Many an event is billed as ``a once-in-a-lifetime experience.'' Halley's comet really is one. Every 76 years or so this celestial celebrity lobs in from the icy reaches of the solar system for a quick vroom around the sun, to the accompanying oohs and ahs of terrestrial rubberneckers the world over. The good news is: Right now Halley's comet is heading our collective way. It's passing between Jupiter and Mars; by this December, under ideal conditions, it will be visible to the naked eye.
The naked eye, that is, of a mighty sharp observer, who can tell Cassiopeia from Orion's Belt, and who is willing to do some traveling -- in some cases extensive traveling -- to get a really good view.
The bad news is that this time around, Halley's performance may be less than a star turn. ``It will not be the spectacular object that people described in 1910,'' says Dr. Thomas Nicholson, director of the Museum of Natural History in New York. He compares the head of the comet in brightness with the stars in the Little Dipper, coming up in the Little Dipper's favor.
Most of a comet's light is just reflected sunlight; thus, the nearer the comet is to the sun the brighter it is, and the more spectacular its flaming tail. Halley will be brightest in February 1986, but Earthlings won't be able to see it then, because it will be hidden behind the sun.
To make matters worse, in the northern United States the comet will be low on the horizon, and visible for only short periods of time. At 40 degrees north, just south of New York, during the optimum time, the comet's head will rise no more than 10 degrees above the horizon. At 45 N, about the latitude of Toronto, it will be 5 degrees above the horizon -- invisible for all practical purposes.
For a better view, you need to go south of latitude 40 degrees north -- and the farther south the better. Anyplace beyond 20 degrees south is prime viewing area. The farther south you go, the longer and higher the comet will be above the horizon; that means in addition to a better view, you'll have more time to look for it as well.
``That's not to say that nobody in New York will be able to see it,'' says Dr. Nicholson, adding that if he weren't going to be out of the area leading a tour group, ``I'd see it from New York, if the weather cooperated.''
According to Dr. Nicholson, March and April will be good viewing months, with mid-March to mid-April -- particularly the last week in March and the first week in April -- being the best time. (Most of the tours aimed at comet enthusiasts are scheduled for the first week in April.) A ``distressingly bright morning moon'' will interfere somewhat between March 26 and April 2 or 3.
Dr. Nicholson suggests using binoculars, especially for identifying the comet's structure and tail. He recommends 7-by-50 prism binoculars, sometimes called nightglasses, which are on the heavy side for steadier viewing. Also, you'll need a good star map. Halley ``will be within the limit of the naked eye to see it. But even if you're very familiar with the stars you will need to know exactly where to look,'' he says.
Of course, the non-stargazer may be wondering: Why would anyone travel very far to see this pinprick? The answer lies in the historic interest of the comet. British Astronomer Royal Edmund Halley was not the first to sight the comet named for him. It was first recorded in 240 BC and spotted in every one of its 29 subsequent visits. But Halley was the first to recognize that what he had seen was the same comet recorded by earlier observers.
The importance of Halley's discovery is that it confirmed that comets are in orbit around the sun. That's why stargazers -- plus many people who never give the night sky a second thought in the normal course of things -- will want to get as good a look at it as possible.
Organized tours that star the comet are heading mainly for Australia, New Zealand, Chile, Peru, and South Africa. These tours include lectures by scientists, visits to observatories, and overnight stays far from city lights, which would obscure the best view of the comet. ``The nice thing about comets is that the comet will be moving slowly through the sky night after night. If it's cloudy tonight, the comet will be close to the same position the next night,'' says Dr. Nicholson.
If you're thinking of going on a tour, you should first assess your level of interest in comets. In other words, how many hours, on how many nights, will you find Halley a riveting spectacle? Some tours will spend a fair piece of time in a remote place that may not have an abundance of other attractions. Other tours may not focus on the comet as much as the die-hard would like.
Even the most basic ``Halley's Comet Tour'' should offer an overnight stay in a place with little chance of rain, and no light or dust or other pollution. An appropriately credentialed person should be provided to make sure everyone is able to locate the comet and to answer questions. The latter is particularly important as, according to Dr. Nicholson, at the time of year Halley's is most visible, the Southern Hemisphere will offer ``a spectacular sky with many interesting objects'': the Southern Cross, the southern branch of the Milky Way, and the ``Clouds of Magellan'' -- our nearest neighbor galaxies. Tour offerings:
Here is a list of highly reputable tours, some geared for the hard-core enthusiast, others for the average person with a strong interest in comet-viewing:
Sky & Telescope magazine is offering two slightly different tours to see the comet at Ayers Rock in Australia. According to editor Dennis di Cicco, who will be leading one of the trips, this location was chosen not only for its perfect latitude but also for its clear air. One tour keeps you a little longer -- four days instead of two -- in dark skies; both include trips to Sydney and Tahiti. (Two-week trips cost $2,799, plus connecting flight to San Francisco; for more information, call Fresh Pond Travel, 489 Concord Ave., Cambridge, Mass. 02138; 1-800-645-0001; or in Massachusetts,  661-4882.)
Charles Morris, John Bortle, and Daniel Green, who are among the world's foremost observers of comets, will also be leading trips to Australia to the Ayers Rock and Alice Springs area. Mr. Morris is especially excited about Halley's because it's going to be so well placed. ``It will be almost overhead,'' he says. (Right now the two CHASE 86 Tours are $3,300, including air fare from Los Angeles; April 5-19. Call  505-0448, or write Astronomy Tours International, 4311 Overland Ave. Culver City, Calif. 90230.)
Mountain Travel is offering a 10-day trip to Peru, leaving on April 2, which includes four days in the colonial town of Arequipa, whose 8,000-foot elevation and usually clear skies should result in good comet-watching. (The cost is $1,390, plus air fare to Lima. Write 1398 Solano Ave. Albany, Calif. 94706 or see a travel agent).
The Astronomical League, a nonprofit federation of 160 astronomical societies, is also headed for Arequipa. (A basic tour, ``very much devoted to people who want to see the comets,'' costs $547 per person per week double occupancy plus air fare.) Three other tour plans are also available. For brochures and travel information, call 800-821-9010 outside Missouri, 800-892-9035 in Missouri. Scientific questions should be sent to George Ellis, PO Box 330811, Fort Worth, Texas 76163.
During the first week in April, Dr. Nicholson will be lecturing aboard the yacht Illyria for the Museum of Natural History's Discovery Tours, on a four-week cruise that starts in Singapore and ends in Athens. The trip is not billed as a Halley's comet cruise; for one thing, though the timing is perfect, the cruise will never get as far south as 20 degrees S. The view of Halley's will be as good as you can get, but it won't be above the horizon as long as it would be farther south, says Dr. Nicholson. The cruise, an elaborate luxury trip for 100 passengers, lasts from April 4 to to May 7. (It costs $8,995 to $10,245 per person double occupancy, plus air fare; includes many land excursions. Call  873-1440 or write Central Park West at 79th St., New York, N.Y. 10024.)
An impressive roster of experts will be lecturing on Sun Line's Stella Solaris and Stella Oceanis in January, March, and April. Two recently announced cruises that still have plenty of space will sail in March from Fort Lauderdale to the glamorous city of Rio de Janeiro (18 days for $3,450 to $6,600 per person double occupancy).
Royal Viking has eight trans-canal and Pacific sailings between Dec. 19 and April 9 that will offer views of the comet and lectures by astronomers. Carl Sagan will be aboard two sailings. (The 14-day cruises range from $2,744 to $10,022 per person double occupancy; 21-day cruises are $3,864 to $15,351. Prices include air fare.)
For the budget do-it-yourselfer, one New Zealand tour operator, Rainbow Adventure Holidays, is offering a variety of tours it calls ``Skywatch Downunder.'' One 7-day tour ($1,275 per person, double occupancy) includes car, lodging, and air fare from Los Angeles; Another, for 14 days ($1,285 per person) gets you a four-berth van, air fare from Los Angeles, and unlimited mileage. For details contact New Zealand Tourist Office, 10960 Wilshire Blvd., Suite 1530, Los Angeles, Calif. 90024,  477-8241.