More on Meteorites and Mass Extinctions
Meteorite impacts have been happening throughout Earth's history, and a Mars-sized body colliding with the Earth and blasting things into space is the best explanation for the formation of the moon. Big impacts were common early on in Earth's history. Penn State's Evan Pugh University Professor Jim Kasting (since retired), helped show that the heat from many of the huge impacts during the first few hundred million years of Earth's existence would have been enough to evaporate the whole ocean, and that as recently as about 3.8 billion years ago impactors may have been big enough to evaporate the sunlit upper layer of the ocean. Since then, collisions have been much smaller. The dinosaur-killer was larger than any others that fell to Earth during the most recent ~2 billion years (with some slight uncertainty because the record of an old event that hit entirely in the ocean might have been subducted), but the energy released by the dinosaur-killer was probably only enough to evaporate an inch or two of the ocean (a few centimeters). Even so, the dinosaur-killer wiped out a lot of species only because it hit special rocks that contained abundant sulfur and carbon. If other similar-sized meteorites had hit the Earth, almost all of them would have failed to cause a similarly large mass extinction, because almost all of them would have hit rocks that would have had less influence on the climate.
Please note that almost every major event in Earth’s history has been blamed on a meteorite by somebody at some time. Then, in almost all cases, additional scientific research showed that a meteorite was NOT responsible. The meteorite hypothesis then should have been moved to a footnote or dropped entirely. But… in online searches, the meteorite hypothesis lives on, waiting to trap unsuspecting individuals with outdated or inaccurate information. Often, one or a few scientists continue to push the idea, or nonscientific groups take over that job. Here, we have tried to give you the best information, representing an immense body of scientific work.
So, in the last couple of billion years, we have evidence of only one huge event in Earth’s history that was caused by a meteorite impact. But, smaller meteorites have caused small events, and many large rocks still are whizzing around out in space, so large impacts remain possible. An event like the one that formed the Meteor Crater, Arizona (see the picture below), which is 3/4 mile across, would be catastrophic for anyone living in or near the impact site, even though it would not have global consequences.

One scientific estimate found that your chances of being killed by a meteorite impact are about the same as being killed in the crash of a commercial airliner. Commercial airliner crashes kill a few hundred people per decade, and a meteorite might wait ten million years and then kill hundreds of millions of people, so the statistics are hard to compare, but the number is interesting. In comparison, recent statistics indicate that roughly 1.5% of deaths in the USA are from guns, and slightly over 1% from car crashes and other transportation-related fatalities. All other “accidental” deaths are much, much rarer but often get more press coverage, including tornadoes, hurricanes, earthquakes, food poisoning, bee stings, shark attacks, and many others. Part of this is because we have taken precautions against many of these others, such as accurate weather forecasts that allow millions of people to get out of the way of hurricanes. In the long term, smoking, over-eating, under-exercising, and other poor health habits are more important triggers of early death.
Scientists are coming up with ways to divert asteroids that might hit us, to help avoid such collisions. If we see an asteroid coming from far enough away, we need to turn its path only a tiny bit to miss the Earth. One idea is to hit the asteroid with a bag of dust that would spread across one side, changing the reflectivity of the surface; the difference between reflecting and absorbing the sunlight would cause a tiny push that would steer the asteroid. Another idea is to send a spacecraft to sit next to the asteroid for a year; the tiny gravity of the spacecraft, tugging on the asteroid, would turn it a tiny bit. In the year 2022, NASA proved that they could change the orbit of an asteroid by hitting one with a "kinetic impactor" Planning to avoid the fate of the dinosaurs may save us someday.
Really, despite the immense drama of the meteorite that killed the dinosaurs, and its critical role in the events that led to us, other issues are more important now. Let’s go visit a few of those, in the Arctic and Yellowstone, in Module 12, after you have a chance to explore the Enrichment.