Q&A: Asteroid Impacts

I've solicited questions from readers in the past, and now someone's taken me up on it! Fifth grader Steven Wu asks:

"I wonder if I could ask you a few questions about the
impacts of killer asteroids. Is there a serious threat from asteroid impacts? Is it possible it could destroy the Earth?"

Follow up:

First: Is there a serious threat from asteroid impacts?

Yes, there is a very real possibility of an asteroid hitting the Earth. Very small impacts happen rather frequently. For instance, on October 7, a small asteroid called 2008 TC3 entered our atmosphere and made a bright flash over northern Sudan, in Africa. The asteroid exploded in the atmosphere, creating a fireball with as much energy as a small nuclear bomb! Small fragments of it may have survived to hit the ground beneath. You can see a picture of the trail left by the explosion here.

No one was hurt by this, though. The asteroid was only about two meters across, and exploded high up in the atmosphere. Most things that hit Earth are very small, and the very small ones tend to burn up and come apart as they push through our atmosphere and get heated up by it. No one has died from any asteroid or comet hitting the Earth in at least the past century, and maybe much longer. (There are a few cases where a small rock fragment hit a car or a house, though.)

However...

Bigger asteroids are certainly out there. They're not nearly as common as the little ones, which is why our Earth (fortunately!) doesn't get hit by them very often. When such things do happen, however, they can have catastrophic results. There's a location called Meteor Crater, Arizona, where 50,000 years ago, an asteroid about 50 meters across hit the ground. The hole it left behind is about 1.2 kilometers across, and 200 meters deep! The impact would have caused a huge fireball, earthquakes... let's just say that it's a good thing there probably weren't any people in Arizona at the time.

In 1908, something maybe 40 meters across shot through the atmosphere over Tunguska, Siberia, in Russia. The asteroid - like the one over the Sudan - probably came apart in the atmosphere. However, it exploded much lower down in the atmosphere, and the shock of the explosion knocked down trees over 800 square miles! Again, it's fortunate that not many people were living in Siberia then.

Clearly, large things do strike Earth from time to time - enough to do significant damage. Fortunately, the larger they are, the more uncommon they are; so the 40-meter and 50-meter types probably happen only once a century or so, while the really nasty ones - a kilometer or larger - probably happen only once every million years, or longer. Which brings me to your next question:

Second: Is it possible it could destroy the Earth?

That depends on what you mean by "destroy the Earth". If you're talking about something that would make the Earth come apart into pieces, then no, there probably isn't an asteroid that size out there. (You'd need something closer to the size of Mercury for that kind of effect - maybe 5000 km across.)

If you mean "something that could destroy all life on Earth"... then the answer is maybe. You'd need an asteroid many tens of kilometers across, and there aren't very many of those left, let alone any that could cross paths with Earth as we go through the solar system. This might happen once during the entire lifetime of the Earth, and is EXTREMELY unlikely to happen in the next, oh, BILLION years.

However, if you mean "something that could destroy many species of plants and animals on Earth, and maybe wipe out most or all human life" (called "mass extinctions") then I'm afraid it is indeed possible. It is very very VERY unlikely, but it's possible. Asteroids a few kilometers in size can at least cause widespread devastation, and there are estimated to be a few hundred such asteroids in existence that are close enough to possibly hit Earth someday. Those might hit Earth once every ten million years or so. For mass extinctions, you'd need something over 10 km in size, and such impacts would only happen once every few HUNDRED MILLION years. So, as I said, it is (thankfully) very unlikely - but it could happen.

We are lucky to live at the time we do, however. Nations around the world have become alert to this possible danger. There are programs in many countries to watch for dangerous asteroids, so that we could do something about it if we saw one that was likely to hit Earth. NASA runs a "Spaceguard" program, for instance, with the goal of tracking almost all near-Earth asteroids over 1 km across, They have found almost six thousand asteroids, including both small and large ones; chances are high, now, that these programs would spot a large asteroid before it could collide with us, and we might be able to do something to prevent the collision! In fact, the program spotted 2008 TC3 - the one that hit over Sudan - a day before it actually hit us, and that was only a little, mostly-harmless asteroid. The larger ones are, fortunately, easier to spot!

Read more about NASA monitoring of these "Near-Earth Objects".