Name a comet, any comet.
Chances are you'll come up first with Halley, even if you don't pronounce it correctly. It rhymes with valley, but is often mispronounced as in daily, and occasionally even as in folly.
But that's understandable. Not many of us have ever seen Halley's comet. It last came around our planet in 1910, proving a real visual spectacle, with the Earth brushing the streaking comet's tail of dust and Chicago newspaper headlines warning housewives to shut their doors and windows.
Now the time for Halley is arriving again. The comet visits our celestial neighborhood every 76 years or so on a wildly elliptical orbit extending from the sun to past Neptune. It's still a good half-billion miles away, between Mars and Jupiter, inbound at 35,000 m.p.h.
But coming it is. In late January, amateur astronomer Stephen J. O'Meara became the first person since 1911, when Halley bid goodby, to make a sighting by direct viewing through a telescope.
Viewed From Hawaii
O'Meara used a 24-inch telescope at Hawaii's Mauna Kea, a prime viewing spot thrusting 14,000 feet high into the dark Pacific skies, to spot Halley at a level of brightness less than a billionth of that of stars in the Milky Way. The details, to be published later this month in Sky and Telescope magazine, where O'Meara works, will give heart to the thousands of amateurs around the world who are part of the International Halley Watch project headquartered at the Jet Propulsion Laboratory near Pasadena.
California Institute of Technology astronomers, using photographs made with computerized electronic light detectors attached to the 200-inch telescope on Palomar Mountain, first identified Halley on its latest inbound run in October, 1982. They had searched the skies for two years based on data extrapolated from sightings throughout history.
Several subsequent photographic sightings have whetted the appetite of 900 scientists worldwide, anxious to carry out the first-ever detailed experiments on an object thought to remain from the primordial days of the universe.
The Soviet Union already has launched two space probes that will rendezvous with Halley in March of next year. Two Japanese and one European craft will be sent later this year in time for the March dates. While the U.S. government nixed an American probe because of the cost, American scientists will be participating in important ways with those of other nations.
"Comets are great big puzzles," Stephen Edberg, coordinator of the Halley Watch, said in an interview. "We're reasonably sure they are primordial pieces of matter, some kind of icy solid that generates atmospheres and tails."
Where Comets Come From
Comets are thought to originate in a cloud of material in the so-called Oort belt, about one light-year from the sun just outside our solar system. If the material is somehow disturbed by a star or other object, a piece may be kicked into the vicinity of Jupiter or Saturn where the massive gravitational pull of those planets can force the material into a permanent orbit as a comet.
"But the nitty-gritty--their composition, their size, whether they rotate, etc.--is pretty much unknown," Edberg said. "After the research is all compiled on Halley, a lot of ideas will probably have to be revised."
While the 29th recorded visit of Halley is expected to be a scientific boon, it may be a visual boondoggle, at least in living up to expectations engendered by the 1910 visit. Then, Halley came within 15 million miles of Earth and was closest within a few days of passing around the sun (called perihelion), the time when a comet is normally brightest.
39 Million Miles Away
This time, Halley will be almost 60 million miles from Earth as it nears the sun in late November and will still be 39 million miles away during its closest point in April, 1986. The basic reason is that the Earth will be on the opposite side of the sun as the comet approaches perihelion. In 1910, and even more spectacularly in 837--when the comet approached to within 3 million miles of our planet with a 90-million-mile-long tail--the comet was between Earth and the sun.
"Visually speaking, this will be the worst pass in 2,000 years," Edberg said. "But that's not to say it won't be interesting to see."
O'Meara said that Halley appeared little more than a speck with an almost imperceptible haze around it when he saw it through the Mauna Kea telescope in January. That haze is Halley's coma, the result of the sun heating the nucleus, or head, of a comet and and causing gas and dust to be released.
In December, 1984, astronomers at Palomar, again using photographic plates, first confirmed the coma. As the comet nears the sun in the fall, the sun's action will become strong enough to push the gas and dust emitted from the nucleus backward, creating the telltale tail.