When considering your future plans, how much weight should you give to the possibility of an asteroid (or comet) striking the Earth in what ought to be your lifetime? Probably not a lot, but that doesn’t mean it won’t happen.
How often do asteroids strike?
Some people like to compare science to magic, and one way they are similar is that when asking questions, it’s important to be precise with your terms. Just as tales of magic are filled with examples where someone comes unstuck because they don’t consider the loophole in a prophecy, badly worded science questions can lead to very silly results.
We know an asteroid will strike the Earth soon, for almost any meaning of the word soon, because it happens all the time.
On October 22, an asteroid burned up in Earth’s atmosphere over the eastern Pacific Ocean. If you didn’t hear about it, that’s because it was only about a meter (3 feet) wide. Known as 2024 UQ, the only reason this got some media coverage was that this object, also known as A11dc6D, was spotted beforehand.
Until recently an asteroid being spotted in space in time to predict its impact was a very rare event, but 2024 UQ was the third time it had happened this year, so the stories were muted.
Meanwhile, a great many more asteroids arrive unheralded. There’s no minimum size for something to be considered an asteroid, so arguably all the bits and pieces of left-over comets that produce a light show during recent meteor showers like the Orionids or the Taurids count.
What about actually hitting Earth?
You might legitimately argue that these objects don’t hit the Earth, just the atmosphere.
Even if hitting land or sea is a requirement, meteorites are common enough. The first of the three predicted encounters with our atmosphere, asteroid 2024 BX1 produced fragments that are known as the Ribbeck meteorites, after the German village nearest to where they landed. Approximately 200 were collected, and even though the total mass was less than a large bottle of soda, that’s a lot of Earth-hitting going on.
What about something dangerous?
Ok, time to stop playing with words. You might say: you know what we mean is something large enough to do serious damage.
Some of the meteorites we are talking about could be a problem if you happened to be in exactly the wrong place, but they’re not the sort of thing people worry about.
The larger an object is, the fewer of them there are in the inner Solar System, and therefore the less likely one will run into us.
There are two ways to calculate the chances of something dangerous hitting Earth.
One is to observe every object with an orbit that brings it close to Earth and calculate the chances of one hitting us. There’s a certain chaotic element to these objects’ paths, so complete certainty is not possible. Theoretically, another space rock might hit Apophis with just the right force to redirect it so it ends up hitting Earth in 2029 or 2036, but the chances are tiny. You’ve got a good life if this is the biggest thing you have to worry about.
If an object the size of the dino-killer asteroid (roughly 10 kilometers or 6 miles) was rattling around the inner Solar System with a chance of hitting Earth in the next few centuries, we would know about it. The risk is higher for something large enough to be locally damaging. Early today, an object 100 meters (330 feet) across passed the Earth at a comfortable distance of about 5.4 million kilometers (3.4 million miles), or 14 times as far away as the Moon. That’s big enough to take out a city if it hit, and the fact that the object’s designation is 2024 VQ4 tells us we only discovered it this year, indicating there are still plenty of gaps in our knowledge.
That’s not a unique example, yesterday it was 2023 WK3’s turn to pass us at 16 lunar distances, and it’s almost three times wider than 2024 VQ4.
NASA estimates there are about 25,000 objects in this size class in the inner Solar System, and we’ve found about 40 percent of them. Since none of the ones we know about present much danger for a long time, the odds are that the unknown ones don’t either, but we can’t be sure.
We’re probably very close to discovering all the asteroids larger than a kilometer in diamater, but those a seventh that size are a different matter.
Even when we’ve been observing long enough to have all 25,000 tracked, this can never be a complete method for assessing the danger. It doesn’t take into account the possibility of something, probably a comet, racing in from the outer Solar System giving us at best a few years’ warning. We think long-period comets, which haven’t come close to the Sun for centuries, strike less often than those that give us a sporting chance to spot them. Still, these may account for a larger proportion of those threats great enough to do planet-wide damage.
The other way to work out the danger is to look at the historical record.
We’ve had one explosion dramatic enough to injure 1,500 people, even if no one was killed. It’s reasonable to expect something like that will happen again in the next few decades, even if we have no idea when.
The only recent example of something significantly larger than the Chelyabinsk event is Tunguska in 1908. We don’t know if incidents like that occur once a century or once a millennium, but it is very unlikely to be more common than that.
What about a dino-killer?
As far as we know, the Earth has only been hit once by something large enough to cause worldwide destruction since animals emerged about 600 million years ago. The Earth has had at least five mass extinctions in that time, but only the last one seems to be from an asteroid, not volcano.
On that basis, we can estimate the chance of something like that happening again in any given year to be about one in 600 million. Even over a century, the chance is still considerably less than one in a million.
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