Out there, in the far recesses of the Solar System, a great existential threat lies in wait for planet Earth: the Oort cloud. Formed at the outset of the Solar System, it largely consists of the remnants of the primitive material that led to the formation of our Sun and the planets. Whatever wasn’t either boiled off by the Sun or locked up into the planetary, lunar, asteroidal, or Kuiper belt objects we have today remained in a spheroidal cloud, anywhere from a thousand times the Earth-Sun distance to one or two light-years away.
Today, these bodies, mostly a mix of ice-and-rock, remain in slow, quasi-stable orbits in the deepest recesses of our Solar System. But every once in a while, a chance gravitational encounter will perturb the orbit of a particular object, and send one careening into the inner Solar System. Even though they have periods that can rise into the millions of years, the wrong gravitational “nudge” from another massive body could send any one of these on a collision course for Earth.
While Comet Bernardinelli–Bernstein, the most massive comet ever discovered, isn’t going to hit Earth on this current pass through the Solar System, the far future is anyone’s guess. Here’s what would happen if a collision were to occur.
There are three major concerns whenever an object is going to impact the Earth in terms of the damage it will do.
- How massive the object is. More mass equals more energy imparted into the Earth, which translates into more destruction. If you were to double the impactor’s mass, the energy imparted into the Earth would also double.
- How fast the object is moving. The faster the object moves, the greater the amount of kinetic energy it brings with it, and that energy gets dissipated into the Earth upon impact, causing the damaging effects we correctly have a healthy fear of. If you double the impactor’s speed, the energy imparted into the Earth quadruples; the energy scales as the square of the impactor’s relative velocity to Earth.
- What the object is made out of. Composition isn’t everything, but an object that’s “more rocky” is generally more dangerous than one that’s “more icy” for a few reasons. Asteroids are more likely to reach the ground and create an impact crater, while comets are more likely to create airbursts. Comets have more volatiles, so they’re more likely to split into smaller fragments, some of which might miss Earth entirely, and the ones that do hit us will certainly dissipate some of their energy in the atmosphere. Finally, asteroids contain a greater fraction of elements that are absolutely toxic to ingest or inhale, so they’re a greater threat to life as well.
There are, of course, other concerns, like the location of impact as well as the angle of impact, but those are only relevant when you have smaller impacts: the kinds that are unlikely to cause mass extinctions. In general, if you had an object strike the Earth that was on the order of a kilometer in diameter or more, it would pose the type of existential threat that would not only bring an end to human civilization, but to a tremendous fraction of the species currently extant on Earth today.
As a reference point, the object that struck the Earth 65 million years ago, causing what we historically know as the fifth great mass extinction, was almost certainly an asteroid and not a comet. The knowledge we’ve gained of Chicxulub crater, including its size, as well as the layer of iridium-rich ash found all over the globe in sedimentary rock strata, strongly indicate that the impactor was an asteroid. Asteroids are also much more likely to strike Earth than comets are, as asteroids:
- are pretty much all in the same plane as the planets to begin with,
- are in relatively close proximity to our Solar System’s greatest gravitational perturber, Jupiter,
- and come to Earth from much less far away than comets do, making a direct hit more likely.
All told, what we know as the K-Pg extinction event was likely caused by a rocky object originating from the asteroid belt that was approximately 10 kilometers in diameter.
You might think that’s impressive, and sure, in some ways, it very much is. But here are some facts that might put that event, catastrophic as it was, into a little bit of perspective.
- The typical density of an asteroid is somewhere between 2 and 3 grams per cubic centimeter, which means, for the 10-kilometer wide asteroid that struck our planet 65 million years ago, its total mass was somewhere around a few ~1015 kilograms, or a few trillion tonnes.
- Asteroids, when they get perturbed so that they cross into the inner Solar System, typically cross Earth’s orbit with a speed of around 25 kilometers-per-second. Given that Earth orbits the Sun at about 30 kilometers-per-second, and that both asteroids and Earth typically orbit the Sun in the same general direction, the typical impact velocity of an asteroid that hits Earth is around 17-20 km/s.
- Putting those factors together, the total energy of that impact was somewhere in the ballpark of 1024 J, give or take whatever the uncertainties are.