In September, NASA performed its inaugural test to alter the course of an asteroid with the objective of assessing our ability to deflect a potentially hazardous asteroid on a collision path with Earth. Then, on February 13, an asteroid heading towards Earth was detected several hours before disintegrating into a meteorite upon impact, causing no significant harm. It, however, produced a spectacular display in the night sky before landing near the English Channel. Dr. Alex Filippenko, a Professor of Astronomy at the University of California, Berkeley, explores the topic of asteroids impacting Earth and the Moon in his video series titled "Understanding the Universe: An Introduction to Astronomy." A fundamental question addressed is the number of asteroids that have struck our planet throughout history.
How Do the Moon’s Asteroid Impacts Differ from Earth’s?
According to Dr. Alex Filippenko, studies of the Moon have revealed that collisions with asteroids have occurred throughout its history, predominantly during its early stages, within the first few hundred million years. The Moon's craters are preserved due to the absence of erosion factors such as rain, wind, and rivers. While some erosion can happen through subsequent impacts or settling dust, the craters generally endure indefinitely. Determining the number of asteroid impact events on Earth is more challenging. Erosion caused by various natural phenomena like wind, rivers, extreme weather events, and tectonic activity tends to bury and destroy asteroid craters. However, despite these factors, scientists have identified around 180 surviving craters. In terms of meteorites falling to Earth, the frequency depends on the definition used. It is estimated that millions of rocky fragments from space burn up in our atmosphere annually. However, the occurrence of a meteorite successfully reaching the Earth's surface happens only a few times each year.
Why Are Craters So Big?
When a rock is dropped onto the ground, the impact area is relatively small. However, meteorite impact craters are much larger in size. According to Dr. Alex Filippenko, when a rock or meteorite hits the surface, it abruptly comes to a stop, causing its kinetic energy (energy of motion) to be absorbed by the surrounding material. The immediate vicinity of the impact point may vaporize, but the energy from the impact propagates outward, excavating a larger area of material. The resulting impact crater is significantly larger than the size of the rock or meteorite itself because the sudden and rapid transfer of kinetic energy energizes the surrounding material, causing it to be forcefully ejected from the point of impact. For instance, if a one-kilometer meteor were to strike the Earth, it would create a crater ranging from 10 to 20 kilometers in diameter.