Space & Innovation

Why Haven't Meteoroids Killed Us All?

If space is full of rocks and other debris, why aren't they colliding with us all the time? How does our atmosphere protect us?

The season of summer nights and shooting stars is upon, which is all good and fuzzy and romantic. But what is it exactly that keeps those shooting stars from impacting Earth, spreading fiery death, and harshing our mellow?

As Julian Huguet explains in today's DNews report, several factors are at work. Shooting stars are technically termed meteoroids. They're bits of cosmic debris, left behind by asteroids and comets, entering our atmosphere.

Some meteoroids become visible when they enter our mesosphere -- when we first see them light up, they're typically around 70 to 120 kilometers away. As Earth's atmosphere gets denser, air friction heats up the incoming meteoroid, which can be traveling upwards of 70 kilometers per second. Do the math, and that's around 156,600 mph.

Those speeds heat up the object to as much as 1,600 degrees Celsius, causing them to burn up and leave that streak of light we call a meteor. Well, we hope they burn up.

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Most meteoroids are very tiny indeed; smaller than a grain of sand. NASA estimates that hundreds of tons of this cosmic space dust burn up overhead every day. But even pebble-sized meteoroids can ignite brightly enough to produce light we can see.

It's the big ones we have to worry about. Meteoroids larger than one meter in diameter are classified as asteroids. Still, if they're less than 25 meters across, they usually burn away in the atmosphere. Larger ones typically break apart into smaller chunks upon entry, generating bolides which can give off intense flashes of light. NASA estimates that the really troublesome incoming space hazards -- asteroids the size of a football field -- hit the Earth once every 2,000 years or so.

It's all about the atmosphere. Not only does Earth fragile atmosphere give us the air we breathe, and protect us from UV rays, it deflects meteoroids and asteroids that would otherwise pound us into submission.

Double Secret Bonus Trivia: Earth's atmosphere is protected, in turn, by the planet's magnetic field, which deflect charged particles from the sun. Without the magnetic field, our asteroid shield would be blown away by solar wind.

-- Glenn McDonald Read More:

Earth Sky: How High Up Are Meteors When They Begin to Glow?

NASA: Near Earth Object Program

Discovery News: Large-ish Meteor Hits Earth... But No One Notices