字幕列表 影片播放 列印英文字幕 There’s a bunch of rocks floating up there in space. What separates “ohhh, pretty lights,” from “ahhhh we’re all gonna die!!!”? Hey everyone, Julian here for DNews. Shooting stars have a magical ethos. We’re 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 stars” is a misnomer, because they’re not stars zipping about. They’re meteoroids, bits of dust usually left by comets or asteroids. We see them at 70 to 120 km away, when they’re 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 they’re 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 they’re 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 they’re itty bitty. So imagine how much energy meteoroids have when they’re 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 they’re classified as asteroids, but so long as they’re 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.