Over the ages, a million rocks that make up the central 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 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 daily by the truckload.
Why do meteorites fall to Earth?
Meteorites fall to Earth due to a combination of factors related to their origin and their interaction with Earth’s atmosphere. Here’s a brief explanation of why meteorites fall to Earth:
Meteoroid Origin: Meteoroids are small rocky or metallic objects in space, ranging in size from tiny dust particles to larger rocks. They can originate from various sources, including asteroids, comets, or the Moon. When these objects enter Earth’s vicinity, they are called meteoroids.
Atmospheric Entry: When a meteoroid enters Earth’s atmosphere, it encounters resistance and rapidly decelerates due to the air molecules in its path. The high-speed entry generates intense heat through compression and friction with the atmosphere, causing the meteoroid to heat up and ionize the surrounding air molecules. This glowing phenomenon is what we observe as a meteor or shooting star.
Atmospheric Interaction: As the meteoroid travels through the atmosphere, the intense heat causes it to vaporize and disintegrate, creating a glowing trail of ionized gas known as a meteor or fireball. Most meteoroids burn entirely up during atmospheric entry, resulting in dispersed small particles that pose no threat to the ground.
Meteorite Survivors: Some meteoroids are large or dense enough to survive atmospheric entry without entirely disintegrating. These surviving fragments, known as meteorites, are the ones that reach the Earth’s surface. Meteorites can vary in size from small pebbles to significant boulders, and their composition can provide valuable insights into the formation and evolution of our solar system.
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. Upon entry, they sometimes break apart with a bright terminal flash called a bolide. Bolides are harmless, but the optical energy they give off as light can be 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 from. Once meteorites continue to come to Earth, they will spread out, 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. 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 on Hiroshima. The meteor left a crater, likely creating local firestorms raised by superheated air. Also, particles from the impact may have cooled Earth’s climate.
Tiny things always hit planet Earth, but they’re usually not fast enough or large enough to worry anyone. Observations suggest that anywhere from 1 to 300 metric tons of dust hit the Earth daily. Assuming 100 tons, evenly across the whole surface, we’d gain around 0.02 nanometers yearly to the Eearth’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 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.
The solar wind would have blown away the asteroid shield if we didn’t have it. So the Earth is shielding the atmosphere, which is shielding the Earth. Mars doesn’t have much 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. Moreover, 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 averages out over time.
Any rock inside the empty band orbits the sun three times faster than Jupiter. As a result, it repeatedly brings it closest to Jupiter at the same two places in its orbit. This so-called “orbital resonance” distorts the shape of the asteroid’s orbit and eventually destabilizes into a potentially Earth-crossing path.
Hebe feeds more space rocks into Jupiter’s reach than any other asteroid, 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 huge 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.
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.”