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24 September 2014
Science & Nature: TV & Radio Follow-upScience & Nature
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An asteroid could cause huge loss of life if it struck a highly populated area
First shown: BBC Two, Thursday 23 January 2003
Averting Armageddon
Next on Horizon
How would London cope if a Dirty Bomb was exploded in Trafalgar Square? Horizon reveals exclusive research. 30 January, 9pm, BBC Two

Averting Armageddon - programme summary

The Earth is under constant bombardment. Each year, many fragments of debris hit our planet. Fortunately for us, most are so small that they burn up harmlessly in the atmosphere.

The scarred surface of the Moon - Image courtesy of Mike Whitecross
The scarred surface of the Moon tells us it has been hit many times

However, there are hundreds of larger asteroids orbiting near the Earth. Many scientists now believe that one of these hit the Earth 65 million years ago, killing the dinosaurs, along with 90% of all life on the planet. What is more, it is only a matter of time before the Earth is hit again.

Experts warn that nuclear weapons might not destroy an approaching asteroid. But Jay Meloch thinks he can use the power of the Sun to nudge an asteroid away from the Earth.

A violent Solar System

Until recently, no one took the asteroid threat very seriously. Yet the evidence that we are in danger is on our own doorstep. We need only look at the cratered surface of the Moon to realise that it has been pounded by impacts throughout its history.

Why then, if collisions were common, was the surface of the Earth not scarred in a similar way? Unlike the Moon, the geography of the Earth is constantly changing, as continents move, and the landscape is constantly reforming. However, scientists realised that many features they had once dismissed as extinct volcanoes could in fact have been made by asteroid impacts.

Shoemaker-Levy 9 hits Jupiter
Comet Shoemaker-Levy 9 hits Jupiter

Then in 1994, something happened which brought home how immediate the danger is. Astronomers realised that comet Shoemaker-Levy 9 was heading straight for the planet Jupiter. The spectacular - and violent - impact created an explosion the size of planet Earth, and was the first time a collision between two astronomical bodies had been observed. If Jupiter had been hit, then the Earth could be next.

Mapping the asteroids

Over 90% of the asteroids in the Solar System lie in a belt between Mars and Jupiter. Most of these are in stable orbits around the Sun, and will never come anywhere near the Earth. However, collisions between asteroids, or the effects of Jupiter's large gravitational field can sometimes deflect asteroids off course, and send them into the inner Solar System.

NASA has been conducting a survey of near Earth objects, and we now know of many asteroids whose orbits cross our own. So far, no imminent collisions are predicted; astronomers are though keeping a close eye on a rock they call 1950 DA, which will either hit, or pass very close to the Earth, in 2880.

"The chance that we are going to be hit by a large comet or asteroid is 100%"

Dr. David Levy - Astronomer

However, there is a long way to go before we have found all the asteroids that are a potential threat. We still don't know for certain that an asteroid will not hit in the near future.

Will nuclear weapons save the Earth?

The most obvious strategy to protect the Earth against an asteroid might seem to be to try to destroy it with nuclear weapons. This plan has two fundamental problems. Firstly, you would have to attach a bomb larger than any yet created, to a very powerful rocket. This might be nearly as dangerous as the asteroid you were trying to destroy.

More importantly however, a nuclear blast might not destroy a large asteroid completely, but merely split it into chunks. Instead of one large impact, you might end up with several smaller ones, which would end up doing nearly as much damage.

Could we use nuclear weapons in a 'gentle' way?

If we cannot destroy an approaching asteroid, then the only other tactic would be to try to nudge it forward just enough to make it miss the Earth - like stepping on the accelerator of a car to make it miss a train at a level crossing. If we had enough warning, then only a very small deflection might be enough.

Rather than firing a nuclear weapon directly at an asteroid, could we explode it nearby so that the blast gives the asteroid the nudge it needed to miss the Earth? For a while, scientists thought they had found a solution. But then some surprising results forced them to think again.

Asteroids like sponges

Three years ago, the residents of Tagish Lake in northern Canada witnessed a bright explosion in the sky, as an asteroid burned up in the atmosphere above them. Jim Brook was lucky enough to find debris from the impact. The first thing he noticed was that it was far lighter than he expected it would be. Like a sponge, the chunks of debris were mostly air.

"I was very surprised at how light it was"

Jim Brook discovers an unusual asteroid fragment

Dan Durdan makes his living by firing ball bearings at asteroid samples - meteorites - to study what happens when they are hit. When he tested samples similar to the Tagish Lake meteorite, he was surprised to see that, rather than shattering or being deflected, these less dense asteroids simply absorbed the impact of the blast.

These results were worrying. This could mean that many asteroids would not be deflected by a nuclear blast. Trying to deflect an asteroid with a blast might have no effect, and would keep it coming on its deadly trajectory.

What can a spinning asteroid tell us?

The question now facing scientists was: how many asteroids near Earth are like the Tagish Lake samples? The only way we can know for sure what an asteroid is made of is by landing on it. But could there be a clue in the way the asteroids behave?

Most asteroids spin around as they travel through space. Some spin slowly, and others quickly. But if an asteroid is spinning too quickly, there is a danger that it will tear itself apart if the material from which it is made is not strong enough. By surveying the spin of asteroids near the Earth, scientists can make a very rough estimate of how dense the asteroids are.

There was good news and bad news. Asteroid 1950 DA was spinning quickly, and so is likely to be fairly dense - and could potentially be deflected with a nuclear device. But the majority of asteroids surveyed were slow spinners. Clearly, scientists were going to have to come up with another solution if they wanted to protect the Earth.

The power of the Sun

Jay Meloch has suggested a radical new way of dealing with a dangerous asteroid. He wanted a surer, more controlled way of diverting a large body - with a gentle push instead of a blast. His idea was to find a way of harnessing the biggest power source in the Solar System - the Sun.

In the same way as you can use a magnifying glass to set fire to a sheet of paper, you could focus the Sun's rays onto a point on the surface on an asteroid. The spot where the Sun's rays met would heat up, blasting particles of the asteroid into space. This would act like a rocket engine, and might be enough nudge the asteroid out of harm's way.

The scientific community ridiculed his suggestion - until Meloch received a phone call from someone who took his idea very seriously. The US military already uses collectors like Meloch's to gather radio waves. Meloch may well have come up with a suggestion that will one day save the Earth.

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