Astronomers preparing to probe Halley's Comet in 1985 have been taken by surprise. Suddenly, they have a comet on their doorstep to study right now. Unknown until late April, it should pass within 2.9 million miles of Earth around 8 a.m. Eastern daylight time Wednesday. That's the closest any comet has come since Lexell passed within 2.1 million miles in July, 1770.
What's more, instead of the handful of instruments that will be packed into Halley-probing spacecraft, cometary scientists can work with the giant telescopes, powerful radars, and other extensive astronomical equipment on Earth.
Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics says the comet has appeared too suddenly for astronomers to tool up properly. But many of them around the world are scrambling to make the most of this unexpected research opportunity.
''The important thing,'' Dr. Marsden says, ''is to make some study of the nucleus. Something like the icy nucleus is there, but we have never seen it.''
He was referring to the ''dirty snowball'' theory of comets. The nucleus is actually the main part of a comet. The brilliant head and sweeping tail that symbolize comets really are gases and dust boiled off the nucleus as it nears the sun. Over three decades ago, Fred Whipple, professor emeritus at Harvard University, proposed that a comet nucleus consists mainly of ice laced with dust - that is, a dirty snowball.
This concept of the nucleus is strongly supported by what astronomers have learned about comets. But comet nuclei are very compact. Typically, their diameters range from a few hundred feet to a few miles. They are far too small to be seen from Earth at the distances they usually pass.
This time, however, the comet is close enough for many kinds of instruments to make detailed measurements of the nucleus, including the detectors on the Infrared Astronomical Satellite (IRAS) - an international orbiting observatory that records the infrared (heat) energy radiated or reflected by astronomical objects. Among other things, astronomers should, for the first time, be able to determine directly the chemical composition of a comet through spectroscopic measurements. They might even be able to photograph the nucleus.
But they will have to work with great care, warns Steven Edberg, an astronomer at the NASA-Jet Propulsion Laboratory (JPL). The comet is moving rapidly relative to Earth; it moves across the sky at two degrees (four times the diameter of the full moon) per hour. It is hard to keep instruments trained steadily on such a fast-moving object, Dr. Edberg says.
JPL called attention to the comet after it was recognized in data from the IRSA. The comet was also discovered and photographed by Genichi Araki in Japan and George Alcock in England. It now is known as Comet IRAS-Araki-Alcock.
Comets are lightweights as solar system objects go. Although their masses have not been determined directly, they are estimated to range from 10 million tons to 10,000 billion tons. That compares to about 6,000 billion billion tons for Earth. At this writing, the new comet's size was not known. But it appears to be a small, standard comet, well within this mass range.
Astronomers believe comets have undergone little change during their 5 billion-year history. They are thought to be the purest samples now available of the primordial nebula from which the solar system formed. This is a major reason why the US National Academy of Sciences has designated comets as a priority target for space exploration in the 1980's.
So far, two comets have been selected - Halley's and Giacobini-Zinner. The European Space Agency is sending a probe to within a few hundred miles of Halley's nucleus. The Soviet Union is sending two probes within 6,300 miles of the nucleus. And Japan plans to send a probe that will pass ten times farther away.
The US, which has no Halley's probe, has diverted the Sun-observing satellite ISEE-3 to pass through the tail of comet Giacobini-Zinner, within about 1,900 miles of the nucleus, during a 100-day observing period in 1985. It will also make Halley observations, but at distances of about 86 million and 19 million miles during two observing periods in 1985 and 1986.
Thus, the spacecraft probes will still be the preferred means of getting comet data.