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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 nightand you really shouldchances

  • are pretty good that within a few minutes youll see a shooting star. Itll zip

  • across the sky, a fiery dot leaving a long glowing trail behind it. Theyre one of

  • the most exciting and fun things youll see when you look up, and they always get

  • a gasp and a squeal of delight from people someone who’s stargazing.

  • What youre 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 youll see is due to a meteoroid that’s tiny, probably

  • smaller than a grain of sand! How can that be?

  • It’s because theyre 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 secondEarth’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 itll have four times the kinetic

  • energy.

  • Meteoroids may usually be small, but theyre 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, theyre

  • 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 youre 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

  • formed!

  • 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 youre

  • interested, make sure you getem 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 TNTas 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. Were 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, were 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.

I love astronomy. You may have noticed. But there’s one really frustrating aspect of

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流星クラッシュコース天文学#23 (Meteors: Crash Course Astronomy #23)

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    Serena に公開 2021 年 01 月 14 日
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