Comets and Earth: A Collision Course?

No comets have been confirmed to have collided with Earth in recorded history. However, comets like asteroids can pass close to Earth and their orbits can bring them into proximity with our planet. While there have been instances of smaller fragments from comets entering Earth's atmosphere resulting in meteor showers, these are not considered direct collisions. The most notable event related to comet fragments is the Tunguska event in 1908, which is believed to have been caused by the airburst of a small comet or asteroid over Siberia.

The Prevalence of Cometary Collisions

Comets do collide with Earth. We don’t know how many are poised to do so because a cometary orbit makes it spend most of the time moving slowly far from the sun, so we can’t see them.

Impact Craters on Earth

Approximately 200 impact craters are known to exist on our planet. Most of these are quite eroded, and most are from asteroid collisions. However, some of them bear traces of having been caused by comets. Such cratering has taken place over the last 4.5 billion years, so we can be reasonably certain we are not getting struck in the near future. The last really big strike was 66 million years ago, when a mountain-sized asteroid struck what is now Mexico’s Yucatán, causing worldwide mass extinctions. It is worth noting that pieces of this puzzle did not begin to fall into place until about 1980!

Notable Craters

Check out the Meteor Crater in Arizona near Flagstaff. Also, don’t miss Lake Manicouagan in Quebec, Canada, and the Pingialuit Crater in Northern Quebec. You should also learn about the one off the Yucatán, which led to the extinction of all the large dinosaurs.

The Role of Comets in Earth's Geological History

Consider the Moon, our next-door neighbor. The generally accepted current theory of the formation of the Moon proposes that the Earth-Moon system was formed by a giant impact very early in the history of the solar system. This very early Moon was much closer to the Earth just outside the Roche limit and quickly became tidally locked to the still hot Earth. The young Moon lost its atmosphere in a geologically short period of time and all its water.

Water on the Moon and Earth

However, recent probes have determined that with the Moon's axis nearly perpendicular to the light coming from the Sun, the lunar polar craters are cryogenic traps. They never get any sunshine and are looking out at the background radiation of the Universe at 4 Kelvin. Over the last 4.5 billion years, this water condensed from the temporary atmosphere the Moon had whenever it was impacted by a comet, including those from the Kuiper Belt. If the Moon has been hit by a comet, then the Earth, an even bigger target, has most certainly been struck as well.

Conclusion

While the chances of a comet colliding with Earth in the immediate future are slim, the geological evidence tells us that both comets and asteroids have indeed collided with our planet in the distant past. Understanding this history helps us better prepare for and potentially mitigate the risks we face.