Word: orbit
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...their great numbers would make them a major component of the so-called dark matter that astronomers believe constitutes most of the substance of the universe. Dark matter has never been seen directly, but its powerful gravitational influence is evident from the way galaxies spin on their axes and orbit one another. There is at least 10 times as much dark matter as visible matter, in fact, and perhaps as much as 100 times as much--which would be enough eventually to halt and reverse the universe's headlong expansion. Even at the low end, dark matter's dominant gravity...
When Japanese amateur astronomer Yuji Hyakutake first caught sight of the comet through a pair of binoculars on Jan. 30 (it was his second comet discovery; the first came just a month earlier) there was no reason to think it would be especially bright. But when professionals calculated the orbit, they realized that Hyakutake would be approaching to within a mere 9.3 million miles of Earth, only 40 times as distant as the moon...
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...
...controlled crash. That would give NEAR's sensors one last, very detailed look at Eros' surface and perhaps reveal even more scientific information. Will they do it? Says APL's Coughlin: "We aren't even going to consider that question until we've successfully gotten there and gone into orbit. But we certainly aren't ruling...