By Jonny Lupsha, Wondrium Staff Writer
Meteorites of all shapes and sizes have struck Earth and the Moon. The most obvious examples of this are visible on the Moon’s surface, as evidenced by the myriad craters dotting the Moon. How many have hit Earth?
In September, NASA conducted its first test to affect an asteroid’s orbit. The purpose of the test was to see if we could deflect a large, dangerous asteroid heading for Earth, should such an event occur. Then, on February 13, an asteroid traveling toward Earth was spotted several hours before breaking up in a fiery impact as a meteorite. It was too small to cause damage, although it lit up the night sky before landing near the English Channel.
In studying objects from space, one of the first questions that arises is “How many asteroids have hit Earth?” In his video series Understanding the Universe: An Introduction to Astronomy, Dr. Alex Filippenko, Professor of Astronomy at the University of California, Berkeley, examines the history of asteroids on Earth and the Moon.
How Do the Moon’s Asteroid Impacts Differ from Earth’s?
“We know from studies of the Moon that collisions have occurred over all of its history, mostly when it was young, mostly in the first few hundred millions of years,” Dr. Filippenko said. “You can see the old [craters] on the Moon because there’s essentially no erosion. There’s no rain, no wind, no rivers. There are some erosional processes, like craters can be destroyed by the impact of additional things that hit them, or by the dust from other impacts that settles upon them.
“But basically, other than that, the craters pretty much live forever.”
The number of asteroid impact events on Earth, on the other hand, is much harder to determine. Erosion from wind and rivers, extreme weather events, subduction of continents under other plates, and other phenomena bury and destroy asteroid craters. However, not all hope of understanding asteroid impacts is lost. Despite erosion, scientists know of about 180 surviving craters.
The number of meteorites that fall to Earth, however, depends on your definition of “falling to Earth.” It’s estimated that millions of rocky shards from space burn up in our atmosphere every year, though a meteorite successfully hitting the Earth only happens a handful of times per year.
Why Are Craters So Big?
If you’ve ever dropped a rock from your hand onto the dirt, or even onto sand on the beach, you may notice that the impact area around the rock is rather small. So why are meteorite impact craters so big?
“The rock hits, and it stops on a dime, so all of the energy of motion—the so-called kinetic energy—gets absorbed into the material,” Dr. Filippenko said. “The material right around the rock gets vaporized, but a lot of material right around the rock, at greater distances, feels this impact, feels the energy of the impact, and gets excavated.”
Regardless, the impact crater is much larger than the rock itself because the kinetic energy from the rock was transferred suddenly and quickly, energizing material around the rock and sending it flying outward from the point of impact. If a one-kilometer meteor hit Earth, it would make a crater 10 to 20 kilometers across.
Understanding the Universe: An Introduction to Astronomy is now available to stream on Wondrium.