Facts about Comet Hyakutake

MILWAUKEE--The most spectacular celestial event in more than 20 years will capture the attention of Milwaukeeans over the next couple of weeks: An unusual comet will drift through the sky, becoming brightest as it passes near the Big Dipper and North Star on the night of March 24-25. See finder chart.

The Comet

Comet Hyakutake, discovered last January by a Japanese astronomer, is already visible to the naked eye when observed from a site far from city lights. The comet lies to the south, in the constellation Libra, at about 3:30 a.m. Because moonlight makes it difficult to see the comet, the best observing times are about 90 minutes after the comet rises, but before the Moon rises. The comet is high in the southeast by about 12:30 a.m. The accompanying table gives the times of moonrise during the comet's apparition.

The comet is currently a faintly glowing milky patch of light, drifting slowly northward through the stars. Next week, it will begin to move much more rapidly toward the North Star (Polaris) and brighten dramatically. By the 25th, it should be easy to spot from within the city (though probably not downtown).

The reason for this dramatic behavior is that the comet is heading toward a close encounter with Earth: On the 24th, it will lie within 9.3 million miles of our planet, a hairsbreadth in astronomical terms. (Comet Hyakutake will make one of the closest passes by a comet on record.)

What to Expect

Think of a comet as a drop of milk on a plate. If the plate is black, and the milk is rich, the drop will be easy to see. If the plate is grey or white, or the milk has been watered down or smeared across the plate, the drop is less apparent.

The plate corresponds to the night sky, of course. In the city, it is far from black; it gets darker the farther you travel from the city limits. No comet looks like pure milk, except for a tiny spot in the middle.

At its best, Comet Hyakutake is going to be a fairly rich spot of milk, but spread out to about four times the width of the full moon. Its tail will be fainter, but still noticeable, particularly when viewed with binoculars from a dark site.

How to Look

The first is your observing site. The farther from city lights you are, the darker the sky will be, and the easier it will be to spot the comet.

The second point is related. Your eyes are the most important observing instrument you have. In darkness, the human eye undergoes chemical changes that increase its sensitivity to light. This process take about 30 minutes to complete, but is instantly reversed by exposure to white light (say, from a flashlight or auto headlights). To protect your "night vision" - and that of those you're with - use only a flashlight whose lens has been covered with red cloth or plastic, or several layers of paper from a paper grocery bag. If you must turn on another light - even the dome light of your car - at least warn others so they can close and cover their eyes.

By the way, remember that the night sky offers more than the comet. You can expect to see several meteors each hour you're out. Also, try to learn some of the constellations, especially Sagittarius, which is rising in the southeast by about 4 a.m. Comet Hale-Bopp, another promising comet, lies in the constellation Sagittarius, and will be bright enough to spot with binoculars this summer.

The Nature of Comets

Comets are lumps of dusty ice created in the early days of the Solar System. They spend most of their lives billions of miles from the warming rays of the Sun. When a comet wanders inside the orbit of Jupiter, however, its ices turn directly to gas (subliming as dry ice does), which begins to glow like the gas in a fluorescent light. The icy lump, or nucleus, is ordinarily only a few tens of miles across, but the glowing head, or coma, can be hundreds of thousands of miles across. Some gas and accompanying dust trails behind the comet, pushed by radiation from the Sun; the resulting tail often stretches millions of miles from the coma. The gas in the tail glows with a bluish light; the dust, which merely reflects sunlight, has the yellowish hue of the Sun.

In general, comets emit the most light, and glow most brightly, when nearest the Sun. However, a comet's apparent brightness as seen from Earth depends on both its distance from the Sun and its distance from Earth. The orbit of Comet Hyatukake is highly tilted, and the comet will dive south after its closest approach to the Sun. For this reasons, the second half of the comet's appearance, when it is emitting the greatest amount of light, will be visible only to persons in the southern hemisphere. Thus, the time of closest approach to Earth - the night of March 24-25 - is the best time for northerners to see the comet.

Questions and Answers

Q.: When and where was the comet discovered?

A: The comet was discovered by Japanese astronomer Yuji Hyakutake on January 31, 1996. He was using 25 X 150 binoculars, and saw the comet when it was 170 million miles from Earth.

Q.: I heard the comet will strike the Earth. Is there any danger?

A.: While the Earth has certainly been struck by comets in the past, the comet will come no closer than 9 million miles. The comet presents zero danger to us.

Q.: Do I need any special equipment to observe the comet?

A.: The comet will be bright enough to detect with the naked eye when viewed from a dark site, such as a county park. The view will be much improved with binoculars. 7 X 50 binoculars will be the most comfortable and give the brightest image, though 7 X 30 binoculars will be adequate.

Q.: How bright will this comet be?

A.: Predicting comet brightness is a notoriously tricky problem (remember Comet Kahoutek?). Moreover, astronomers use the same terminology for stars, planets and comets, though the light of a comet is spread out over a much larger area. So, while the comet may be brighter than the North Star, it won't gleam as brightly as the star, whose light is concentrated into a single point. It should, however, be easily detectable even without binocular or a telescope when viewed from a reasonably dark site, such as a county park.

Q.: Where should I observe the comet?

A.: Best viewing will be enjoyed away from the lights of the city and suburban shopping strips. The comet will be in southern skies until about March 23, and until then areas south and west of metro Milwaukee will be best. During closest approach, the comet will pass overhead and into the northern sky, so an observing site north of the city will put the bright lights behind you.

Q.: How can I learn more about this event?

A.: The current program at the UWM Planetarium is devoted to this comet. Planetarium programs are given Friday nights at 7 and 8:15 p.m. Admission charge is $1.00 (75 cents for UWM students). The UWM Planetarium is located at the north end of the Physics Building, 1900 E. Kenwood Blvd.

Q.: I heard about another comet that's supposed to be even more spectacular next year. What's the story?

A.: That would be Comet Hale-Bopp, discovered last year. Because it is already unusually bright at its current large distance from the Sun, this comet is thought to be either unusually large or unusually active. Unlike Comet Hyatukake, this comet will not make a close approach to the Sun. However, it will make a relatively close approach to Earth. If the comet continues to brighten as it has, it could easily rival or exceed the brightness of Hyatukake when it makes its close approach to Earth, in the spring of 1997. You can learn more about this comet at the UWM Planetarium show mentioned above, or on the World Wide Web. Popular astronomy magazines like Astronomy and Sky & Telescope have had articles about this comet, and will undoubtedly have more in the future.