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  • There’s a bunch of rocks floating up there in space. What separatesohhh, pretty lights,”

  • fromahhhh were all gonna die!!!”?

  • Hey everyone, Julian here for DNews. Shooting stars have a magical ethos. Were told to

  • make a wish when we see them streak across the sky, but you should probably already thank

  • your lucky stars. If we didn’t have an atmosphere, they would be a lot more dangerous.

  • First of all, “shooting starsis a misnomer, because theyre not stars zipping about.

  • Theyre meteoroids, bits of dust usually left by comets or asteroids. We see them at

  • 70 to 120 km away, when theyre just starting to dip into our mesosphere and the air starts

  • becoming dense enough to give the meteoroid some resistance. And it only gets more dense

  • the closer to Earth they get. These meteoroids are traveling fast, between 11 and 71 km per

  • second, and the friction from rubbing against all that air at those speeds heats them up

  • as much as 1600 degrees celsius, causing them to burn up and leave a streak of light called

  • a meteor.

  • Meteoroids the size of sand grains and smaller enter our atmosphere every day by the truckload.

  • NASA estimates a hundred tons of dust hits us every single day. But theyre so small

  • they don’t have enough momentum to keep up their speed when they hit the air. Instead

  • they slow down and make it all the way to earth.

  • If you want to see a show, there are about a dozen major meteor showers a year, and they

  • happen predictably because the Earth keeps passing through the wake of the same comets.

  • Some particles the comets left behind are about the size of pebbles, so theyre big

  • enough to keep up their speed and ignite. It’s pretty impressive that we can see them

  • as far as 120 km away. That should tell you a little bit about how much energy they have

  • at those speeds, even though theyre itty bitty. So imagine how much energy meteoroids

  • have when theyre the size of a car. They cause huge fireballs, brighter than the planet

  • Venus. Meteoroid size varies a lot, they can be up to a hundred meters across before theyre

  • classified as asteroids, but so long as theyre less than 25 meters across, the atmosphere

  • usually burns them up. Sometimes upon entry they break apart with a bright terminal flash,

  • called a bolide. Bolides are harmless, but the optical energy they give off as light

  • can be in the rage of a hundred thousand gigajoules, a hundred times more energetic than the bomb

  • dropped on Nagasaki. Bolides are detected about 28 times a year.

  • So obviously our atmosphere is doing us a solid. More than just giving us the air we

  • breathe and protection from UV rays, the atmosphere is running interference on meteoroids and

  • asteroids too, making sure our planet isn’t as cratered as my face was in high school.

  • But who watches the watchmen? Turns out the atmosphere needs a little help too from earth’s

  • magnetic field. The magnetic field produced by the spinning iron core diverts the charged

  • particles the sun is constantly shooting into space away from our atmosphere. If we didn’t

  • have it, our asteroid shield would be blown away by the solar wind. So the Earth is shielding

  • the atmosphere which is shielding the Earth. Isn’t that sweet? Mars doesn’t have much

  • of an atmosphere because it has no magnetic field, and would you look at that, it gets

  • hit with meteorites 200 times a year.

  • Even with the atmosphere intercepting most space rocks before they can impact the surface,

  • Earth still gets hit with a biggie every now and then. NASA estimates an asteroid the size

  • of a football field gets through and causes significant damage to the local area once

  • every 2000 years. If it’s over a kilometer wide it could have global impacts; the meteorite

  • that wiped out the dinosaurs was estimated to be 10 km across. NASA isn’t leaving our

  • fate to chance, and is working to track dangerous Near Earth Objects. They hope to one day send

  • astronauts to study a dangerous asteroid before sending said asteroid into a harmless orbit

  • around the moon. Yes, it sounds like NASA got the idea from a Bruce Willis and Ben Affleck

  • movie.

  • So now you know why meteoroids burn up, but what if they make it all the way to earth?

  • Trace has the scoop on meteorites here.

There’s a bunch of rocks floating up there in space. What separatesohhh, pretty lights,”


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B1 中級 美國腔

為什麼流星體沒有殺死我們所有人? (Why Haven't Meteoroids Killed Us All?)

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    羅紹桀 發佈於 2021 年 01 月 14 日