By The Great Courses Staff

A meteorite unexpectedly struck western Cuba on Friday, February 1. It wasn’t the first and it won’t be the last. Small celestial bodies have covered the moon in craters and have wiped out the dinosaurs. Here is what you need to know about asteroid and meteoroid collisions.

Scientists currently know of 800 or so asteroids that are 1 kilometer (km) in diameter or greater and cross the Earth’s path as we orbit around the sun. They believe 1,100 exist. If just one were to collide with Earth, the damage would be equivalent to over 100,000 megatons (MT) of TNT or several thousand H-bombs detonating at once. On average, an asteroid this size hits our planet once every million years.

Meanwhile, more than 100,000 asteroids with diameters greater than 100 meters (m) cross Earth’s path. One of these is estimated to collide with us every 10,000 years or so—far more often than their big brothers. “That would produce an explosion of 100 to 10,000 MT of TNT, so that’s like several or a dozen H-bombs going off,” said Dr. Alex Filippenko, Professor of Astronomy and the Richard and Rhoda Goldman Distinguished Professor in the Physical Sciences at the University of California, Berkeley. Unfortunately, despite a lack of radiation, an impact of this magnitude would cause far worse problems than a nuclear weapon.

Ground Zero and Beyond

Craters extend far beyond the size of the meteorite that causes them. “The reason for that is that the rock hits and it stops on a dime,” Dr. Filippenko said. “So all of the energy of motion—all of the so-called ‘kinetic energy’—gets absorbed into the material. A lot of the material around the rock at greater distances gets excavated. The excavated amount is much larger than the size of the rock.”

Dr. Filippenko explained that a 1-km asteroid impacting a city may form a crater up to 20 km in diameter, while a 10-km asteroid would affect an area 1,000-km in radius. The detritus sent outward from the point of impact would cause enough friction rubbing against the air to create a major heating effect. “The rest of the Earth would essentially be like an oven set to ‘broil,’” Dr. Filippenko said. “The people at Ground Zero, in a sense, are the lucky ones. They don’t know what hit them. They died instantly and that’s it.” After the impact, the rest of us would face widespread fires, then a phenomenon scientists call “impact winter.”

During an impact winter, the collision sends enough dust into the air to block out the sun. The resulting cloud of particles causes temperatures to drop, which in turn makes vegetation die out. Without plentiful vegetation, herbivores starve. Without enough herbivores to eat, carnivores starve. At some point during this chain reaction, carbonates like carbon dioxide mix with oxygen and water and form carbonic acid. Carbonic acid falls down to the Earth in the form of acid rain. Environmentally, these scenarios are nightmares.

Obtaining Scientific Evidence

The Cuban meteorite gives us cause to look back and wonder how we obtained such knowledge about these collisions. The last mass extinction occurred during the Cretaceous period and eradicated the dinosaurs. However, geological strata of the Earth also suggest a larger effect. “There’s this abrupt layer where two-thirds of all living species on Earth died quite suddenly,” Dr. Filippenko said. “You can tell from the strata pretty much exactly when this event happened, and it was very, very sudden.”

In 1992, a comet that had been orbiting Jupiter for several decades began to break apart. Two years later, in July, 1994, it gave us a clear visual of the biggest explosion ever seen in our solar system. Over the course of a week, 20 or more comet fragments struck Jupiter and released the energy equivalent of more than 40 million MT of TNT.

Cuba, the dinosaurs, and Jupiter’s comet shower have led us to ask about the risks Earth faces from small celestial bodies hurtling through space. Fortunately, after the 1994 Jupiter comet event had led to widespread media coverage and public concern, astronomers then obtained government funding to research techniques for planetary defense. If astronomers detect a threat to the Earth, they plan to warn the public—it’s even possible that NASA would be able to send a spaceship that would reach the incoming celestial body and deflect it. “So don’t get too worked up,” Dr. Filippenko said.

Dr. Alex Filippenko contributed to this article. Dr. Filippenko is Professor of Astronomy and the Richard and Rhoda Goldman Distinguished Professor in the Physical Sciences at the University of California, Berkeley. He earned his B.A. in Physics from the University of California, Santa Barbara, and his Ph.D. in Astronomy from the California Institute of Technology.