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Comets :

Comets

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On February 9, 1986, the most famous comet of all time will make the closest approach to the Sun in its current 76-year trip around our star. For a few months, before and after that date, the comet will be visible from our vantage point on the Earth — but, alas, not as well as it was in 1910. We begin the inaugural issue of The Universe in the Classroom with an in-depth look at Comet Halley (pronounced to rhyme with "Sally'') and its upcoming "not-so-close encounter'' with the Earth.

What exactly are comets? :

What exactly are comets? Comets are literally cosmic icebergs — chunks of ice with dust and rocky particles embedded in them. This "dirty ice'' includes not just frozen water, but many other frozen substances as well. A typical comet iceberg is only a few kilometers wide, very small compared to most of the objects astronomers study.

Why does Comet Halley come back regularly? :

Why does Comet Halley come back regularly? The comets we see follow a variety of paths across the expanse of our solar system. Some make a single pass into the region of the Sun and then retum to the deep freeze of the outer solar system, never to be seen again. Others may have long elliptical paths that will bring them back to us, but only after many thousands or millions of years have passed.

Why is the comet named Halley? :

Why is the comet named Halley? The British astronomer Edmond Halley (1656-1742) was using Isaac Newton's ideas of gravitation to analyze the motion of bodies in the solar system. He noticed that the records for the bright comets of 1531, 1607, and 1682 showed that all three comets had very similar orbits. He drew the bold conclusion that all three were really the same comet, trapped by the gravitational pull of the outer planets, and predicted that the comet would retum in 1758-59. The comet was found again on Christmas night 1758 and was then named in the late astronomer's honor.

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The coma and tail of a comet, indicated on a photograph of Halley's Comet taken on May 8, 1910. (Mt. Wilson Observatory photograph.)

How well will we be able to see Comet Halley this time around?:

How well will we be able to see Comet Halley this time around? Unfortunately, this will be one of the less favorable appearances of the comet and significantly less impressive than the one in 1910. How dramatic a comet appears to us on Earth depends on several factors: how bright it really is, how close it is to the Earth when it crosses our orbit, on which side of the Sun it makes its closest approach to our star, and how high above the horizon it is when we see it. The news on many of these fronts is not very promising.

Here in the United States, when will be the best times to see the comet? :

Here in the United States, when will be the best times to see the comet? For much of January 1986, the comet should be faintly visible low in the western sky for a short while after sunset. Then it will disappear behind the Sun and re-emerge from the Sun's glow in the morning sky after the first week of March. Since the comet will be favoring the Southern Hemisphere at this time, the farther south you are, the better your view will be.During the second week in April the comet will be very low in the predawn sky and impossible for many of us in the U.S. to see. It will become visible later in April in the evening sky and will be getting fainter and fainter as the month goes on.

Other than providing a show in the sky, why are comets of interest to us? :

Other than providing a show in the sky, why are comets of interest to us? Astronomers believe that comets formed at the same time as our solar system, almost 5 billion years ago. Here on Earth (and on many of the other planets), geological processes, volcanoes, and weather long ago erased all traces of this remote epoch that gave us birth. Comets, on the other hand, spend most of their time in the "deep-freeze'' of the outer solar system. The material in the comet "iceberg'' is thus well preserved and could tell us a great deal about what things were like in that ancient time. Astronomers can also use comets as probes of the present conditions in the solar system. By watching a comet's coma and tail develop and change, we can learn about the flow of particles and energy from the Sun, the details of the magnetic fields and particles between the planets, and the detailed characteristics of the comets themselves.

How will astronomers be studying Comet Halley?:

How will astronomers be studying Comet Halley? To coordinate observations of the comet around the world, astronomers have formed The International Halley Watch, con sisting of professional and amateur astronomers in many countries. All observations will be sent to IHW to establish a single archive that will be the richest record of a cometary encounter ever assembled.Not only will Halley be observed with all sorts of telescopes on Earth, the advent of the space age makes it possible for the first time to send space probes to make close-up measurements of the comet's properties. A small armada of spacecraft will monitor Halley, including five that will fly by the comet in March of 1986. Japan and the Soviet Union are each sending two probes to Halley, while the European Space Agency (the European equivalent of NASA) is sending one. The U.S. decided not to send a fly-by craft to Halley, but we are redirecting an older probe to fly by another comet (Giacobini-Zinner) in September 1985. In addition we will be making observations of Comet Halley with a special package of instruments called Astro 1, to be carried aloft by the Space Shuttle in early March of 1986.

Is there any danger from Comet Halley? :

Is there any danger from Comet Halley? Before we understood the nature and orbits of comets, people worried about the effects a comet might have on us. Today we know enough about these objects to provide a reassuring "environmental impact statement.'' Unless a comet physically collides with the Earth (which Halley will be far from doing), these small chunks of dirty ice pose no danger to us. Their effects are on the mind, stimulating our curiosity and kindling our imaginations.

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