If theoretical scientists are correct, comets are the frozen remnants of the material that formed the sun and the planets -- ice, gases and rock.
They are probably the most primitive objects in the solar system and hold the clearest record of the chemical and physical processes that created the sun and planets 4.6 billion years ago.
Astronomers speculate that billions or trillions of comets reside billions or trillions of miles beyond the planets as a gigantic swarm known as Oort's Cloud. Only a few journey to the inner solar system and even fewer are captured into orbit to return again and again as does Comet Halley (it rhymes with valley).
The baffling question: How did Oort's Cloud get where it is?
''I've had many theories myself, but I don't believe any of them now,'' said Harvard astronomer A.G.W. Cameron, an expert on the origin of the solar system.
Astronomer Jan Oort proposed in 1950 that a sphere of ''comet-stuff'' surrounded the solar system about 6 trillion miles away.
As astronomers work backward to determine the orbit of a comet that wonders in the vicinity of the sun, it becomes clear that the greatest distances are between 50 and 150,000 astronomical units of about 93 million miles each.
This would put the outer edge of Oort's Cloud halfway to the nearest star. The cloud is assumed to be a sphere, not a band like the asteroid belt between Mars and Jupiter, and the comets hurtle inward and in very eccentric orbits.
Theoretically, about every 10 million years a random star in our region of the galaxy will pass near or through this cloud of comets and set off a major disturbance.
Some of the comets are torn away and leave the solar system. Others fall toward the sun in parabolic orbits, pass just once around it, and head back into the far regions of space never to be seen again.
Other comets plunge directly into the sun, also never to be seen again.
Others are pulled into elliptical orbits by the gravity of the larger planets.
To account for the number of known long-period comets, Oort's Cloud must contain at least a billion to a trillion comets.
Astronomer Fred Whipple of the Smithsonian Astrophysical Observatory proposed in 1950 that comets are made of a nucleus of chunks of cosmic dust and the ices of water, carbon dioxide, ammonia and methane.
As the sun evaporates the ices, changing them from solids directly to gases, the imbedded cosmic dust escapes into the surrounding space and shines by reflected sunlight. This is the visible coma.
A comet has a dust tail and a plasma tail, which forms when atoms in the coma become ionized and the ions are swept back along with the dust by the solar wind.
For larger comets, the visible tail can grow to a length of 100 million miles, but the solid matter of the entire tail could fit in a good-sized suitcase.
The closer a comet gets to the sun, the more mass it loses. The more mass it has to begin with, the more returns it can make.
''At an approximate age of 16,000 years -- its lifetime in the solar system -- Comet Halley probably weighs about 100 billion tons,'' said Gail Cleere of the U.S. Naval Observatory.
In three hours of pioneering exploration in September, NASA's ICE spacecraft gave scientists a new perspective on comets, revealing that some, at least, are anything but benign.
ICE's encounter with Comet Giacobini-Zinner showed that they are turbulent masses that greatly disturb their environment.
''The appearance of the comet, from our point of view, is that it is the source of energetic particles and it's a place where all sorts of particle acceleration occurs,'' said Fred Scarf, a space physicist with TRW Inc.
''This is something that I think is completely unexpected.''
ICE also discovered that the dust around the comet was less dense than some expected, and its tail was three times thicker than predicted.
A fleet of spacecraft will conduct far more extensive investigations of Halley's comet next year.