enters our atmosphere, its velocity is translated into energy, which in turn is transferred to the air around it. As it screams through the upper atmosphere, a meteoroid rams the air violently—a rock moving at Mach 50 is going to compress the air a lot. The air gets squeezed so quickly and at such high pressure that it heats up thousands of degrees and starts to glow.
As you can imagine, all that hot air is like a blast furnace. The meteoroid, traveling just a few inches behind that rammed air, feels that heat. It can’t last long in those conditions, and if it’s small it usually burns up in a matter of seconds. We see a bright glow, a streak across the sky that lasts for a moment or two, and then it’s gone, adding its nearly insignificant mass to the Earth’s.
To a stunned observer, a meteor looks like it’s traveling just over his head, but in reality the action is occurring fifty or more miles above the ground. At that height the air is very thin, yet still thick enough to stop small, dense particles. But what if the particle is bigger than a pea, or a grape, or a watermelon? What if it’s the size of, say, a couch, a car, a bus?
For a bigger object, things are very different. If it’s a few yards across, instead of simply burning up, that chunk of space debris gets squeezed by the air pressure as if it’s in a vise—the pressure can top out at over a thousand pounds per square inch at meteoric speeds. This pressure can flatten out the incoming object in a process called pancaking for obvious reasons. But a rock can only take so much of that before it crumbles and falls apart. Within seconds, instead of one big rock coming in, we now have hundreds or thousands of little ones, all still moving at velocities of several miles per second, and all dumping their energy into the air around them. They compress further, fracture, heat up, and so on . . . and within a fraction of a second we have a whole lot of rubble releasing a whole lot of heat all at once.
This is, by definition, an explosion.
So medium-sized meteoroids blow up in the atmosphere. Again, this usually happens fairly high up, depending on how tough the meteoroid is; ones made of metal can take more punishment and penetrate deeper into our atmosphere, but may still explode many miles above the Earth’s surface. The energy involved is impressive: a rock only a meter across can explode with the force of hundreds of tons of TNT. In fact, military records indicate that such an explosion from an incoming chunk of rock is seen on average once a month!
Since meteoroids explode so high up in the atmosphere, you’d expect we’d be safe from things that size.
Well, not exactly. Under some conditions, the incoming rock may break up, but some chunks can survive. If the main mass slows enough before it explodes, then smaller fragments can slow even more without totally disintegrating. These can make it all the way to the ground. Metallic meteoroids have even more structural strength and can remain intact all the way down as well. If they do survive and impact the ground, they’re called meteorites. 1
Small meteoroids that make it down to the ground usually aren’t moving terribly fast when they impact. In fact, their initial velocity is completely nullified by our air, leaving them to fall at what is called terminal velocity. It’s as if they were dropped off a tall building or from a balloon; they wind up impacting at maybe one or two hundred miles per hour. Scary, sure, but not too scary.
Still, you wouldn’t want to get hit by a rock moving that fast. For comparison, they hit the ground faster than even a professional baseball player’s pitch. In November 1954, a woman named Ann Hodges from Sylacauga, Alabama, was actually hit by meteorite. It was fairly small, about the size of a brick and weighing just over eight pounds. It punched through her roof, bounced off a wooden radio cabinet, and smacked into her where she was lying on the couch, taking
As you can imagine, all that hot air is like a blast furnace. The meteoroid, traveling just a few inches behind that rammed air, feels that heat. It can’t last long in those conditions, and if it’s small it usually burns up in a matter of seconds. We see a bright glow, a streak across the sky that lasts for a moment or two, and then it’s gone, adding its nearly insignificant mass to the Earth’s.
To a stunned observer, a meteor looks like it’s traveling just over his head, but in reality the action is occurring fifty or more miles above the ground. At that height the air is very thin, yet still thick enough to stop small, dense particles. But what if the particle is bigger than a pea, or a grape, or a watermelon? What if it’s the size of, say, a couch, a car, a bus?
For a bigger object, things are very different. If it’s a few yards across, instead of simply burning up, that chunk of space debris gets squeezed by the air pressure as if it’s in a vise—the pressure can top out at over a thousand pounds per square inch at meteoric speeds. This pressure can flatten out the incoming object in a process called pancaking for obvious reasons. But a rock can only take so much of that before it crumbles and falls apart. Within seconds, instead of one big rock coming in, we now have hundreds or thousands of little ones, all still moving at velocities of several miles per second, and all dumping their energy into the air around them. They compress further, fracture, heat up, and so on . . . and within a fraction of a second we have a whole lot of rubble releasing a whole lot of heat all at once.
This is, by definition, an explosion.
So medium-sized meteoroids blow up in the atmosphere. Again, this usually happens fairly high up, depending on how tough the meteoroid is; ones made of metal can take more punishment and penetrate deeper into our atmosphere, but may still explode many miles above the Earth’s surface. The energy involved is impressive: a rock only a meter across can explode with the force of hundreds of tons of TNT. In fact, military records indicate that such an explosion from an incoming chunk of rock is seen on average once a month!
Since meteoroids explode so high up in the atmosphere, you’d expect we’d be safe from things that size.
Well, not exactly. Under some conditions, the incoming rock may break up, but some chunks can survive. If the main mass slows enough before it explodes, then smaller fragments can slow even more without totally disintegrating. These can make it all the way to the ground. Metallic meteoroids have even more structural strength and can remain intact all the way down as well. If they do survive and impact the ground, they’re called meteorites. 1
Small meteoroids that make it down to the ground usually aren’t moving terribly fast when they impact. In fact, their initial velocity is completely nullified by our air, leaving them to fall at what is called terminal velocity. It’s as if they were dropped off a tall building or from a balloon; they wind up impacting at maybe one or two hundred miles per hour. Scary, sure, but not too scary.
Still, you wouldn’t want to get hit by a rock moving that fast. For comparison, they hit the ground faster than even a professional baseball player’s pitch. In November 1954, a woman named Ann Hodges from Sylacauga, Alabama, was actually hit by meteorite. It was fairly small, about the size of a brick and weighing just over eight pounds. It punched through her roof, bounced off a wooden radio cabinet, and smacked into her where she was lying on the couch, taking
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