Frontiers of Science:

Comets! Wayward Wanderers of Space

By Dr. Sherwood B. Idso

Print Share

    Do you know what comets are? One dictionary defines them as “luminous heavenly bodies, often having a long nebulous tail and following an orbit around the sun.” But does that really tell us what a comet is? Actually it is only a description of what a comet looks like and what it does.

    If we were to ask a scientist who studies comets to explain just what a comet is, we might get an answer like this: “A comet is a vast swarm of very tiny dust particles about 100,000 miles in diameter that are grouped together in the shape of a ball.” But we are just as apt to get an answer like this: “A comet is a dirty snowball made up of a single chunk of ice and dust that is usually less than one mile in diameter.”

    How could two definitions be so different? Are there really scientists who have such opposite views? Yes, indeed!

    Perhaps the main reason for the different theories is that we have never seen a comet at close enough range to answer the question once and for all. Thus, at the present time some scientists believe that all of the characteristics of comets we have been able to observe with our earth-based telescopes can be explained by both theories. These ideas are called theories—neither opinion has yet been established as fact. So let’s not make up our minds yet, but take a quick look at some of the known facts.

    A comet may be considered to have two basic parts: a head and a tail. The main part of the comet, or head, is composed of a nucleus and a coma. The nucleus is the basic material responsible for the comet’s existence. Therefore, it may be either the dust swarm or the dirty snowball of the scientists’ theories.

    The coma is a secondary feature composed of gas and dust driven out of the nucleus by radiation from the sun. It encircles the nucleus and can sometimes be very large. The coma of Halley’s famous comet in its 1910 passage near the Earth measured 250,000 miles in diameter!

    Streaming out from the head of the comet, always in a direction pointing away from the sun, is the feature that gives the comet its characteristic appearance—it’s tail. Again, it is radiation pressure from the sun, sometimes called the solar wind, that sweeps material out of the coma to form the comet’s tail. The tail of Halley’s Comet in 1910 was almost 20 million miles long; and there have been other comets whose tails have stretched to almost 100 million miles.

    Comets orbit the sun much like planets, but in more exaggerated elliptical paths. The most frequent “round tripper” is the Comet Encke that makes a complete circuit every 3 1/2 years. It is followed by about 60 other comets that take between four and nine years to make complete revolutions. The outermost points of the orbits of these comets all lie close to the orbit of Jupiter and are believed to have been “captured” by that giant planet. Beyond these relatively close neighbors, however, there may be as many as 100 billion comets belonging to our solar system, with orbits requiring millions of years for completion.

    Every year brings a group of new comet discoveries. In 1975, for instance, there were 13 sightings of previously unknown comets. Several of them were made by amateur astronomers. One of these persistent sky-watchers actually discovered two in one night! Clearly, this is an area where one does not have to be a professional scientist to make a significant finding.

    But what will be the next milestone in unraveling the mystery of comets? Probably a spacecraft encounter with one. In 1986 Halley’s Comet will make its closest approach to Earth in 76 years. And there are plans to attempt a close flyby of it or Comet Encke, possibly continuing the flight on to the actual nucleus of another comet named Comet Temple. Maybe then we’ll get an answer to the perplexing question of just what it is that makes up a comet. I like the dirty snowball theory. How about you?

    Close-up view of head of Halley’s Comet during its 1910 passage of earth. (Hale Observatories photo.)

    The most famous of all comets, Halley’s Comet, is shown here as photographed from Honolulu, Hawaii, in May of 1910. (Hale Observatories photo.)

    Another famous comet, Comet Kahoutek, is shown here as photographed on January 14, 1974, by the 48-inch Schmidt telescope of Hale Observatories.

    Graphical timetable showing orbit of Halley’s Comet and indicating time of its next, closest approach to Earth.