It was love at first sight.
As soon as astronomers spotted their intended across 650 million miles of interplanetary space, they knew the encounter would be an affair to remember. Even two years before its closest approach to Earth, Comet Hale-Bopp shone brightly in the frosty darkness of the outer solar system.
Now, 19 months after its discovery, Hale-Bopp is close enough that, even without a telescope, early risers can see it hovering on the eastern horizon just before dawn. By April, it should be prominent just after sunset on the northwestern horizon.
"It will be a beautiful and splendid object," said Michael Mumma, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md. "It's going to be quite a good show."
Because of Hale-Bopp's size and brightness, astronomers can study it in more detail and over a longer period than any comet in memory. Their investigations since the comet's 1995 discovery already have revealed a great deal about what comets are made of and how they're put together.
Understanding such things could reveal important clues about the origin of Earth and the other planets, because comets are thought to be pristine remnants from the solar system's earliest days.
"The comets are leftover material," said Anita Cochran, an astronomer at the University of Texas at Austin. "The comet as we see it today is indicative of the conditions then, without having undergone earthquakes and erosion and the other things that are common on the big planets."
Comets also are thought to have seeded Earth with some of the molecules important to the development of life, so studying the composition of Hale-Bopp and other comets could even offer clues as to how life began.
Like many remarkable scientific discoveries, Comet Hale-Bopp popped up in the middle of the night. In the wee hours of July 23, 1995, both Alan Hale of the Southwest Institute for Space Research in Cloudcroft, N.M., and Thomas Bopp, an amateur astronomer from Glendale, Ariz., reported seeing a fuzzy spot in the constellation Sagittarius.
Within hours, the International Astronomical Union had announced the existence of the comet - and named it for its two discoverers.
Astronomers were astounded that an object so far away - Hale-Bopp was beyond the orbit of Jupiter at the time - could be so bright. They predicted the comet would be quite a sight when it passed by Earth and the sun in spring 1997.
Those predictions have been toned down a bit, but not much. Astronomers still expect the comet to be as bright as the brightest stars by late March, when Hale-Bopp will pass Earth at a distance of 123 million miles.
"Hale-Bopp has just been a monster in terms of its brightness ever since its discovery," said Harold Weaver, a research scientist at Johns Hopkins University in Baltimore.
In October 1995, Mr. Weaver and several colleagues used Hubble Space Telescope pictures of the comet to estimate its size. By their best guess, Hale-Bopp is a whopper - 25 miles in diameter. Halley's comet, which last passed this way in 1986, is only a quarter that size.
But big doesn't necessarily mean bright. A comet's brightness is generally determined by the amount of material that spews off it as it hurtles through space. That stuff consists of dust, water vapor and other substances that remain solid in the frigid reaches of the outer solar system, but turn to gas as the comet is heated during close approaches to the sun.
Fortunately, Hale-Bopp's gas and dust production is as impressive as its size.
"Hale-Bopp is going to be the most productive comet that we've ever observed," Mr. Weaver said.
When it was out beyond Jupiter, French astronomers reported last year, Hale-Bopp was producing about a ton of carbon monoxide a second. The release of carbon monoxide also liberated dust, which the Hubble telescope saw spiraling off the comet as it hurtled through space.
During the first few months astronomers observed it, the comet brightened at 18- to 36-day intervals, apparently burping up as much as 10 times more gas at some times. Each burp probably represented the rotation of the comet as a particularly active part of its surface spun into view and then turned away, Mr. Weaver said.
As the comet came nearer, the burping was replaced by a constant torrent of activity. In mid-1996, active areas began popping up all over the comet, presumably because it got close enough to the sun for water to boil off its surface.
"Now, instead of looking at this spiral structure, you get something more like a porcupine," Mr. Weaver said.
Astronomers expect the comet to reach peak brightness in March and April, when even the most amateur of astronomers should be able to spot it on the northwestern horizon.
"People can go out and look at it after getting home from work and just before dinner," Mr. Weaver said.
Comets such as Hale-Bopp and Hyakutake, which passed within 10 million miles of Earth last year, circle the sun in lopsided orbits that carry them through the inner solar system every few millennia. On Hale-Bopp's last visit to the solar neighborhood about 4,000 years ago, the Babylonian astronomers charting the courses of the five planets that they knew - Mercury, Venus, Mars, Jupiter and Saturn - may have seen it.
Experts differ in their estimates of exactly how bright Comet Hale-Bopp will be this time around. But it will certainly be a better show away from city lights, said Dan Green of the Harvard-Smithsonian Astrophysical Observatory in Cambridge, Mass.
"It should be about as bright or a little brighter than the comet last March," Mr. Green said, referring to Hyakutake, which was visible last spring as a fuzzy streak in dark skies.
Comet Hale-Bopp won't get nearly as close to Earth as Hyakutake did, however. So instead of stretching across a good portion of the sky, its light will be concentrated into a fairly compact spot and perhaps a short tail.
Nobody can be sure exactly what it will look like, however, because comets are unpredictable beasts. They can shed gas and dust furiously one week, only to dry up and disappear from view the next.
"They're very much like cats," Mr. Weaver said. "They both have tails, and they end up doing what they want to do."
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