Halley's comet is made mostly of ice but its surface appeared boiling hot when it made its closest approach to the sun.
The tail of the comet comes not from a uniform shedding of particles but from intense jets of hot gas and dust that spew from parts of the surface.
The comet's ice is made of water, confirming the "dirty snowball" hypothesis of recent years, but it also contains dry ice, frozen carbon dioxide.
The dirty snowball is not spherical but elliptical, about 9 miles long and 5 to 6 miles wide. It is tumbling slowly, completing a rotation approximately every 53 hours.
Dust particles from the nucleus, the so-called coma, surround the snowball at distances of up to 5,000 miles.
These are among the chief findings of the most ambitious scientific examination of a comet ever undertaken. Five space probes, launched by the Soviet Union, a consortium of European countries and Japan, flew near the comet in March, one within 370 miles, and radioed back to Earth millions of bits of data never before available. Although the comet was a disappointment to millions of casual observers because it was six times farther away than in its previous appearance, the spacecraft more than made up for the increased distance.
Today, 10 weeks after the probes made their closest approaches, the British journal Nature is publishing 35 articles in which scientific teams report their early findings. Such rapid reporting is almost unheard of, especially since the authors come from so many countries.
"This unique exercise in the rapid publication of technical data," the editors of Nature said, "has been made possible by means of an agreement with the three agencies responsible for the five space probes." These were the European Space Agency, the Japanese Institute of Space Science at the University of Tokyo and the Intercosmos program of the Institute for Space Research at the Soviet Academy of Sciences in Moscow.
The United States sent no spacecraft to meet Halley but did conduct observations through Earth-orbiting satellites whose studies are to be published later.
One of the more controversial findings was a Soviet observation that the comet's solid core, or nucleus, is hot enough at its surface to boil water. This poses the mystery of why the icy nucleus did not vaporize long ago.
The answer, the scientists report, seems to be linked to another new observation -- the surface of the nucleus is mostly black, reflecting no sunlight. The visible part of the comet is composed almost entirely of its coma of dust, gas, vapors of water and carbon dioxide.
A team of Soviet, French and Hungarian scientists speculated that, as the original surface of the snowball vaporized, the larger bits of rock and other solids were left behind, forming a layer of rubble.
The dark rubble accounts for the black surface of the nucleus and, the researchers speculate, insulates the icy core from the sun's heat. The heat that does reach the ice melts and vaporizes it, and the vapors seep up through the rubble to escape and form the coma.
The vapor and dust-sized particles that escape with it are then swept back by the solar wind to form a tail that can stretch millions of miles. Solar wind consists of a thin mixture of atoms and subatomic particles continually flying out of the sun. Its force is weak, compared with air wind on Earth, but strong enough to move the comet's particles.
Some scientists dispute the finding of high surface temperatures. Walter Huebner, a comet specialist at Los Alamos, N.M., National Laboratory, said the Soviet sensors might have been fooled by the high temperature of the dust jets from the nucleus. If so, the rubble may be much cooler, though still not below the freezing point of water.
In any case, it appears that the rubble, which is presumed to grow thicker each time the comet passes near the sun, has kept the comet from disintegrating in millions of round trips.
Although this suggests that Halley's comet should be slightly dimmer with each 76-year passage, the spacecraft also discovered that the gas and dust emerged in jets of varying intensity. The jets are generally confined to the side illuminated by the sun, but the slow tumbling exposes different surfaces.
As a result, strong jets may appear only when surface regions having thin or unusually porous rubble face the sun. This would explain the sudden changes in brightness.
Analysis of the dust particles suggests they are similar to a form of meteorite called a carbonaceous chondrite. Unlike meteorites made chiefly of silicate-based stone or metals, these are rich in carbon, hydrogen, nitrogen and oxygen.
The findings confirm a theory that comets formed early in the history of the solar system from a swirling cloud of gas and dust. Close to the sun, the heavier elements condensed into planets. But on the colder outskirts, lighter elements condensed into comets.