieces that plunged into the planet’s atmosphere at speeds of more than two hundred thousand kilometers per hour. Gigantic explosions sent enormous balls of fire above the planet, and dark scars marked the face of Jupiter for weeks afterward. It was a spectacular event but also a little scary because if a comet can hit Jupiter, it means a comet could hit us. And we know Earth has been hit by comets many times in the past.

In 1908, a comet struck northern Russia. Fortunately, there were no people living in the area, but the impact flattened trees in the surrounding forest for hundreds of kilometers in all directions. If such a powerful event happened today near a city, it would be one thousand times more devastating than the first atomic bomb dropped on Hiroshima.

No one knows exactly how many comets are out there, but there are certainly more comets in space than there are people on Earth. Several robots have been sent to take pictures of comets. One robot, called Stardust, flew right through a comet tail and picked up pieces of the dust that were brought back to Earth for analysis. Another robot, Deep Impact, launched a heavy slug that smashed into a comet to see what was inside. As expected, the inside of the comet was made of a lot of water, ice, and dust, as well as some carbon compounds. And another robot, named Rosetta, followed a comet all the way around the sun and sent a lander down onto the surface of the comet itself. From these robotic adventures, we’ve learned that most comets are about five to ten kilometers across, which is the size of a small city.

If you could ride a comet, you’d get a tour of the entire solar system, passing by tiny, icy Pluto, followed by the giant blue-green planets Neptune and Uranus. You’d continue on past Saturn, with its magnificent rings. Lastly, you’d pass mighty Jupiter, the king of the planets. By that time, you’d start to feel warmer as you’d be closer to the sun. Then things would get really interesting. An amazing transformation would take place as your comet begins to warm up. Gases would erupt from below the surface in great geysers, shooting ice crystals into the sky. Snow would fall up! And those snow geysers would be seen from the Earth as a gorgeous comet tail. The funny thing is that pictures of comets make it seem like they’re flying through space headfirst. While that’s sometimes true, they can also fly tailfirst, meaning the tail sometimes wags the comet rather than the other way around.

A comet tail is like a flag that always blows in the direction of the wind. In space, there is a kind of wind of electrified particles that blows out from the sun in all directions called the solar wind. This is the force that always keeps the comet tail pointing away from the sun no matter what direction the comet is moving. As the comet comes in from the edge of the solar system and heads almost directly toward the sun, the tail is streaming out behind. But when it swings around the sun, the tail swings, too, always pointing away until the comet is flung back out into deep space tailfirst.

If you ever happen to see a comet, you are fortunate, because they are among the most beautiful of all celestial objects to cross our skies.