Word: comets
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Dates: during 1990-1999
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That makes it the closest comet since 1983 and gives it a leg up on visibility (Halley's came no nearer than 39 million miles). Better yet, Hyakutake's trajectory will place it high in the northern sky when it reaches peak brightness this week, so that it will be visible for most of the night over most of the northern hemisphere...
Finally, the shape of Hyakutake's orbit tells astronomers that it has been here before, perhaps 9,000 years ago. That's crucial: the only reason a comet is visible at all is that its tiny core, just a few miles across, is surrounded by a giant cloud of gas and dust that can spread over a million miles. The cloud spews from the comet itself, as the sun's heat turns its dirty, icy surface into dirty water vapor mixed with other gases. If a comet is on its maiden voyage to the inner solar system, though, its surface...
Hyakutake's status as a return visitor makes it ideal for amateur stargazers but a bit less enticing to scientists. Comets are believed to be leftover material from the formation of the solar system 4.5 billion years ago. Millions are still orbiting lazily in a halo called the Oort cloud, far beyond Pluto, where they are perfectly preserved. It's only occasionally that one changes orbit and plunges into the relative warmth of Earth's neighborhood. And the more often a comet returns to be heated, the more its structure and chemical composition are altered...
Although a truly primordial comet would tell them more about the original chemistry of the solar system, astronomers are still thrilled to see any comet this close. "We can't exactly reach out and touch them," says Green. "We have to wait for them to come...
...bright star overall, but its light is diluted over an area twice the size of the full moon; it's a glowing blob rather than a fiery point. There won't be much of a tail at first, but one will grow over the next few weeks, making the comet even more prominent through April. And after it fades from view later in the spring, comet lovers have a treat to look forward to next year: a second new comet, called Hale-Bopp. Astronomers are predicting--cautiously--that this one could be even brighter...