University Lowbrow Astronomers

The Lives of the Comets.

by Kurt Hillig
Printed in Reflections:  February, 1997.

Just what are comets, anyway?  Where did they come from?  Why do they have such weird orbits?  What are they made from?  How may comets are there?  What do they mean?

These and many other questions have puzzled astronomers, astrologers, and just plain folks for thousands of years.  The recorded history of comets goes back nearly as far as recorded history itself.  In pre-telescope days, comets were often thought to be messengers from the Gods, or omens of doom.  (This is due to the “full moon effect” - the claim by many delivery room nurses that there are more babies born when the moon is full than at other times.  The real reason for this is that people are more likely to remember events that are accompanied by another dramatic event.  So the appearance of Halley’s comet over the Battle of Hastings in 1066 “confirms” that comets are associated with catastrophe.)

To the ancient Greeks, comets had a less mystical explanation:  they were exhalations from the Earth, or perhaps hairy, wandering stars (the word “comet” derives from the word for hair, just as “planet” comes from the word for wanderer).  The natural and supernatural interpretations clashed - at least in western cultures - until the Renaissance, and the invention of the telescope (and, indeed, long after).  With Newton and the mathematics of gravity, comets moved completely into the world of the natural - although we still find them to be rather extraordinary sights to see!

But are they extraordinary?  Just what are they, really?  To answer this, we need to look back in time - not to the beginning of recorded history, but to the darkness before the dawn of the solar system.  Set the Wayback Machine for five billion BC, Sherman!

Before the Sun was born, we were a cloud.  A big cloud, tens of light-years across, with the mass of a thousand Suns, filled with dust and gas, an organic soup of molecules as tenuous as the best laboratory vacuums today.  Something like M8, or M42, in other words!  Shock waves from nearby supernovae rippled through it, compressing it; and where the density rose, the self-gravity of that knot of matter drew in more material, and it started to condense.  After a few tens or hundreds of millions of years, the proto-Solar System was a dense vortex a few thousand AU across.

In the center, a star was beginning to form.  Further out, smaller eddys were condensing to form bodies anywhere from a millimeter to tens of thousands of kilometers across.  Heat from the evolving proto-star kept the volatile materials - hydrogen, ammonia, carbon monoxide, etc., from condensing in the inner regions; but at the outer edges, shielded from the inner heat by the dense cloud, the comets grew.  At temperatures only a few degrees above absolute zero, they slowly grew; dry-ice hailstones at 15°K.

In the inner part of the nebula, the star caught fire, and the innermost clumps were baked dry; but the outer planets swept up the escaping gas, and grew, and played catch with the millions of smaller bodies that littered the system.  Over the next billion years or so, the inner planets were bombarded with rocks, and with the comets that came in too close; something big hit Uranus-to-be, hard enough to knock it on its side; something the size of Mars struck the proto-Earth, breaking loose what would someday be called Luna.  Another billion years, and something big brushed past Neptune, flinging its moons across the sky; two of the escapees formed a partnership, calling themselves Pluto and Charon.

Meanwhile, back at the 1000 AU ranch we call the Oort Cloud, the primordial comets slowly wove their way thorough space.  Barely feeling the tug of the Sun, they creep along like snails; too far out to have been caught in the vortex, they move in all directions.  Every few million years, a wandering planet, flung out aeons ago from some stellar system (not necessarily ours), or perhaps a wandering star, comes close enough to wake them from their sleep, and send a few drifting in towards us; and after another few thousand years, some make it close enough for us to see.

It is estimated that there are as many comets orbiting our Sun as there are stars in the Milky Way; and they range in size from a few tens of meters to perhaps a hundred kilometers or so.  Some are more rock, some are mostly gas; some are captured when they come in, and forced into short-period orbits where they evaporate quickly, leaving trails of dust that form our brightest meteor showers; some are flung out to travel forever across the universe.  Some fall straight into the Sun, or into Jupiter; many have hit the Earth (it is thought by some that most of our water and atmosphere came from comets).

Some, like Hyakutake and Hale-Bopp, come by only rarely, with periods of thousands, tens of thousands of years.  Like dirty snowballs that spend most of their time in a deep freeze, they still retain much of the volatile gas that they started with; and when the geometry is right, they make close passes by both the Sun and the Earth, yielding spectacular sights.  Surrounded by a swarm of pebbles, the smaller grains and dust trail out along the orbital path to yield a tail yellow from reflected sunlight.  The evolving gas, driven by the solar wind, ionized by solar ultraviolet, and warped by the solar magnetic fields, provides a blue-green fluorescence that can glow for a hundred million miles.  Comet quakes calve off icebergs, exposing fresh surfaces that erupt in towering jets.

And we on Earth - a species which has existed for a mere fraction of the lifetime of the comets - we stand outside at night in awe and in silence, and we dream, and wonder....


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