Phil Plait: How Can We Defend Earth From Asteroids? What's six miles wide and can end civilization in an instant? An asteroid — and there are lots of them out there. Blogger Phil Plait explains what we must do to avoid them.

How Can We Defend Earth From Asteroids?

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It's the TED Radio Hour from NPR. I'm Guy Raz.

Do you look up at the sky every night?

PHIL PLAIT: When I go outside, and it's clear, yeah I look up. It's - it's a habit, and it's something I wish more people did. You may see something that will profoundly affect you - Jupiter and Saturn, Venus, the constellations. There's some talk - there's a very bright comet that may swing by the earth later in 2013, that could be extremely bright. It could be, you know, one of these once-in-a-lifetime comets, and if you never look up, you'll miss that.

RAZ: That's Phil Plait. He's an astronomer and a science writer, and he wants you to pay attention to asteroids for a simple reason.

PLAIT: We are bombarded every day. There's a hundred tons of material in the form of very, very, very small grains of rock, like sand, and you see these every night. If you go out and look up and wait an hour, you'll probably see two or three shooting stars. That's what this stuff is. And a hundred tons sounds like a lot, but you have to realize, the Earth is 6.6 sextillion tons, so you could bombard the Earth every day with a hundred tons of stuff for a gazillion years, and not really add even a fraction of the weight that we have here. The real problem isn't the amount - it's how it's delivered. When you have a hundred tons and you spread it out over the vast surface area of the Earth, it's not a big deal. But if you collect several hundred tons into a single rock, and then let it hit the Earth, that's when you start getting into trouble, and that's the sort of thing I'm worried about.

RAZ: Now, there's a very specific rock Phil Plait is worried about, and that rock has a name, which Phil Plait described in his TEDx Talk in Boulder.


PLAIT: You've probably heard about the asteroid Apophis. If you haven't yet, you will. Apophis is an asteroid; it was discovered in 2004. It's roughly 215 yards across, so it's pretty big. Big size, you know, bigger than a football stadium, and it's going to pass by the Earth in April of 2029, and it's going to pass us so close that it's actually going to come underneath our weather satellites. The Earth's gravity is going to bend the orbit of this thing so much, that if it's just right, the Earth's gravity will bend it just enough that seven years later, on April 13th - which is a Friday, I'll note - in the year 2036 - you can't plan that kind of stuff - Apophis is going to hit us.

RAZ: It will hit us.

PLAIT: Now, that was the thinking a few years ago. Now the thing is, the orbit of this object in 2029 is not precisely known. We do know very well that it's not going to hit us, but by exactly how much, we don't know. And if it gets a little bit closer than we expect, by literally, you know, a mile or less, the Earth's gravity will bend the orbit a lot and then seven years later it'll miss us. And if it's too far away, by, I don't know, a mile - it's probably even less than that, seven years later, again, it will miss us. But if it's right down the pipe - if it hits this bull's-eye in space, which is what we call a keyhole - it passes through that area. Then the Earth's gravity is exactly enough to bend the orbit such that, seven years later, this thing will come back and hit us.

RAZ: So, imagine you're outside that day and you're looking up.


PLAIT: Given that it's moving at 20 miles per second - and that's a guess, it could be moving faster or slower - and the atmosphere is very roughly a hundred miles high, and it's coming in at an angle, there might be five or six, seven, eight seconds, something like that, where you'd see this thing coming in if you happen to catch it early on. So, you'd have several seconds to go, whaaa? And then, there would be a tremendous flash of light. You might see the explosion. It would look very much like a nuclear bomb. You'd see a mushroom cloud, and then a shockwave would come rumbling through the ground a few seconds later, followed by the compression of air. The shockwave through the air, and after that, you know, debris raining down, all kinds of - all kinds of craziness, and it gets worse the bigger these things are.

RAZ: At 250 meters across, Apophis is nowhere near as big as the six-mile-wide asteroid that wiped out the dinosaurs, but it would still obliterate everything within a couple hundred miles. And if it lands near a major city, that would be bad. Now the odds of that happening?


PLAIT: Are one in a million, roughly - very, very low odds, so I personally am not lying awake at night worrying about this at all. I don't think Apophis is a problem. In fact, Apophis is a blessing in disguise, because it woke us up to the dangers of these things. This thing was discovered just a few years ago and could hit us a few years from now. It won't, but it gives us a chance to study these kinds of asteroids. We didn't really necessarily understand these keyholes, and now we do and it turns out that's really important, because how do you stop an asteroid like this?

RAZ: What Phil Plait points out is that we can't move the Earth ...


PLAIT: We can't more the Earth. At least not easily, but we can move a small asteroid. And it turns out, we've even done it.

RAZ: That was back in 2005. NASA launched a space probe called Deep Impact right into the path of a comet, which was orbiting the sun...


PLAIT: At 10 miles per second, 20 miles per second. We shot a space probe at it and hit it. Okay? Imagine how hard that must be, and we did it. That means we can do it again if we need - if we see an asteroid that's coming toward us, and it's headed right for us, and we have two years to go, boom! We hit it.


PLAIT: The problem is, what happens if you hit this asteroid, you've changed the orbit; you measure the orbit then you find out, oh yeah, we just pushed it into a keyhole, and now it's going to hit us in three years. Well my opinion is, fine. Okay? It's not hitting us in six months, that's good. Now we have three years to do something else, and you could hit it again - that's kind of ham-fisted - you might just push it into a third keyhole or whatever. So you don't do that. And this is the part ...


PLAIT: ... it's the part I just love.


PLAIT: After the big macho Rrrr, bam...we're going to hit this thing in the face, then we bring in the velvet gloves. There's a group of scientists and engineers and astronauts and they call themselves The B612 Foundation. For those of you who've read "The Little Prince" you understand that reference, I hope. The little prince who lived on an asteroid, it was called B612 ...


RICHARD KILEY: (Singing) Shine for me again, little prince...

PLAIT: ... These are smart guys - astronauts, like I said, engineers. If we see an asteroid that's going to hit the earth and we have enough time ...


UNIDENTIFIED MAN: (Singing) As we get closer and closer...

PLAIT: ... then what we do is we launch a probe, it doesn't have to be huge - a couple of tons, not that big - and you park it near the asteroid. You don't land on it, 'cause these things are tumbling end over end. It's very hard to land on them. Instead you get near it. The gravity of the asteroid pulls on the probe ...


PLAIT: ... and the probe has a couple of tons of mass. It has a little tiny bit of gravity. But it's enough, it can pull the asteroid, and you have your rocket set up. And you basically - these guys are connected by their own gravity, and if you move the probe very slowly - very, very gently - you can - very easily finesse that rock - into a safe orbit.


PLAIT: ... and we have the technology to do this. This probe actually can't use chemical rockets. Chemical rockets provide too much thrust, too much push. The probe would just shoot away. We invented something called an ion drive, which is a very, very, very low thrust engine. It generates the force a piece of paper would have on your hand, incredibly light, but it can run for months and years providing that very gentle push. If anybody here is a fan of the original Star Trek, they ran across an alien ship that had an ion drive, and Spock said, they're very technically sophisticated. They're a hundred years ahead of us with this drive. Yeah, we have an ion drive now. We don't have the Enterprise, but we've got an ion drive now.


PLAIT: ... Spock.


PLAIT: So, that's the difference between us and the dinosaurs. This happened to them. It doesn't have to happen to us. The difference between the dinosaurs and us is that we have a space program and we can vote, and so we can change our future.


PLAIT: We have the ability to change our future. Sixty-five million years from now, we don't have to have our bones collecting dust in a museum. Thank you very much.


RAZ: That's the astronomer Phil Plait from his TEDx Talk in Boulder, Colorado. TEDx events by the way, take place in communities around the world. To find out how you can organize TEDx in your city or region, visit



PLAIT: Hello?

RAZ: Hey Phil, it's Guy - it's Guy here. How's it going?

PLAIT: Oh hey, thanks for calling.

RAZ: So I understand you have an update for us.

PLAIT: Yeah, we have some good news. There was a near pass of Apophis in January of 2013, and a bunch of people observed it including with a radio telescope, which gives really precise measurements of the position of this thing. And they found out, in 2029 it's going to not pass through the keyhole. In other words, in 2036 - the big date when we thought maybe, maybe it might hit - it's going to miss, and it's going to miss by lot actually about 20 million kilometers, which is 50 times farther away than the moon. So we are pretty much safe from this particular rock for a long time.

RAZ: That's, that's, that's great news.

PLAIT: Yeah, I'm quite happy about this.

RAZ: Is a part of you a little bit disappointed, though? I mean - I mean, it would be kind of cool if it did hit, I guess, I mean if you were on the safe side of the Earth.

PLAIT: No. No. No. Look, you know, if one of these things could hit the moon, that would be awesome. We could get a good look at it. We could see everything that happens. We could test out all of our ideas about impact energies and all that kind of stuff, and we'd be perfectly safe, and we'd have a front row seat to this awesome event. That would be great and nobody gets hurt. You just can't do anything like that anywhere on Earth. Again, I'm very, very happy with this right now.

RAZ: So, I guess you can take some time off now, right?

PLAIT: Well, I wish but this isn't the only rock out there. There are plenty more, and there are quite a few whose orbits need to be refined better. And some of these overlap where the Earth is going to be, and they could potentially hit us. Apophis is a case where - you know, yay, it's going to miss, but there's a long list of other rocks out there that we need to be keeping our eyes on.

RAZ: All right, cool. Thanks, Phil.

PLAIT: Cool, thank you.

RAZ: Bye.

PLAIT: Bye-bye.


RAZ: You can find Phil's blog at It's called "Bad Astronomy."

Our show today, "The Wonders Above Us," and in a moment listening for signs of life in our galaxy, and maybe even beyond. We're peering into space here on the TED Radio Hour from NPR. I'm Guy Raz.


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