Halley's comet is speeding through the orbit of Saturn on the inward leg of its 76-year circuit through the solar system. This time, Earth astronomers are ready for it.
They are preparing the largest astronomical project ever organized to view a celestial visitor.
Spacecraft from Asia, Europe, and the Soviet Union will greet the comet. They are designed to collect specimens, photographs, and other data while the comet is nearest Earth in late 1985 and early '86. In addition, an international network of 50 observatories and some 100 astronomers will observe it from sites in India, Indonesia, Argentina, Brazil, Chile, and Australia.
A meeting of this ''International Halley Watch'' is scheduled for the last week of August at Patras, Greece, to plan observing strategies. Specialized branches of astronomy have formed teams to make detailed observations of the comet's tail, nucleus, orbit, and brightness. They will use conventional optical telescopes as well as infrared and radio techniques. Their work must be scheduled and coordinated as an integrated research program.
One of the most important goals of the international effort will be to study the comet's exact chemical makeup using spectroscopic analysis. Dr. Peter Wehinger and his wife, Dr. Susan Wyckoff, of Arizona State University have contracts with the US National Aeronautics and Space Administration to head the spectroscopic team.
Spectroscopy is a method of determining chemical composition by dispersing the light coming from a bright object into a spectrum of different wavelengths. Each chemical constituent of that object produces a characteristic pattern of lines of specific wavelengths in the spectrum -- a pattern that is as individual as a person's fingerprints. Since no two chemicals produce the same bands of lines, scientists can determine the comet's composition by analyzing those spectroscopic patterns.
''The key information we are looking for is . . . the exact proportion of water-ice in the nucleus,'' Dr. Wyckoff explains.
Comets had long been suspected of containing water. In 1950, Fred Whipple first speculated that the nucleus, or head, of a comet is composed of ice and dust -- the ''dirty snowball'' model. But his theory could not be verified then.
Drs. Wehinger and Wyckoff provided the confirmation in 1973 while observing the comet Kohoutek from an observatory in Israel. They made a spectrogram that included the ''fingerprint'' of water. It was the first hard evidence that H2O was indeed present.
It is this water-ice that gives comets their grandeur. When a comet nears the sun, the ice melts and forms a cloud surrounding the nucleus and trailing back in the ''tail.'' Light striking the tail makes water molecules glow.
Because comets move through the sky and belie the static, unchanging nature of the heavens, they have been viewed with awe and trepidation. The ancient Chinese described them as ''flowing stars'' and considered them signs of heavenly displeasure with the emperor. Rome saw the presence of Halley's comet in 43 BC as representing a divine chariot drawing the recently assassinated Julius Caesar to his reward. The Anglo-Saxons were so impressed by the ''hairy star'' in 1066 that they considered it a bad omen for their king Harold, who was later defeated by William the Conqueror.
When science succeeded fear and superstition, comets were studied as part of nature rather than as manifestations of the supernatural. Edmund Halley was the first to see an underlying pattern in the appearances of the most spectacular comet. In 1705, he predicted it would return by 1758-59. Halley died before the comet reappeared. Scientists named the comet after him, however, in recognition of his achievement.
For the comet's visit in 1910, astronomers around the world tried to coordinate their observations. But poor communications and primitive equipment hampered their efforts. Although some spectacular photographs were taken, there were no earthshaking discoveries.
Meanwhile, the announcement that Earth would pass through the tail of the comet had aroused worldwide concern, because the tail contains cyanogen, a derivative of cyanide. There was needless fear that Earth's atmosphere would be poisoned and life would end. But the world did not end in 1910. The tail of a comet is too diffuse to pose any threat, no matter what chemical may be present, Dr. Wyckoff says.
Rather than presenting a danger to earthly life, Halley's comet may offer some clue to life's origin.
Besides water, comets are composed of the same interstellar dust and gas that formed the sun and planets. In fact, comets may hold clues to the original makeup of the primordial solar nebula. Also, they may help scientists learn what compounds are common throughout space.
Among the compounds that make up comets are water, ammonia, carbon dioxide, carbon monoxide, and even carbon itself -- all necessary compounds for life, Dr. Wyckoff explains. If those compounds are common in space, it could indicate that the basic chemicals necessary for organic life are not restricted to Earth.
''Life could be rampant throughout the galaxy,'' she says. One theory even holds that comets may have ''fertilized'' Earth with the primitive chemical precursors to life. But Dr. Wyckoff says she thinks such a mechanism was not necessary for life to have evolved on our planet.
Astronomers now have equipment of which scientists in 1910 scarcely dreamed. One of the most exciting aspects of the Halley project will be the use of telescopes to gather data, Dr. Wehinger says.
The United States had planned to launch a 100-inch telescope in late 1985 to observe the comet. Budget cuts halted that project. In its place, NASA will orbit three smaller telescopes on the space shuttle during missions in November 1985 and again in March 1986. These will study the comet when it will be brightest.
In addition, the USSR will launch a Halley's probe carrying both Soviet and French equipment. The European Space Agency and Japan will also send comet-observing spacecraft. These three probes will carry cameras and automated laboratories to take close-up photographs and analyze samples of the comet's tail.
The amount of data is expected to be enormous. The German center for European and Asian observations will use a communications satellite to transmit weekly results to the worldwide headquarters of the Halley Watch at the NASA Jet Propulsion Laboratory in Pasadena, Calif. JPL will redistribute that information to participating observatories in America.
Coordination of the network will be a major logistics feat. One purpose of the International Halley Watch meeting in Patras is to plan this coordination in detail, in an attempt to anticipate potential problems. Then the network and communications systems will be tested in 1984 when the comet Enke passes Earth, Dr. Wehinger says, unless budget limitations prevent it.