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What If a Large Asteroid Was Heading for Earth?

Posted By Paul Comstock On December 2, 2008 @ 10:15 am In Non-Fiction Reviews,Science | 13 Comments

The asteroid “253 Mathilde” which orbits the sun between Mars and Jupiter.

CLR INTERVIEW: Philip Plait is an astronomer who has written for magazines such as Astronomy and Sky & Telescope, as well as many national and international newspapers. His website, Bad Astronomy [1], was named Best Science Blog of 2007. His latest book is Death from the Skies!: These Are the Ways the World Will End… . Below is Philip’s interview with the California Literary Review.

Death from the Skies!: These Are the Ways the World Will End…
by Philip Plait
Viking, 336 pp.

Your book covers several ways in which earth, or at least life on earth, might be destroyed because of a cosmic disaster. I’d like to focus on the one that seems the most likely and also the one that we may actually be able to prevent – an asteroid impact. First of all, what is an asteroid and why would it be heading toward earth instead of orbiting around the sun or another star?

Asteroids are chunks of rock or metal (some are both, actually) that orbit the Sun. The first one was discovered over 200 years ago, and now we know of thousands. The biggest is Ceres which is about 950 km (600 miles) across, but most are much much smaller.

Most of them stay between the orbits of Mars and Jupiter. But over time, the gravity of Jupiter can change their paths, and so some have more oval orbits that take them in closer to the Sun, and some of those intersect the Earth’s orbit. If they happen to be at the same spot in space at the same time as Earth, well, we get smacked. In some cases it’s no big deal; a piece of rock the size of a Volkswagen entered our atmosphere over Darfur in 2008 and all it did was make a very pretty light in the sky as it burned up. But when they get bigger, we can be in trouble.

How large would an asteroid have to be to make it through the earth’s atmosphere? How often does that happen?

The Earth is pelted by 20-40 tons of meteors every single day! But those are usually no bigger than a grain of sand. A piece of rock bigger than about 100 yards across can potentially make it all the way to the ground, but if it’s made of metal – many asteroids are solid nickel-iron – then a piece only 30 yards across can penetrate all the way down.

However, even if it doesn’t physically hit it can be bad. In 1908 a chunk of rock about 30-50 yards across blew up high in the atmosphere, and the explosion set fire to trees and smashed them flat for hundreds of square miles. So this is a threat astronomers take pretty seriously… even if something this big only hits us on average every few centuries.

Is it true that an asteroid is responsible for killing off the dinosaurs? How large was it, and what would be the short and long term effects of an asteroid of that size hitting earth tomorrow?

It’s thought that environmental problems were already starting a mass extinction 65 million years ago, when dinosaurs were around. But then an asteroid 6 miles across – that’s bigger than Mt. Everest! – slammed into the Gulf of Mexico just off the Yucatan Peninsula. The explosion was huge, setting fire to vast amounts of land, and creating a tsunami that must have scoured the Mexican and Texas coasts clean. It launched so much rock into the sky that they went on ballistic arcs, going up out of the atmosphere and then back down, setting fire to forests around the world. It also blasted bromine and chlorine into the air, which destroyed the ozone layer. After that, dust from the impact would have blocked sunlight, causing the Earth’s temperature to drop, killing off quite a bit of plant life and the animals that depended on it.

All in all, it was a global catastrophe of epic scale.

Happily, it’s incredibly unlikely to happen again any time soon. That’s a once-every-hundred-million-year event, and unlike the dinosaurs, we have astronomers to spot such rocks and a space program that we can use to prevent them from hitting us.

Do we know what he odds are of an asteroid hitting earth – one that is capable of doing serious damage to the planet?

Right now the chances are really low. Your personal odds of dying in an asteroid impact over your lifetime are about 1 in 700,000 – you’re just about as likely to die in an amusement park accident. And we know there are no dinosaur-killers out there liable to hit in the next few centuries at least.

Philip Plait, Ph.D.

How much warning would we have?

In general we’d have years of warning. An asteroid big enough to do global damage would be big enough and bright enough to spot a long time in advance. Comets, though, are tougher; they can be harder to detect. Hale Bopp, a really bright comet seen in 1997, was only discovered two years before it passed the Earth. Had it been headed right at us, that’s all the warning we would have had. And a small asteroid, one say 200 yards across (enough to take out a city but not the planet) would be tough to spot until just before it hit us… and we might never notice it before impact. We wouldn’t have any warning at all, except the flash of light in the sky as it came in.

Most importantly, is there anything we could do to stop it?

That’s the good news! We already have ideas on how to stop them. Sending up astronauts to plant bombs on incoming killer rocks isn’t a great idea: it’s hard to do, for one, and there’s no guarantee it would work.

The whole point is to get the rock to not hit us. So a group of scientists, astronauts and engineers formed the B612 Foundation to work on this problem. They came up with the idea of a “gravity tug”: a space probe that can tow the asteroid out of harm’s way. It uses the gravity of the probe itself to slowly move the asteroid into a safe orbit. It takes a few years, but hopefully we’ll have that much warning time. I’ll note we haven’t built anything like this yet, but if we see a rock headed our way, I suspect we’ll be plenty motivated to get it done.

What are we doing, in terms of research, to deal with the possibility of a large asteroid heading our way?

The first thing we need to do is look to the skies better. We have several telescopes scanning the heavens now, and more on the way. Some of these will look at phenomenal areas of the sky for moving objects, and should spot anything dangerous. NASA wants to have 90% of all potentially hazardous objects greater than 140 meters in size spotted by 2025.

Once we find one headed our way, well, we’ll have to see. The gravity tug should do the trick if there’s enough time. If there isn’t, we may have to launch a rocket at the thing and simply smash into it – the energy of impact is actually far larger than the detonation of a nuclear weapon. If we can do that early enough it may shatter the asteroid, and most of the pieces will miss us.

But again, it’s no guarantee. Our best bet is to find these guys, and find them now. The amount of money we need to spend on this is not terribly high, and the payoff is saving our civilization. That seems like money well spent to me.


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[1] Bad Astronomy: http://blogs.discovermagazine.com/badastronomy/