What are comets?

Comets are frozen bodies composed of various ices and dust, or, to quote astronomer Fred Whipple, they are a "dirty snowball." They typically move through the solar system in orbits ranging from a few years to several hundred thousand years. Comets are not on fire. As they near the sun, the sun's heat sublimates the comet's ices and releases dust particles which produces the comet's tail. Comets rarely come within a few million miles of Earth and, thus, have a slow apparent motion across our sky. Typical comets remain visible for weeks and up to a few months. What is it that makes comets exciting to astronomers? Here are a few of the reasons:

Their appearance is unpredictable

Comets can suddenly brighten or fade from view in a matter of hours. They can lose their tail or develop multiple tails. Sometimes they can even split into two or more pieces, so that, through a telescope, several comets can be seen moving together across the sky.

They are ancient

Comets represent some of the oldest, basically untouched objects in the solar system. Their very composition seems to represent the original makeup of the vast nebula that ultimately condensed to form the sun and planets.

Recent years have revealed comets have shaped the progress of life as we know it here on Earth. Many astronomers are convinced that early collisions between Earth and comets brought the vast amounts of water that now make up Earth's oceans. These oceans enabled life to get a foothold. On the flip side, the dinosaurs are certainly an example of how collisions between Earth and comets can also bring extinction to different life forms. Evidence is growing to show that other periods of mass extinction may have also been caused by such collisions.

Like a time machine

Finally, comets are like a time machine. It is thrilling to watch a comet like the famous Halley's Comet with a 75-year period and think about what life was like when the comet was last seen. Similar thoughts fill your mind when watching comets which may have last traveled through Earth's skies several hundreds, or thousands, or even millions of years ago.

The typical comet is less than 10 kilometers across. Most of their time is spent frozen solid in the outer reaches of our solar system. At this point, the comet is nothing more than the nucleus. Except for a few suspected dead comets, and a couple of suspicious asteroids that occasionally show gas emissions like a comet, the nucleus is never really seen from Earth. By the time a comet becomes bright enough to be seen from our planet, it is mostly showing the coma.

The nucleus

After the spacecraft Giotto photographed the nucleus of Halley's comet back in 1986, we now know that a comet's nucleus probably has a surface that is best described as a black crust. Although the length of the nucleus of Halley's comet is about 12km, it is believed that comet nuclei can range from 1 km to perhaps 50km across. Comet Hale-Bopp of 1997 had a nucleus that was perhaps 40km across.

The black crust of the nucleus helps the comet absorb heat, which in turn causes some of the ices under the crust to turn to a gas. With pressure now building beneath the crust, the serene, but frozen landscape begins to bulge in places. Eventually the weakest areas of the crust shatter from the pressure beneath, and the gas shoots outward like a geyser and is referred to by astronomers as a jet. Any dust that had been mixed in with the gas is thrown out as well. As more and more jets appear, a tenuous gas and dust shell forms around the nucleus and this is called the coma.

The nucleus of Halley's Comet as photographed by Giotto Spacecraft in 1986. Note that the active areas are spewing dust and gas into space. This material will form the coma and tails of the comet.

This is a drawing of the region surrounding the nucleus of comet Hale-Bopp on 1997 March 10. This shows an intense emission or jet coming from the nucleus which is forming features called "hoods" or arcs within the coma. The hoods are features formed as a result of the rotation of the nucleus. In the case of Hale-Bopp, a new hood was formed nearly every 12 hours.

The coma

Comets can typically display a coma several thousand kilometers in diameter, with the size being dependent on the comet's distance from the sun and the size of the nucleus. The latter is important because since jets generally spring up on the side of the nucleus facing the sun (that side gets warmest), and since large nuclei have a greater surface area facing the sun, then there is the potential for larger numbers of jets and greater amounts of gas and dust feeding the coma. One of the largest comets in history was the Great Comet of 1811. It was one of the few comets in history to be discovered with a relatively small telescope at an unusually great distance from the sun, in this case over half-way to the planet Jupiter's orbit. The nucleus has been estimated as between 30 and 40 kilometers in diameter. At one point during September to October 1811 the coma reached a diameter roughly equivalent to the diameter of the sun and was a very notable naked-eye object seen by people around the world.

Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars. At this distance the particles streaming out from the sun provide enough force so as to act as a wind and will literally blow the gas and dust particles away from the nucleus and coma. This disruption is the process responsible for a comet's tail, the most spectacular feature of a comet.

This edited (false simulated color) photograph of Comet Hale Bopp was provided by Valenti Sallares and edited by Gregorio Drayer for educational purposes. It was taken on March 27th, 1997at 4 a.m. from Astronomical Observatory of Senterada (OAS) at the Vall Fosca (Dark Valley) in the Pallars Jussà, Catalunya, Spain, N 42º19'43" E 0º59'20". sallares@cdlcat.ictnet.es

The tail

When a large comet moves well inside the orbit of Earth, there is a potential to witness a long tail. The current record holder for longest tail length is the Great Comet of 1843. Its tail extended more than 250 million kilometers. What this means is that if the comet's nucleus was placed in the center of the sun the tail would have stretched passed the orbits of Mercury, Venus, Earth, and Mars!

A comet can develop two kinds of tails:

The dust tail

With a length of up to 10 million km long, it is composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye

The ion tail

With a length of up to 100 million km long, it is composed of plasma and laced with rays and streamers caused by interactions with the solar wind. The solar wind is a low density stream of charged particles (mostly electrons and protons) emitted by the Sun which propagates throughout the solar system at about 450 km/sec.

Where do comets come from?

Our solar system began as a vast cloud of gas and dust. Several billion years ago, this cloud slowly rotated around our very young sun and particles within the cloud collided with one another. During this time some objects were obliterated by these collisions, while others grew in size and were to later become the planets.

Throughout this early period, comets probably filled the solar system. Their collisions with the early planets played a major part in the growth and evolution of each planet. The ices that make up comets appear to have been the very building blocks that formed the early atmospheres of the planets, and scientists now very strongly believe that it was the collisions of comets that brought water to our world and enabled life to begin.

Over the years, comets actually became rarer within our solar system. They no longer fill the skies as they did 4 billion years ago, and today a prominent naked-eye comet can be expected only about once a decade. Astronomers with powerful telescopes can see many more comets, but even in this case it is still rare for as many as 15 or 20 comets to be detectable in the sky at any one time.

Today, most comets are located outside our solar system in part of the original cloud of dust and gas that has remained virtually untouched for billions of years. These regions are referred to as the Oort Cloud and the Kuiper Belt.

The Oort Cloud was first theorized by the Dutch astronomer Jan Oort in 1950. His investigation of the orbits of comets with very long orbital periods brought him to conclude that a large "cloud" of comets existed far outside the solar system, possibly within the range of 5-8 trillion kilometers (or more) from the sun. The total number of comets within this belt was estimated as a trillion. It is thought that objects within this cloud are occasionally ejected either by collision with one another, or by the gravitational forces of stars. Many of the ejected objects probably never cross the paths of the planets, and still more do not come close enough to be seen with even the largest telescopes. However, a few do manage to travel into the inner solar system and are subsequently seen from Earth. This cloud remains a theory only, as it has never been directly detected.

The Kuiper Belt is a region first theorized by the Dutch-American astronomer Gerard Kuiper in 1951. Seeing that Oort's cloud of comets did not adequately account for the population of comets with short orbital periods (making complete orbits around the sun in less than 200 years), Kuiper conjectured that a belt of comets probably existed outside the orbit of Neptune within the range of 30 to 50 astronomical units (2.8 to 4.6 billion miles) from the sun. Collisions and perturbations by the planets of our solar system are believed to be the reasons for the ejection of bodies from this belt.

Naked eye appearance

Comets are invisible except when they are near the Sun. Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto; these are seen once and then disappear for millennia. Only the short- and intermediate-period comets (like Comet Halley), stay within the orbit of Pluto for a significant fraction of their orbits.

Seeing a comet with the naked eye is a somewhat rare event. On the average we get a naked-eye comet once every five or six years and this includes comets that become barely visible to the naked eye. Classic comets with long tails only appear about once every 10-12 years. The motion is very difficult to detect and comparing its place with naked-eye stars over several days is the only way to see it move. In general, comets are best observed with telescopes or binoculars.