Arizona telescope sees a lopsided Halley's comet getting brighter

Although Halley's comet is just beginning to enter the inner solar system, astronomers here at the Kitt Peak National Observatory already are gathering a rich harvest of data. Halley is still beyond the orbit of Mars. It looks like little more than a fuzzy blob, even to the world's fourth-largest telescope -- the observatory's 4-meter ``eye.'' Because of the comet's orientation, the tail it's developing is hidden by the comet's head.

Still, astronomers have already made some significant discoveries.

Dr. Susan Wyckoff of Arizona State University says the most significant finding so far is the confirmation that the comet's nucleus is mainly a big ice ball.

Astronomers knew that Halley's comet had a great deal of water. But they didn't know what proportion of the nucleus would be icy. It now looks like the nucleus is largely ice.

She and a team of fellow astronomers have just finished several days study of the comet with the 4-meter telescope here. They are studying the comet by the light it reflects from the sun and by the light of its own fluorescence, which is beginning to turn on as the comet draws near the sun.

Dr. Wyckoff explains that the comet is brightening so fast since passing Jupiter's orbit that the only model that fits what they are seeing is fluorescense from the comet's water molecules.

Dr. Michael Belton, an astronomer at Kitt Peak, said another major finding from data taken here and elsewhere has been the rapid variations in the comet's brightness that have shown up since Halley was near Saturn. He says this means that the object is rotating and is very irregular in shape. The ratio of its longest dimension to its shortest dimension may be something like 2 to 1.

The astronomers emphasize that Halley's visit this year is the first time that anyone has been able to see a comet nucleus and watch the coma (atmosphere) forming, even with the most powerful telescopes. Usually scientists pick up a comet after it has begun to form its coma, as Halley now is doing. When that happens, you can't see the nucleus. But scientists knew that Halley's comet was coming and where to look for it, so they were able to pick it up before the coma developed.

Astronomers here noted that this is probably the last time scientists will need to wait so long to observe the comet. From now on, instruments such as the Hubble Space Telescope, scheduled to be launched from the space shuttle next year, will be able to track Halley at all points of the comet's orbit, even when it reaches its farthest point beyond Neptune. This will open a new age in comet research.

Referring to the information they are now getting, Dr. Wyckoff said, ``It looks like wonderful data. We're all eager to dive into it and see what it's telling us.''

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