Heralding Hale-Bopp.

As it heads toward an April 1 rendezvous with the sun, Comet Hale-Bopp continues to blossom. Over the coming weeks, skywatchers will see for themselves whether this icy relic of the solar system's birth merits the title Comet of the Century.

Now visible with the naked eye in the northeastern sky before sunrise, the comet by March 20 should appear low in the northwestern sky soon after dusk. It's likely to delight after-dinner viewers through most of April and may be at its brightest during the first 2 weeks of that month. Moonlight should not interfere with observations between March 26 and April 12.

Astronomers chart a comet's journey from the fringes of the solar system using a planetary yardstick-the separation between the Earth and sun, defined as 1 astronomical unit (AU). Even at its closest approach to Earth, on March 22, Hale-Bopp will stay a respectful 197 million kilometers, or 1.3 AU, from our planet. That's 13 times farther away than Comet Hyakutake ventured last spring, when it put on a memorable show for observers away from city lights (SN: 6/1/96, p. 346).

Nonetheless, Hale-Bopp may surpass that performance, according to comet mavens Michael F. A'Hearn of the University of Maryland in College Park and Brian G. Marsden of the Harvard-Smithsonian Astrophysical Observatory in Cambridge, Mass. Hale-Bopp's nucleus, they note, may be a whopping 40 km in diameter-more than 10 times Hyakutake's-if a rough estimate made with the Hubble Space Telescope is correct. The comet also has a far more mercurial nature, which may portend the expulsion of larger amounts of dust. Dust particles, which form a comet's signature tail and its huge shroud, or coma, reflect sunlight extremely well and make for a brilliant apparition.

Because of its distance from Earth, Hale-Bopp will appear smaller than last year's visitor. However, its very compactness concentrates Hale-Bopp's light. As a result, viewers are likely to see a small, sharply defined dust tail rather than the elongated, wispy gas tail exhibited by Hyakutake, says Marsden. Especially near cities, where the night sky never gets completely dark, Hale-Bopp "is definitely going to be more spectacular," adds A'Hearn.

Marsden and A'Hearn predict that the comet will be at its brightest shortly after April 1, when it comes closest to the sun. Solar heating of the comet turns some of the water-ice on or just beneath its surface into jets of vapor that fling out dust, brightening the body. If Hale-Bopp is indeed large, it may take a little time for jets of water vapor and the dust they carry to percolate through the comet's outer layers, Marsden speculates.

A'Hearn compares Hale-Bopp to Comet Bennett, a small, bright comet that graced terrestrial skies in 1970, and says it may become as memorable as Comet West in 1976. Marsden hasn't budged from his original prediction that the comet, discovered in July 1995 when it was well beyond Jupiter's orbit, could become as bright as the star Sirius and even rival the appearance of the Great Comet of 1811 (SN: 8/12/95, p. 103).

Harold A. Weaver of Johns Hopkins University in Baltimore offers one caveat. Analyzing data from Hubble and other telescopes, he and his colleagues, including A'Hearn, find that the amount of dust spewed by the comet during 1996 did not keep pace with the increase in expelled water vapor. Weaver theorizes that if the trend continues, Hale-Bopp will not brighten quite as much as Marsden hopes.