Last May, the world's telescopes focused on comet Hyakutake. It was the brightest comet viewed from Earth since the appearance in 1976 of Comet West. Weaver said that Hale-Bopp is considerably bigger than Hyakutake and may erupt with 30 times more gas and dust. But Hale-Bopp, in its nearest pass of the Earth on April 1, will be more than 100 million miles farther away, somewhat diminishing its light show. Hale-Bopp appears to be about 18 to 25 miles across, about four times bigger than Hyakutake, but the new comet is much more energetic in the release of gas.
Astronomers observe comets in different spectrums in order to measure their chemical content.
Gases spewing from the comet and then lighted by the sun contain chemicals that have specific light signatures. Hale-Bopp is expected to be visible just after sunset in the western sky starting on April Fools day. It will dip behind the sun after a few days and not be visible again until August. When it returns to view, however, it will be more than 200 million miles from Earth and much fainter. X-rays had never before been measured from a comet, but once that radiation was found with Hyakutake, astronomers reexamined achieved data from earlier comets.