Comet Hale-Bopp lights up Michigan’s April sky

March 20, 1997
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ANN ARBOR—April will bring striking views of Comet Hale- Bopp to Michigan sky-watchers, according to University of Michigan astronomer Richard Teske.

Teske recommends looking for the comet about an hour after sunset. “At that hour it should be hanging motionless in the northwest about two hand-breadths above the horizon, its tail extending almost vertically upward,” he said. “The comet’s head will appear to be a fuzzy star as bright as other stars in that part of the sky.” For best results, Teske recommends picking a dark observation site with a low, uncluttered horizon far away from artificial lights.

“Comet Hale-Bopp will remain visible during all of April,” Teske said. “For the first two weeks, it will appear about the same altitude above the horizon when darkness descends. After April 15, it will gradually get lower in the sky from night to night. Seen from a good observing site, the tail looks half as long as the handle of the Big Dipper and actually extends over 20 million miles.”

Comets are creations of the sun, Teske explained. “A comet’s brilliant head, its tail and all of the phenomena we see, are caused by the sun itself. As the comet draws closer to the sun, its head and tail grow more prominent. They fade as the comet recedes from the sun.”

The heart and core of every comet is a solid icy fragment of space debris three to eight miles across, the size of a single Earth mountain. This core is made of frozen water, frozen methane and other ices in which gritty, rocky bits are embedded. Space photographs of the nucleus of Comet Halley showed it to be shaped like an Idaho potato. Astronomers believe a huge number of these chunks exist in the dark, deep freeze of space far beyond the farthest planet Pluto, left over from the formation of our solar system.

“Some of these icy lumps enter the inner solar system from time to time, plunging unseen toward the warming fires of the sun, guided by its gravitational pull,” Teske said. “When a comet nucleus comes closer to the sun than Jupiter’s orbit, its surface ices begin to evaporate in the increasing solar glare. The ice becomes hazy, rarefied gases spreading out for 200,000 miles around the still solid nucleus. Sunlight has caused some of the ice to evaporate; now sunlight causes the gas to glow, making the comet visible. This is the bright head of comets like Hale-Bopp.”

When the comet moves within the orbit of Mars, a tail appears—the result of two additional solar effects. One of these is the solar wind which blows perpetually outward from the sun into interstellar space. “The tenuous gas of the solar wind pushes on the rarefied gas of the comet’s head creating part of the gossamer tail which we see streaming rearward energized by sunlight,” Teske explained.

The second effect is called the “radiation pressure” of sunlight. While the effect is a small one, it is real and important. Planetary scientists must account for its effect upon spacecraft traveling to distant planets, lest the vehicles arrive at their destinations hundreds of miles off-course. “In making a comet tail, the sun’s light pushes on the tiny rocky bits associated with the comet’s nucleus, forcing the bits rearward into the tail. There they reflect sunlight in all directions adding to the tail’s illumination.

“Hale-Bopp will be closest to the sun on April 1 at a distance from it slightly less than our own distance from the sun,” Teske said. “That should make April a time of enhanced brightness and tail length and make for a fine comet show.”

Richard Teske