What powers a comet?

Comet in the desert sky.

Our friends at the Back Bay Astronomy Club in Virginia Beach, Virginia asked "What powers a comet?"

Well, that's a very interesting way to look at comets. It does seem as if they must be under some kind of power like a rocket or a spacecraft with thrusters. Comets travel very fast, loop around the Sun, and have long tails.

A comet is a chunk of material left over from the formation of the solar system. About 4.6 billion years ago, our solar system formed from a vast cloud of gas and dust. Slowly pulled in by its own gravity, this swirling, roiling mass collapsed, and if you could have watched, it might have looked like an explosion in slow motion and reverse. Most of the material in the cloud collected at the center and formed the Sun. Other big clumps of gas and dust became the planets, including Earth. Some of the material that didn't get caught up in the Sun or planets as they were forming gathered into smaller chunks, sometimes just a few kilometers across. Many of these chunks are what we call comets.

Comet Hale-Bopp, Johsua Tree National Park, March 28, 1997.

Because the comets are much smaller and lighter than planets, they are easily pulled this way and that by the gravity of the planets (or, in some cases, even by stars). So when we see them streaking through the solar system, they often aren't following the kind of neat, nearly circular orbits that most planets do.

The solid part of the comet, its "nucleus," is like a dirty, hard-packed snowball with pockets of air trapped inside. When a comet's orbit brings it into the inner solar system, where you and I live, some of the ice and gas are heated by the Sun, and they expand to form a cloud around the nucleus. Although the nucleus may be only a few kilometers across, the cloud, called a coma, can be thousands of kilometers across—even larger than Earth. The coma blocks our view of the nucleus, just as a cloud blocks your view of an airplane flying inside it.

Comet tails point away from the Sun and get longer as they approach the Sun.

Light and other radiation from the Sun push on the gas and dust in the coma, blowing the material away to form a tail that can be millions of kilometers long. Now imagine you are wearing a scarf on a windy day. The direction the scarf blows does not depend upon the direction you are walking—all that matters is the direction the wind is blowing. The tail of a comet is the same—it points away from the Sun, regardless of whether the comet is on the part of its orbit that is taking it toward the Sun or away.

So one answer to what powers a comet is simply that the Sun does. It provides the heat that drives some of the material from the nucleus into the huge coma, and it provides the pressure to push some of that into the long and beautiful tail.

Now what powers the comet's orbit? Well, comets formed in the rotating cloud that produced the solar system, so that got them started on their orbits of the Sun. (This is the same reason Earth and the other planets orbit the Sun.) We already know that the comets are so light that the planets yank their orbits around, sending them just about every direction in the solar system. But still another force acts on comets. When gases inside the nucleus expand under the heat of the Sun, they often shoot through holes or weak spots in the crust of the nucleus, like the steam coming through the hole in a tea pot. The expanding gas and dust escape in jets, having the same effect as thrusters on a spacecraft. This force, like letting the air out of a balloon, will change the comet's path, making its orbit hard for astronomers to predict precisely.

Comet Borrelly, photographed by Deep Space 1 from about 3400 kilometers (2000 miles) away.

Even so, NASA has been able to send spacecraft to meet up with comets to study them. I was lucky enough to be in charge of Deep Space 1 when it flew through the coma of Comet Borrelly in 2001. One of the most thrilling moments of my life was when the pictures from our brave little explorer showed up in mission control. From deep inside the coma, DS1 captured the best pictures ever taken of the nucleus of a comet and the jets shooting out of it. It revealed a strange and fascinating place unlike anything we had seen before. It would have been impossible to see this from Earth because of the coma.

Stardust collects samples from Comet Wild 2.

Another spacecraft, Stardust, visited Comet Wild 2 in 2004. Speeding through the coma, it collected tiny samples of gas and dust and brought this treasure home to Earth in 2006. For the first time, we will have seen the raw material from which the solar system was made—truly the dust of stars. You can play Tails of Wonder and learn more about comets and the Stardust mission.