Show Fizzles Out
Well, the long-awaited apparition of Halley’s comet in 1986 has come and gone, and, let’s face it, it was a flop. In the mind’s eye the comet sparked the imagination; to the naked eye it was more like Kohoutek’s comet than a fireball in the sky.
In 1910, when Halley’s comet got so close that the Earth passed through its tail, mass hysteria erupted. This time the comet was much farther away, and most people, we suspect, never saw it. Those who did--even those who went to the Australian Outback--didn’t see much.
While Halley’s comet was a visual fizzle, it did provide important evidence in support of the “dirty snowball” theory of comets, which holds that the nucleus of a comet is a solid chunk a couple of miles across made up of rocks and minerals and held together by frozen gases.
This model had been little more than a guess until a few weeks ago. “There’s no evidence for it whatsoever,” our friend Al Hibbs of the Jet Propulsion Laboratory was saying on March 17, just as the European spacecraft Giotto was sending back its pictures of Halley’s. Hibbs is as sensible a scientist as there is, and when he says that a theory hasn’t been proved, he’s probably right.
An alternative theory held that the nucleus of a comet was an accumulation of small bodies held together by their own mutual gravity. The first pictures from Giotto didn’t settle it. “The observed data is terribly ambiguous,” Hibbs said. “Both theories can explain the observed results.”
But a few days later, after Giotto’s final pictures had been analyzed, Hibbs called to say, “Forget what I said about the dirty-snowball theory. The comet is solid.” The last pictures before the spacecraft went dead had settled the question.
But comets remain mysterious, and this year’s answers will beget still more questions to be asked when Halley’s comet next appears in 2061--which, by the way, is expected to be even less of a show than this year’s. There may be other reasons to stick around till then, but getting a good look at Halley’s comet isn’t one of them.
The apparition of May, 2134, will be a different story altogether. In that pass the comet will come closer to the Earth than it did in 1910. Maybe by then people will have decided whether to pronounce the first syllable of the name of the comet with a long “a,” like “hail,” or a short “a,” like “hal.”
For the moment, public interest in comets is sure to fade faster than Halley’s tail, which began its disappearing act last week. So long, Halley’s. Sorry that this wasn’t your year.
