Return to Uncle Buzzer Home Page -- Read About Comets on a String
In just about every bad space-rock-hits-the-planet movie, the writers decided they needed something exciting to happen in the early going -- so they have a couple of little fragments from the big rock come crashing into Earth a week or a month beforehand. And, of course, they always hit the Empire State Building or the Eiffel Tower or something. They never hit in the middle of the desert or the jungle or the ocean, because that wouldn't be cool enough.
That kind of impact is not going to happen. We need to explain a little about orbits to see why.
Any space rock that crosses Earth's orbit will be moving on its own path around the Sun, its own orbit. If that rock's orbit crosses the Earth's orbit, that doesn't mean it's going to hit the Earth. Why not? Because the Earth has to be moving through that point in its own orbit at the same moment as the rock. If some other rock wants to hit the Earth, obviously it too will have to cross Earth's orbit just as Earth is moving through that position.
But if a rock is going to do that a week or a month earlier than the main object, so as to make for good drama in the movie, it will have to be in a very different orbit -- different enough that the first rock to hit can't possibly have anything to do with the second rock.
Let's look at this another way. Let's start with the assumption that a main asteroid or comet (let's call it Big Rock) has other rocks (we'll call them Little Rocks) sharing its orbit. There are two way that can happen: Little Rocks could be orbiting Big Rock, or else they could simply be sharing Big Rock's orbit. In other words, the Little Rocks would be moving around the Sun in the same path, but trailing or leading Big Rock by thousands or even millions of kilometers. It's been very well established that lots of asteroids do have their own little teeny moons. However, the vast majority of asteroids just aren't big or massive enough to hold a moon in orbit at any distance of more than about 1500 kilometers.
As it happens, fifteen kilometers a second (KPS) is a pretty fair guess at how fast an asteroid would be going when it hit the Earth. It could be going twice that fast, but let's stick with fifteen KPS. It's the estimated speed of the asteroid that might have killed off the dinosaurs.
So let's assume that our moonlet, Little Rock, is exactly lined up between Earth and Big Rock just as Big Rock is about to collide with our planet. Divide fifteen hundred kilometers by 15 kilometers per second. Even if everything lined up perfectly, it would only impact about 100 seconds before the larger body.
In other words, unless one minute and forty seconds is enough time to establish dramatic foreshadowing, the Litte Rock orbiting Big Rock approach won't work.
A better (but not good) way to get lots of rocks crashing into Earth before the main body hits is to have the space rock be a comet.
Comets are basically giant dirty snowballs -- big lumps of ice with rocks and dust mixed into them. Most comets never come near the inner Solar System, and they have stayed frozen solid for the last four billion years or so.
However, some comets do get bumped out of their remote orbits, one way or another. Some are just kicked loose to sail out into the empty space between the stars. Others fall into orbit that take them toward the inner Solar System, where we live. Some comet pick up so much speed as they fall past the Sun that they are thrown clear of the Solar System forever after a single pass through the inner Solar System.
Others settle into what's called (surprise!) a cometary orbit -- a very long, thin, stretched-out oval, with one end of the oval, or ellipse, wrapped in close around the Sun, and the other end reaching far out into space -- usually well past the orbits of Neptune and Pluto. The Earth takes one year to complete one orbit around the sun. Comets can take decades, or centuries, or even longer.
The comet will spend nearly all its time moving very slowly through the part of its orbit farthest from the Sun, but every time it swings back around toward near the Sun, it will move faster and faster. The Sun will start heating it up -- and its surface will start to evaporate.
The sun continues to heat the lumpy, tumbling comet as it gets closer to the inner Solar System. The rough surface of jumbled-together material gets uneven heating that creates cracks and pockets of gas. Big chunks of stuff can be blown off into space in just about any direction.
Most of these big chunks won't get thrown very far, or with all that much energy. They won't get shot across the Solar System -- just pushed away at low speed. They'll bang into other chunks and particles that have kicked loose.
These fragments will still share the same basic orbit as the main body. They'll just be moving a bit slower or a bit faster, or in very slightly different directions. As time goes by, most of these pieces -- most of them smaller that sand grains, some of them fist-sized, and maybe a very few the size of a house -- will slowly drift farther and farther away from the comet.
As the comet whips around the Sun and then heads back off into the outer Solar System, it cools down, and re-freezes, as do all the bits and chunks that have broken off from it. This process is repeated every time the comet gets close to the Sun. Every time it does, more of the comet is boiled away or blown off.
These melted-off and ejected bits and pieces of rock, ice, dust, and dirt are gradually dispersed over the whole length of the comet's orbital path. After a few thousand or million years, the comet itself is completely evaporated away, leaving nothing behind but the rocks and dust. The entire orbital path is strewn with comet debris, a cloud of dust shaped like a badly-stretched-out doughnut, an oval or ellipse billions of kilometers in circumference and many thousands of kilometers across its cross-section.
Earth's orbit intersects the orbits of various former comets on a regular basis, ploughing right into these clouds of dust and rock. When it does, we get one of the various annual meteor showers -- a night or two of streaks of light moving across the sky.
If, if, if our imaginary Big Rock were a comet that had come in close to the Sun enough times to slough off enough dust and junk to make a good meteor shower, but not enough times so that it had evaporated altogether, we might get a few pretty meteors a few hours before the Big Rock. However, the odds would be very much against Earth piling into a chunk large enough to do us any damage in anything longer than a few hours before the main event at the very outside. Because Earth would still be moving along its orbit after the first impact, it would be almost impossible to work out how the Big Rock comet could also hit Earth if it was in a similar but slightly different orbit, but days or weeks behind the Little Rock.
To put it yet another way, in very round numbers, the Earth is about 13,000 kilometers in diameter, and moves along its orbit at about 30 kilometers a second. Do the math, and you can see that it take less than eight minutes for Earth to move completely through any given point in its orbit. Two objects traveling one behind the other on essentially the same orbit couldn't be more than about eight minutes apart and both hit the planet.
It's certainly possible for two objects to hit the Earth a few weeks apart. (Pebble-sized meteors and dust particles hit the Earth by the thousands every day.) But the odds against the two objects being related to each other in any way would be huge.
Want to learn about the time in 1994 when more than 20 comets hit in less than a week? Want to see why that happening doesn't contradict everything you've just read? Click here!
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