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Comets: From Doomsayers to Delights (Part 2)
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In our previous episode we saw how comets have played a significant cultural role for millennia, usually by scaring people half to death. Only a couple centuries ago was it realized that they were not messengers from the gods, but mere solar system bodies, kin to planets and asteroids. But what exactly were they, where were they from, and why did they have that funny tail? Let's take a look back in the way distant past to find out.
A long time ago a star was born. Around baby Sol was a colossal cloudy disk of scattered building material: rocks, gases, and ices of all kinds stretching out for billions of miles. In some areas in the disk --- eddies in our great swirling whirlpool --- planets were created as gravity swept together some of the dust and gases.
Planets near the hot new sun could only form from rocky material. Far away where it was much colder the newly forming outer planets could use ices and gases as well as the rocky stuff for bulking up their mass.
Way off in the distance, beyond Neptune's orbit, the debris was spread so thin it couldn't get it together to form planets. The original building blocks of the solar system remain there untouched even today in what is called the Kuiper Belt.
Some of the paraphernalia that was being collected into planets were actually evicted. Planets, because of their great gravity, can actually fling poor passing icy rocks right out of the solar system. Much thrown off debris resides in an unimaginably vast and very distant sphere around the sun called the Oort Cloud. This giant nebula of tenuous dirty ice extends nearly a third of the way to the nearest star.
Our comets come from these two reservoirs. Short-period comets, those that come and go quickly and can return in just dozens of years probably come from the Kuiper Belt. The long-period comets, some of which pass through our neighborhood never to return, are from the Oort Cloud.
So how do they get here? Why rouses them to leave their sleepy, dark, distant abode?
Occasionally a few of these ice rocks collide with each
other and get knocked towards the sun. Or, possibly, out in the Oort
Cloud, a star may pass close by disrupting the orbits of some of these
critters and in they fall.
What happens next is what most of us know as the traditional comet, tail and
all.
As this "dirty snowball" makes its way into the solar system proper, it begins to warm. By the time it reaches about the orbit of Mars, some of the so-called volatiles --- substances that can easily evaporate --- start to turn from solid to gas.
This doesn't bode well for the comet's constitution. As the volatiles, including water ice, boil off they carry away some of the dirt as well. The escaping gases and dirt expand out for millions of miles. This would be nothing much more than a vast smoggy sort of atmosphere if it weren't for that sun!
The sun spews off photons of light, of course, but also atomic particles in a phenomenon called the solar wind. Both the light and the wind work together to blow the comet's atmosphere away. This, my friends, produces the tail of the comet.
Consequently the tail, which gives the illusion that the comet is racing through the night sky, is always pointing away from the sun. Even as the comet steers round the sun and back towards the outer worlds, the tail points away. As a result, the tail can actually be out in front of the comet on its trip outward.
Unfortunately, all comets do not perform as advertised. Some have little ice left after coming by so often, so they have little tail material and are rather unspectacular. Some, being loosely held together, actually split up into pieces as they head around the sun.
What will our two visitor comets be like by the end of the month, our best chance to see them? Will they be Hale-Bopps or Big Flops. We'll wait, hope, and see.
Next time: Who these two visitors are, and the fate that awaits comets ...
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2004.05. 2 12:36 PM | Comments (0)
