The Sky, In Fact, Is Falling

“The sky is falling” is a familiar childhood refrain that conjures the image of an over-anxious hen’s doomsday fears. While we typically dismiss thoughts of threatening cosmic circumstances (we have more than enough threats on Earth), the belief that our place in space is comfortably benign is dangerously misguided. The sky, in the form of meteors, does in fact fall regularly.
The space stuff that actually impacts the Earth are meteorites. Meteorites can range from the size of a dust-like particle to miles-wide rock capable of causing extinction. A meteor is the term for the object as it streaks through the Earth’s atmosphere. While meteors are accompanied by tremendous light and heat generated by friction, they do not always collide with Earth. Meteors are produced by asteroids or comets. Annual meteor showers are composed of dust particles, called meteoroids, which are remnants of comet tails that burn up as they pass through the Earth’s atmosphere.
Just looking at the Moon should make us aware of how typical these impacts are.

The Moon lacks an atmosphere, and thereby allows meteors to hit the surface, unencumbered by friction. With no atmosphere, there is also very little erosion. Thus, evidence of surface bombardment is residual.
In 1994, when fragments of Comet Shoemaker Levy hit Jupiter, Earth had a spectacular ringside seat. See This Link for a visual representation. The collision released energy many times that of our own nuclear warheads.

Earth, of course, has also received an ample share of meteor impacts. However, our protective atmosphere and erosion by wind and water rapidly erase evidence of such impacts. The following Wikipedia image illustrates just how ubiquitous meteor strikes are on Earth.

Recent studies estimate that about 17,000 meteors hit the Earth per year, see This Link . The vast majority of these are tiny and have little immediate effect. We are, of course, well aware of the effect of the “big ones”. The most famous is the Chicxulub impactor that hit Mexico 66 million years ago at the end of the Cretaceous Period, ending life for the dinosaurs. It is estimated to have been about the size of a city.
The Chelyabinsk meteor strike on February 15 2013 is the most recorded and most invaluable of all meteor strikes. Russian dash cams, primarily intended by Russian drivers for accident insurance claims, were unintentionally repurposed to record a rare and unique astronomical event. Youtube is replete with a record of this event. While comparing the energy of meteor collisions and nuclear bombs may be misleading (apples and oranges), their effects are similarly detrimental. Because the meteor exploded some 30 miles above the city, rather than on the ground, Chelyabinsk was spared a more horrific outcome.
Most disturbing of this enormously damaging event was the fact that astronomers, who were very active that morning, had no idea of its presence. All telescopes were trained on an unrelated space rock 212DA14, passing Earth from the west at an astronomically close distance of 17,200 miles. The Chelyabinsk meteor, on the other hand, came as a complete surprise. The 20 meter (65 foot) space rock approached Earth from the east; its presence obscured by the rising Sun. This made it nearly impossible to detect beforehand with current observational technology.
This event was actually a wake up call. First, it made us aware that large scale impacts on Earth are not so rare. See This video for the top 15 asteroid impacts in history. Secondly, it galvanized the U.S. to create the National Nuclear Security Administration (NNSA) which partners with NASA, FEMA, and Homeland Security. It is charged with developing and implementing a strategy for planetary defense. (Using the word “Nuclear” in the title is a sure sign that the government takes the issue seriously.)
They initially are looking to detect asteroids or comets that are larger than 50 feet in diameter and that pass within 4.6 million miles of Earth. If you are interested go to this NASA link to read about the planetary defense program: Click here.
The 2022 DART mission, which successfully altered the path of the asteroid Dimorphos by deliberately sending a rocket to collide with it, demonstrates one type of strategy currently being developed, see Here for more information on the DART mission.
Meteor impacts have indeed shaped Earth’s history in profound ways, influencing both its physical environment and its biological evolution. While these collisions are often remembered for their catastrophic effects, their contributions to life’s development are equally fascinating. Next month’s article will delve into just how meteor impacts have played a significant role in our own evolution.