All About Comets

For thousands of years comets have fascinated humanity. They appear suddenly and brilliantly, trailing their tails across the sky like a banner of light, and then disappear as quickly as they had arrived. Ancient people considered them ill omens and signs of poor luck, and as recently as 1910 people believed they could be poisoned by gas brought by the comet. There is something admittedly frightening about their somewhat eerie appearance – against the night sky, they look almost like blazing specters. So, then, what exactly are comets? Where do they come from, and where do they return to?

The ‘core' of the comet is, in truth, fairly small. It's thought that cores are at the maximum four miles across, and one mile is considered a more reasonable estimate. The core is most likely a ball of frozen ice and rocks, sort of like a particularly grubby snowball. For the most part it remains in this inert state when left alone. However, when a comet comes near the sun, the ice on the surface of the core melts into gases that form a cloud around the comet known as the coma. This cloud is ionized by the sun, causing the bright colors.

The tails of the comet form when the solar wind pushes the gases that make the coma out behind the comet, forming the long, flaring gas tail. The other fainter tail that some comets form comes from the solid debris that's been freed from the melting core, which turns into a just-visible ‘dust' tail. This curves because the debris is pushed at different rates by the wind, forming a lagging curve.

Thus, when the comets are close enough to appear in our skies they appear as brilliantly glowing points trailed by two tails – one almost as vivid and one ghostly and faint. They can often be quite bright, even to the point of being visible in daylight, and produce a stunning visual effect. It is easy to see why people of the ancient world were fascinated by them.

Comets come in two varieties – periodic and non-periodic. The comets we see in our skies are ones on extremely elongated elliptical orbits that go from the furthest reaches of the solar system to distances close enough to the sun to cause their tails to form. Periodic comets complete this orbit on a scale we can recognize, such as Halley's Comet, which arrives every 75 to 80 years. Non-periodic comets most likely do have a period, but humanity has only been able to witness one orbit due to the extreme length of their orbit. It might well be thousands of years. Most of the periodic comets have most likely become ensnared by the gravity of one planet or another, shortening their orbit as a result.

The passing of a comet is a brilliant, beautiful event, and some comets like Halley's Comet are true once-in-a-lifetime experience. When the next comet comes close enough to be seen in our skies, make sure you go out and look – the sight of that brilliant streak across the night sky will remain with you forever. Far from being an ill omen, comets are instead a miracle of nature.

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