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I love astronomy. You may have noticed. But there’s one really frustrating aspect of
it: Everything we study is really far away.
Nearly everything we understand about the Universe comes from light emitted or reflected
by objects. It’d be nice if we could get actual samples from them; physical specimens
we could examine in the lab.
Welp, sometimes the Universe can be accommodating, and allows us to hold it in our hands.
Cambot, can we get this up on still store?
If you go outside on a clear, dark, moonless night — and you really should — chances
are pretty good that within a few minutes you’ll see a shooting star. It’ll zip
across the sky, a fiery dot leaving a long glowing trail behind it. They’re one of
the most exciting and fun things you’ll see when you look up, and they always get
a gasp and a squeal of delight from people someone who’s stargazing.
What you’re actually seeing is a tiny bit of interplanetary debris: rock, ice, or metal
ramming through the Earth’s atmosphere, heated to incandescence. Most are faint, but
some can be astonishingly bright; I saw one once that left an afterimage on my eye!
Obviously, shooting stars aren’t really stars. So what do we call them?
Sometimes it seems like astronomers use different names for objects to keep things as confusing
as possible. But really, we do that to separate out different things. In this case, the actual
bit of solid stuff coming from space is called a meteoroid.
The phenomenon of the meteoroid getting hot and blazing across the sky is called a meteor.
And finally, if it hits the ground, we call it a meteorite.
I think the second best way to tick off an astronomer is to mix up meteor and meteorite.
Sometimes astronomers can be pretty pedantic about such things.
Oh, the best way to tick off an astronomer? Ask them, “Hey, what’s your sign?”
Amazingly, a typical meteor that you’ll see is due to a meteoroid that’s tiny, probably
smaller than a grain of sand! How can that be?
It’s because they’re hauling mass. You heard me.
The meteoroid is orbiting the Sun, probably at speeds of a few dozen kilometers per second.
As it approaches the Earth, our planet’s gravity accelerates it an additional 11 kilometers
per second — Earth’s escape velocity. And when it enters our atmosphere it’s moving
incredibly fast, up to 70 km/sec or more.
The energy of motion is called kinetic energy. If you want to get something moving, you have
to give it energy, and if you want it to stop, you have to take that energy away. This kinetic
energy depends on the mass of the object and how fast it’s moving. In fact, it depends
on the square of the velocity; double its speed and it’ll have four times the kinetic
Meteoroids may usually be small, but they’re screaming fast, and have a huge amount of
kinetic energy. As they hit our atmosphere they slow from their ridiculous orbital speed
to nearly a standstill, and all that energy has to go somewhere. It gets converted into
light and heat, and that’s what we see as a meteor.
A big misconception about meteors is that they get hot due to friction with air. Actually,
a far bigger contributor to their heat is compression. One of the most basic laws of
physics is that when you compress a gas it heats up. And a meteoroid coming in at hypersonic
speeds compresses the air in front of it a lot, heating it hugely. The gas can reach
temperatures of thousands of degrees Celsius for a few seconds.
The air radiates away this heat, in turn heating up the meteoroid. The material on the surface
vaporizes and blows away—a process called ablation. That ablated material leaves a glowing
trail behind the meteor, which we call a train. Sometimes it can glow for several minutes,
getting twisted up in high altitude winds, leaving behind an eerie, ghost-like persistent
train. This all happens high above your head, about 90 – 100 km above the ground.
Typically, from any one location, you can see a few meteors per hour. It may not seem
like much, but when you add them up all over the planet you find the Earth is getting pelted
to the tune of about 100 tons of material a day. But again, most of these meteoroids
are teeny tiny.
Those random meteors are called sporadic meteors. They tend to be rocky in composition, and
generally come from asteroids. If two asteroids smack into each other, the collision can eject
little bits of material that then orbit the Sun on their own. If their orbit crosses the
Earth, then you have a potential meteor. It may take a few million years, but at some
point the Earth and the meteoroid are at the same place at the same time, and boom.
But sometimes meteoroids travel in packs. When that happens, we can get meteor showers,
many dozens or even hundreds of meteors per hour. With one exception, those don’t come
from asteroids: They come from comets.
When a comet orbits the Sun, the ice on it turns to gas, dislodging dust and gravel mixed
in. This material leaves the comet and tends to stay more or less in the same orbit as
the comet itself. Over time, that material gets scattered all along the orbit, creating
a puffy ribbon of tiny pieces of space debris around the Sun.
When the Earth plows through that cloud of debris, we get a meteor shower.
From our viewpoint on Earth we see meteors shooting across the sky, apparently radiating
away from a single point. That’s a perspective effect; it’s like driving through a tunnel
and seeing the tiles on the wall and ceiling flying past you, all apparently coming from
a point ahead of you. The point in the sky where the meteors come from is called the
radiant, and the shower is named after the constellation the radiant’s in. So we have
the Perseid meteor shower, the Leonids, the Camelopardalids. Or the Camelopardalids.
And, since the Earth hits a specific comet stream around the same time every year, the
showers are annual. The Perseids are in August, and the Leonids in November.
Watching a meteor shower is easy: Just go outside and look up! Generally, they’re
better after local midnight. The Earth plows into the meteoroids, so facing the direction
of Earth’s orbital motion means more meteors, just like you get more raindrops on the front
windshield of your car than than on the back when driving through a storm. After local
midnight you’re on the part of the Earth facing into the orbit, so you see more meteors.
By the way, if you happen to be on the International Space Station, you have to look down to see
a meteor. In 2011, astronaut Ron Garan photographed a Perseid burning up below him! But don’t
worry: The odds of the Space Station getting hit are extremely low. Space is big.
Oh, and that one exception I mentioned before? That’s the annual Geminids shower, which
occurs in December. That comes from the asteroid 3200 Phaethon, which is on an orbit that takes
it very close to the Sun. It’s possible it gets so hot that the rock vaporizes, making
it act like a comet.
The vast majority of meteoroids are small and tend to burn up in our atmosphere. But
they can be bigger. A bolide, or fireball, is an extremely bright meteor, and those can
be about the size of a grapefruit. Those happen pretty often somewhere over the Earth. I’ve
seen a few myself.
Very rarely, an incoming meteoroid will survive all the way to the ground and become a meteorite.
Sometimes, the immense pressure of ramming Earth’s air causes the incoming meteoroid
to crumble or even explode, raining down dozens or hundreds of smaller pieces. Typically,
they slow rapidly after their blaze of glory, and simply fall the rest of the way to the
ground. The air up there is cold, and their interiors are cold from being in space so
long. So, despite what you might think, meteorites don’t cause fires when they hit the ground.
In fact, they can be quite chilly!
Meteorites are classified into three broad categories: Stony, which are mostly rock;
iron, which are mostly metal; and stony iron, which are a mixture of the two. The majority
of meteorites we find are stony.
The stony meteorites are subdivided into two kinds: Chondrites, and achondrites. Chondrites
contain chondrules, small grains of minerals. These are very primitive, and are thought
to have condensed out of the original disk of material that formed the solar system.
Their age can be found by looking at ratios of elements in them formed from radioactive
decay. The oldest known meteorite formed 4.568 billion years ago: Before the Earth itself
Achondrites don’t have chondrules in them. Most likely they came from a bigger asteroid,
one that was once molten through, mixing the minerals. A big collision disrupted the parent
body, creating the achondritic meteoroids.
Iron meteorites most likely come from the center of a large asteroid, one big enough
that metals fell to the center via gravity. Again, a big impact blew the asteroid up,
scattering its material around the asteroid belt, and with some on orbits that eventually
intersected Earth.
Stony irons are the rarest. Some have green or orange crystals of a mineral called olivine
embedded in a web of metal. Called pallasites, they may be the most beautiful of all meteorites.
I actually collect meteorites. It’s fun but can be a somewhat pricey hobby. If you’re
interested, make sure you get ‘em from a licensed dealer. We have links to some in
the dooblydoo.
Of course, on occasion the meteoroid coming in can be a tad bigger. And when that happens,
well, all hell can break loose.
On February 15, 2013, residents of the Russian city of Chelyabinsk got a rude awakening.
At 9:20 a.m. local time, a rock about 19 meters across came in at a low angle. It got nearly
as bright as the Sun as it slammed into the atmosphere, and the pressure of its passage
broke it up into several chunks, which broke up again. In a moment’s time, the sudden
energy released was equivalent to the detonation of a half million tons of TNT — as much
as a small atomic bomb!
While no one was killed, over a thousand people were injured by flying glass, shattered by
the explosion. No doubt they were at their windows gawking at the huge vapor trail in
the sky when the shock wave hit.
There was no warning for this event; the asteroid was essentially too small to detect while
it was out in space. Well, for now at least. Telescopes are coming online soon that should
be able to find smaller asteroids and give us some warning. Astronomers are more worried
about ones roughly a hundred meters across or bigger; these can do serious damage on
a city-wide scale or larger, but at the moment aren’t easy to spot much in advance.
And what do we do if we do see one headed our way? As of right now, there’s not much
we can do. Studies have been done to determine the best course of action; maybe lobbing a
nuke at it, or simply ramming it with a spaceprobe to change the orbit and make sure it misses
Earth. These ideas look good on paper, but they haven’t been tested yet. We’re still
a few years from that.
The good news is that objects that size hitting the Earth are rare; maybe once every century
or three. But if we do nothing, it will happen eventually. As science fiction writer Larry
Niven points out, the dinosaurs went extinct because they didn’t have a space program.
Hopefully, we’re smarter than they were.
Today you learned that meteors are small bits of interplanetary debris sloughed off by asteroids
and comets. When the Earth plows through the stream emitted by a comet we get a meteor
shower. Meteors burn up about 100 km above the Earth, but some survive to hit the ground.
Most of these meteorites are rocky, some are metallic, and a few are a mix of the two.
Very big meteorites can be a very big problem, but there are plans in the works to prevent
us from going the way of the dinosaurs.
Crash Course Astronomy - hey Crash Course, meteors! Cool!
Crash Course Astronomy is produced in association with PBS Digital Studios. Head over to their
channel for even more awesome videos. This episode was written by me, Phil Plait. The
script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller. It
was directed by Nicholas Jenkins, the script supervisor and editor is Nicole Sweeney, the
sound designer was Michael Aranda, and the graphics team is Thought Café.


隕石 (Meteors: Crash Course Astronomy #23)

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Serena 發佈於 2015 年 11 月 16 日
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