Hyakutake on March 25 1996

Lessons from last time In March of last year (1996), Comet Hyakutake made a celestial spectacle of itself by passing within 14 million kilometers of Earth, producing a stunning sky show for amateur and professional astronomers alike. Look mom! No telescope! To amateurs, the comet was the brightest since at least 1976, when Comet West blazed through the heavens. For pros, Hyakutake was a chance to do big-time science on these elusive, ephemeral (defined) celestial objects that fascinated the ancients. When the mid-sized comet passed close to a slew of new astronomy instruments, scientists made some intriguing discoveries:

			Hyakutake emitted 100 times as many X-rays as predicted. This was surprising because comets don't possess the kind of nuclear furnace needed to produce such energetic radiation. Scientists speculated that the nucleus might reflect X-rays in sunlight, but the rays did not seem to originate at the nucleus. Instead, the X-rays originated about 20,000 kilometers ahead of the nucleus, in a region called the coma. (The coma is the gauzy cloud of gas and dust spewed from the comet's nucleus that accounts for the comet's cotton-ball appearance.) According to experts at NASA's Goddard Space Flight Center in Greenbelt, MD., the X-rays probably were made in a shock wave produced by the collision between the coma and the solar wind (defined). Expect further investigation of the phenomenon when Hale-Bopp cruises by in March.
			A comet's chemical cargo is always interesting for the scarce information it can reveal about conditions when the comet, and the solar system, formed. Signals from Comet Hyakutake were the first to conclusively show signs of two long-expected hydrocarbons: methane (CH4), and ethane (C2H6). The comet also seems to contains an unexpected hydrocarbon, acetylene (C2H2). In all, Hyakutake contains about 1 percent methane, a simple compound that is the major component of natural gas. Methane is quite common in interstellar space (one possible origin of comets), but it had never been seen conclusively in previous comets.
			Unfortunately, information on the chemical soup didn't mesh well with existing ideas about how comets had formed. The high levels of ethane (about 1 percent of the comet's mass) made it unlikely that Hyakutake was formed in interstellar space. But the ratio of methane to ethane didn't do much for the theory that comets were formed closer to the sun, from ice that had already been vaporized by solar heat. But don't shed a tear for the poor old theorists. At least they've got some data to chew on. Keeps 'em honest, don't you think?

Gloom and doom? Once upon a time, comets were feared as ominous omens -- a streak of light that might herald a war or a plague. Later, they were seen as messengers from a simple, ancient solar system. Nowadays, they seem to be telling us that the solar system has a more complex history. In other words, we now have indications that comets can form in or around the solar system, and then some are "reprocessed" by heat and radiation when they approach the sun. All of these factors could account for comets' varied composition, and could eventually lead to a more complete -- and more accurate -- picture of celestial history. This kind of increasing complexity is to be expected as data floods a field of astronomy that has long suffered a data shortage, says astronomer Lucy McFadden of the University of Maryland. "As our ability to study comets becomes better, we see more details. In the 1970s, we developed the ability to measure how much gas and dust comets emit. Now we can get two-dimensional images in various spectral regions, so we see more details of gas jets and dust jets." Ironically, she says, the confusion can be attributed to technical progress: "We're using finer glasses." Did you think our solar system is kind of, er, special?

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