It’s interesting how some people choose to look down, to delve deep into the earth for precious metals and stones. These people have often dug down for their homes. If we want to get poetical about it, there are folks whose necks are bowed down to scour the ground for treasure or trash – muckrakers, as Teddy Roosevelt famously called them. They contribute to the rich tapestry of life, but it’s hard not to feel sorry for them sometimes. Because . . .
Then there are the stargazers. The men and women and children who spend their lives looking up into the heavens. Looking for portents and wonders. Who hasn’t spent a summer evening laying on the grass looking up into the starry skies, wondering what it was all about? Do that enough and you’ll see a shooting star. Or maybe a comet coursing through the heavens.
Scientists tell us that a comet is a leftover chunk from the very beginnings of the universe. They are not space debris like an asteroid, or formed from interstellar dust – nor have they been blown off of some exoplanet way out there. They are part of the original fabric of the universe. And as such, modern researchers are eager to get samples of them to analyze and theorize about. Astrologers of old were also very much interested in comets. But not for scientific purposes. They wanted to forecast the future from the bright comets they could see in the sky with their naked eye. After all, there’s always been big money for those willing to convince the rich and powerful that a bright old blob in the sky means they’re going to get more rich and more powerful. In 1066 William the Conqueror was told that Halley’s Comet, which happened to be passing by at the time, was his lucky rabbit’s foot. And it was – he conquered most of England that same year.
Comets contain a bit of rock and metal, but by far the most common elements of a comet are frozen water, along with congealed carbon monoxide, carbon dioxide, ammonia, and methane. Astronomers often refer to comets as dirty snowballs. But they are snowballs with a purpose, so to speak. Unlike asteroid and meteors, which have no set course and just go meandering and crashing through the universe, every comet has its set trajectory. A closed loop. So that mathematicians, once they know the elliptical curve of the dirty snowball, can predict where it will be, and when. That’s why Halley’s comet is so well known. A British astronomer named Halley figured out its course nearly two hundred years ago. Mark Twain wanted to go out with Halley’s comet, since he was born in 1835 when it flew past Earth, and so he arranged his affairs to die peacefully in bed in 1910 when the comet was again scheduled to whiz past.
The so-called tail of a comet is caused by the sublimation of frozen gases in the comet whenever it passes close by a star such as our Sun. As the gases warm up they revert to their gaseous state and trail for thousands of miles behind the hurtling comet. In deep space comets have no tail at all, and so are much harder to track. Astronomers with a sense of humor thus have jokingly labeled comets as ‘hairy stars’ because of the cloud of gases formed around them when they heat up. Which cloud envelopes the comet in a fuzzy smog. But if you want to get technical about it, the Latin name for the smoggy atmosphere that surrounds comets near the Sun is coma. Which is Latin for hair. Never forget that if you want to make something sound important and mysterious, just slap on a Latin or old Greek name.
Scientists learned that trick long ago.
The reason a comet’s tail becomes so bright and visible is that as the sublimated gases and dust leave the comet they are charged by the solar winds the Sun constantly throws out. This is the same phenomena that illuminates the Northern and Southern lights.
Over the years writers have speculated on what would happen if our planet passed through the tail of a comet as it went past the Sun. They had great fun imagining how these weird interstellar gases and powders would alter the atmosphere – sometimes poisoning us and sometimes causing us to suddenly grow super smart and sprout a third eye on the forehead. Well, the Earth actually DID pass through the tail of Halley’s comet in 1910, and it was a big flop. Even though scientists were able to detect minute amounts of cyanogen, a deadly gas, in the comet’s tail. Nobody developed super powers and nobody keeled over dead. So don’t lose any sleep over that particular topic the next time it goes viral on the internet.
Since comets lose mass whenever they’re near our Sun or any other star, how is it they keep going and don’t just dwindle away?
There are two parts to that answer.
First, most comets are in no hurry when it comes to their elliptical orbit. It may take them millennia to complete one complete circuit. So they only start to disintegrate once every hundred-thousand years.
The second reason is that while they’re out in the freezing reaches of deep space they may be able to pick up some dust and gases, since they do posses an admittedly weak gravity.