The Seasonal Almanac shares astronomical events in December 1996 and January 1997. This issue includes information on the long-anticipated arrival of Comet Hale-Bopp, one of the largest, brightest comets in 1,000 years to be visible from earth.
The wait is almost over. For a year and a half, professional and amateur astronomers have been watching for the arrival of Comet Hale-Bopp, what they think is one of the largest or most intrinsically active comets of the past thousand years heading toward our inner solar system. It will never get close to Earth and it will not zoom in so near the sun as to be wildly stirred. But this comet looks to be so powerful it won't have to get near Earth and sun to become spectacular in our skies, possibly more spectacular than any comet in a millennium.
As winter progresses, we should see the mighty visitor brighten and get a little higher in the east before dawn each day, then as it reaches its brightest, switch over to shine for several hours after nightfall in the west in late March and April. It could sport a tail so bright that much of it will be seen even from big cities for two whole months.
The name of the mighty visitor? Since it was discovered independently, by American amateur astronomers Alan Hale and Thomas Bopp, it is known to all the world and all posterity as Comet Hale-Bopp.
A Word of Caution
Before we get too excited about the prospect of grandeur in our skies, though, there must always be a strong word of caution: comets are fickle. We can calculate where they are going but the brightness and tail appearance of a comet is difficult to predict. Caribbean cruises to see Comet Hale-Bopp have been planned. The prospect of it has already generated entire books, T-shirts, and tabloid TV and newspaper features. But a similar great preparation was made for the infamous Comet Kohoutek in late 1973. As some of us still recall, Kohoutek proved to be about 100 times dimmer than initially expected and thus was a very modest glimmer to the naked eye.
Scientists have no reason to believe that Hale-Bopp will fall short as Kohoutek did. At the time of this writing, Hale-Bopp has been brightening about as steadily and dependably as a comet can. Nevertheless, we should remind ourselves of what a great comet scientist once said: "Never bet on a come." There is a strong likelihood that Comet Hale-Bopp will be as bright and spectacular as forecasted. But anyone who says he or she can absolutely guarantee that is just plain wrong.
The Most Mysterious Objects
When you consider what comets are, you begin to understand why predicting their brightness can be so difficult. Comets are the most mysterious objects in the solar system. They are not meteors ("shooting stars")—the brief streaks of fight we see zip across the sky when a usually tiny piece of space dust or rock enters our atmosphere at tremendous speed and burns up from friction with the upper atmosphere's gases.
Comets are at heart mountains of dusty, rocky ice miles across. Most of the time, a typical comet is so far from the sun that it is nothing but this huge chunk of ice, which we call the comet's nucleus. But when the nucleus begins to approach the sun, the surface ice is heated and, in the vacuum of space, some of it turns directly into vapor, producing a cloud of dust and gas which may be up to a million miles across. That cloud is called the coma (Latin for "hair"; comet means "long-haired"). Together, the tiny hidden nucleus and giant visible coma are called the head of the comet. But in the case of many comets, the pressure of sunlight and the solar wind also pushes the dust and gas away to form the most famous part of these objects—the tail.
Now, consider the fact that comets shine by both reflected light (sunlight off their dust) and emitted light (mostly a kind of fluorescence which occurs when atomic particles from the sun strike the comet's gases). Consider that their ice, while mostly frown water, also contains a great variety of strange ingredients. Consider that only certain spots of the comet nucleus's surface may be exposed ice, while the rest may be covered with a mantle of dust or other matter. Then ponder the fact that this nucleus is not only spinning around but usually wobbling as it spins, and is so light and fragile that a planet's gravity may push it here and there, and gravity or heating may cause big chunks of it to break off and gush extra gas and dust. All of these factors—and others!—can affect the brightness and tail of the comet we see in our sky.
Comets as Destroyers and Creators
Why should we bother with comets if they are so strange and tricky? Because they can be, temporarily, the largest and fastest, and almost the brightest and hottest, objects in our solar system. Because some of them are the most unspoiled pieces of original solar system matter in existence. Because scientists suspect that comets may have played crucial roles in helping to create, destroy, and modify life on Earth (maybe on other worlds, too).
Some scientists believe that comets helped bring huge quantities of needed water to Earth in the early days of our planet's history, and possibly even the organic molecules from which life itself originated. But comets have probably been the greatest danger to the continued existence of life on Earth. Most scientists now believe that either a big comet or asteroid crashed into Earth 65 million years ago, killing off not only the dinosaurs but about 75 percent of all species on Earth.
Fortunately, collisions of this magnitude only seem to happen at intervals of tens of millions of years. But lesser collisions are tremendously more common and these could be quite devastating to the country or part of a country where the strike occurs.
Preparing for the Arrival of Hale-Bopp
Of course, yet another reason to learn about comets is in order to better enjoy the display of beauty offered by a great comet like Hale-Bopp. Our diagrams show where and when to look for Hale-Bopp this month and next. But I will give much more information in this column in the February/March issue—months when the comet should at last brighten to extraordinary prominence and return to the evening sky.
What I want to discuss here and now are important general points about preparing for the comet and starting to observe it.
A first question that almost everyone asks is: Should I have a telescope? My answer to that question, now as always, is: Only if you already know some constellations and planets and have practiced observing with the naked eye and binoculars. There's no denying that some of the most fantastic views of Comet Hale-Bopp will be closeups through telescopes. But it's also true that the naked-eye sights of a great comet—hanging among the constellations, or above the twilight glow, or off to the side of a crescent moon and planets—are as glorious as anything in all of astronomy. Furthermore, binoculars are highly portable and often inexpensive optical instruments which can show you many of the details that a small telescope can and give a typically much wider field of vision.
Should you buy a telescope to see Comet Hale-Bopp? The answer is yes, if you've done your preliminary reading and observing, and feel that you are ready to spend $300 to $500 or more (almost all telescopes for less money are essentially junk). The best way to figure out which telescope to buy is to go to a meeting or observing session of a local astronomy club, ask questions, and look through different telescopes.
I'd say even more important than the optical instrument you use will be finding a place to observe that has as little light pollution as possible. Light pollution is excessive or misdirected outdoor lighting. Most people don't realize that the light escaping uselessly from poor lighting fixtures can seriously diminish our view of stars and comets for 10 or 20 miles away from a typical city of 50,000, and 30 or 40 miles away from a city of 500,000. We expect much of the tail of Comet Hale-Bopp to be bright enough to see even in twilight, let alone in city light pollution. But for the best mornings or evenings of the comet you should consider driving to a country site to see the full glory of Hale-Bopp and the starry sky it hangs in. Remember that when the moon is bright or skies are hazy, the comet and stars will be diminished in splendor.
Among the Greatest Comets in History
The great comet of 1744 displayed a huge fan of up to 11 tails. The sun-grazer comets of 1843 and 1882 skimmed the solar surface at speeds in excess of a million miles per hour and shined so bright they were easily visible beside the sun in broad daylight like daggers of light. The incredibly broad tail of the great comet of 1861 extended almost three-quarters of the way across the sky.
Comet Hale-Bopp should take its place among these and other great comets of history. It may shine as bright as the planet Jupiter (much brighter than last spring's Comet Hyakutake) and display a tail of both gas and dust many times brighter than Hyakutakes's gas tail was last March. If you want to read much more about Hale-Bopp, the dozens of other great comets in history (including Haley's), how a comet may have killed off the dinosaurs, or what special features to look for in a bright comet, you may wish to check out my new book, Comet of the Century (New York: Springer-Verlag, 1996).
Good luck with your early views of Hale-Bopp! (The December/January Almanac appears on page 19 of this issue.)