Over the ages, a million rocks that make up the main asteroid belt between Mars and Jupiter have sent chunks of space debris large and small crashing into Earth. After analyzing many of these meteorites, scientists have discovered that more than a third of them have the same chemical signature. It suggests that they’ve broken off from a single asteroid.
The meteoroids travel fast, between 11 and 71 km per second. The friction from rubbing against all that air at those speeds heats them as much as 1600 degrees celsius. It causes them to burn up and leave a streak of light called a meteor. Meteoroids the size of sand grains and smaller enter the atmosphere every day by the truckload.
Why do meteorites fall to earth?
Meteoroid size varies a lot, and they can be up to a hundred meters across before they’re classified as asteroids. After that, they’re less than 25 meters across, and 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. Bolides are detected about 28 times a year.
- A thing moving faster than the speed of sound will build up air pressure called RAM pressure.
It happens way up in the atmosphere 30 to 50 kilometers. It depends on how fast an object is going and what angle it comes. Once meteorites continue to come to Earth, they will spread out and slow down and eventually fall more or less in free fall to the Earth.
When an object hits the surface at high speed, the soil doesn’t move out of the way but solidifies. This is because all kinetic energy dissipates through the soil rapidly and creates a blast crater. Meteors can hit the ground with massive force and kinetic energy. The 1908 Tunguska Impact in Siberia flattened 2,000 square kilometers of Russian forest with an explosion 1,000 times greater than the bomb dropped on Hiroshima. The meteor left a crater, likely created local firestorms raised by superheated air. And particles from the impact may have cooled Earth’s climate.
Tiny things hit planet Earth all the time, but they’re usually not fast enough or large enough to worry anyone. Observations from satellites suggest anywhere from 1 to 300 metric tons of dust hit the Earth every day. Assuming 100 tons, evenly across the whole surface, we’d gain around 0.02 nanometers every year to the earth’s radius.
You can see some of these things as they fly through the atmosphere, anything over 2 millimeters in size. It can create a streak of friction-spawned burnup. Scientists call them shooting stars, but they’re just dust and bits of rock.
- The magnetic field produced by the spinning iron core diverts the charged particles the sun is constantly shooting into space away from the atmosphere.
If we didn’t have it, the asteroid shield would be blown away by the solar wind. So the Earth is shielding the atmosphere, which is shielding the Earth. Mars doesn’t have much of an atmosphere because it has no magnetic field.
By using special telescopes to look at the mineral makeup of asteroids, the main thing is Hebe. Hebe is big compared to other asteroids in the asteroid belt. But it still only makes up a tiny fraction of the total mass. And it’s been in lots of meteoroid-making collisions. It might have broken off a huge chunk of rock called Jebe.
Instead, Hebe owes its unique status to its location at the edge of an empty band in the asteroid belt. As asteroids on either side of this band orbit the sun, they pass Jupiter and get a little extra gravitational pull. But that pull comes at a different place with each orbit. So it kind of averages out over time.
Any rock that finds itself inside the empty band orbits the sun exactly three times faster than Jupiter does. As a result, it brings it closest to Jupiter at precisely the same two places in its orbit repeatedly. This so-called “orbital resonance” distorts the shape of the asteroid’s orbit, and eventually, it destabilizes into a potentially Earth-crossing path.
Hebe feeds more space rocks into Jupiter’s reach than any other asteroid, thereby sending more rocks rocketing toward us than anything else. Fortunately, most of them miss us, like the one with 100,000 times more destructive power than the Hiroshima bomb.
- In 1976, a boulder the size of a Toyota Camry crashed into northern China.
- In 1868, ten tons of pea-sized meteorites peppered northeastern Poland.
Scientists are researching ways to divert a really big one if it were on a collision course with Earth. But the anti-armageddon plan is still decades away from realization. So there’s still time for a mega-meteorite to turn us into dinosaurs. It’s enough to give you the Heebie-Jeebies.
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McSween, Harry, Meteorites and their parent planets (2nd ed.). Cambridge: Cambridge University Press.
“Introduction: What is a Bolide?”. US Geological Survey, Woods Hole Field Center.
McSween Jr, “A new type of chondritic meteorite found in lunar soil.” Earth and Planetary Science Letters.
Rubin, Alan E., “The Hadley Rille enstatite chondrite and its agglutinate-like rim: Impact melting during accretion to the Moon.” Meteoritics & Planetary Science.
The Meteoritical Society, Committee on Meteorite Nomenclature. “Guidelines for Meteorite Nomenclature.”
Chapman, Clark R, “The Comet/Asteroid Impact Hazard: A Systems Approach.”