Hard-Hat ZonePlanet-building is like a giant snowball fight. The loser of the fight -- the body that gets pummeled the worst -- is the winner: It becomes the largest planet. Nowadays, the snowball fight is much less intense than it was during the turbulent youth of the solar system. But it is not over yet. The planets are still growing slowly as comets and meteoroids collide with them. Events such as the 1994 impact of Comet Shoemaker-Levy 9 into Jupiter remind us that the Under Construction signs in our solar system can't be taken down just yet. The snowball-fight metaphor is especially apt when you consider what comets actually are. In the 1950s, American astronomer Fred Whipple suggested that comets are dirty snowballs: clumps of ice mixed with interplanetary dust. His hypothesis was verified in 1986 when the European Space Agency's Giotto spacecraft flew within a few hundred kilometers of Halley's comet and took photographs of the actual dirty snowball, 15 kilometers (9 miles) long by 8 kilometers (5 miles) across.
For most of its orbit, a comet remains far from the Sun, little more than the dirty snowball that it is -- what is usually called a bare nucleus. Its glory days begin when the nucleus approaches the Sun and starts to feel the heat. Gases and dust start erupting in geysers reminiscent of Old Faithful at Yellowstone National Park. The gas and dust form a large cloud, called the coma, around the nucleus. This coma is what gives comets their fuzzy appearance. If the comet gets close enough to the Sun, radiation pressure from sunlight pushes some of the dust away from the Sun, creating a long tail of dust. The solar wind, a stream of charged subatomic particles from the Sun's surface, blows gas from the coma into a separate tail, known as either the ion, gas, or plasma tail.
These processes are most active when a comet is closest to the Sun, near what is called the perihelion -- Latin for "near Sun" -- point of its orbit. As the comet swooshes around the Sun and begins to recede, its activity decreases. The geysers blow their last gust; the coma disperses into space; and the comet is once again a bare, unloved, dirty snowball, until such time as it approaches the Sun on its next go-around.
Astronomers know of over 1,000 snowballs that have made the journey at least once. Most of these have been discovered this century, when astronomers started looking for them with powerful telescopes, which reveal much dimmer comets. Some 120 comets have been observed at two or more returns. Each year, observers discover a dozen or so new comets; in addition, a dozen or more comets that are already known make their passages around the Sun.
Most of these objects are extremely dim. Perhaps one-third to one-half are visible in the telescopes used by amateur astronomers, but the majority of these are dim, unimpressive objects -- no more than a faint fuzzball. Perhaps three to five comets each year are bright enough to be picked up in a pair of binoculars; of these, perhaps one will reach the point where sharp-eyed observers at dark sites are able to see it with their naked eyes. The truly great comets, those easily visible to the naked eye of anyone who looks skyward, usually occur only once in a decade.
Stars with trains of fire. For King Harold and the Saxons, the appearance of a comet in 1066 was a portent of doom; for Duke William and the Normans, the same comet was a blessing from heaven. Later that year, William's army defeated Harold's forces at the Battle of Hastings. William's wife, Queen Matilda, commissioned this tapestry, the famous Bayeux Tapestry, to commemorate her husband's victory. Today we know that the comet was Halley's comet on one of its recurring visits.
For over 25 years, I have been part of the latter group. So far, I have observed over 200 of these fuzzy visitors. On an average night, two or three comets are visible through the moderately large telescope that I own, and on a semi-regular basis I observe any that are visible. I have also spent time trying to discover a comet of my own, but after years without any success, I gave this up a few years ago.
On Saturday night, July 22-23, 1995, I planned to observe the two comets that were then visible in the night sky. After finishing with the first one about midnight, I had an hour to wait before the second comet rose above my house. Because the night was especially beautiful, with the summer Milky Way arching brightly overhead, I decided to pass the time by observing a few star clusters and gas clouds toward the center of the Galaxy.
When I turned my telescope toward one such star cluster, catalogued as M70, I immediately noticed a dim, smaller object in the same field of view. A check of my star atlases, and of the various catalogs of clusters and clouds, revealed that nothing should be in that position. Then I checked the known comets; none were in that location either. After about an hour, I saw that the fuzzy object had shifted its position relative to the background stars -- a sure sign it was a comet.
At once, I reported my find to the Central Bureau for Astronomical Telegrams in Cambridge, Mass., the clearinghouse for reporting and announcing discoveries of comets and other such objects. An amateur astronomer in Arizona, Thomas Bopp, happened to be looking at M70 at about the same time. He, too, noticed the comet, and duly reported it. In keeping with the tradition, the comet was named after the two of us: Comet Hale-Bopp. After all the years I had spent hunting for new comets, and failing to find any, I found one after I had given up the search.
Both Bopp and I had every reason to believe that our comet would be like 99 percent of all the other comets that are discovered: a relatively dim object that would fade from view a couple of months later. But then we saw the first orbital calculations. These calculations, performed by the Minor Planet Center in Cambridge, Mass., applied Newton's law of gravitation in order to translate our position measurements into a distance.
Comet Hale-Bopp was an almost unprecedented 7 astronomical units (1,060 million kilometers, 665 million miles) from the Sun -- halfway between the orbits of Jupiter and Saturn. At this distance, very few comets are visible with any telescope in the world, let alone with a relatively small instrument.
When you're a jet you're a jet all the way. As Comet Hyakutake approached the Sun, jets of dust erupted from the Sun-facing side of its nucleus. Paul Boltwood took this unusually close-in image of the inner coma of the comet using a 7-inch refractor and a homemade CCD camera. The faint streaks in the image are stars. His movie and other educational materials are available on video tape and CD-ROM from Cyanogen Productions at 1-800-835-6794 or firstname.lastname@example.org. © 1996 Paul Boltwood.
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