Can we survive the next major asteroid impact?

We have been around for a couple of million years or so and we tend to take it for granted that this happy situation will continue indefinitely, but will it?

We know from fossil records that on a fairly regular basis, to the order of 26 - 30 million years or so, mass extinctions occur. Various theories have been proposed to explain this. One current theory is that every 30 million years the Earth is subject to heavy bombardment by asteroids, or comets. There is geological evidence of a thin layer of material at a depth representing an age of 65 million years that appears to be the result of an enormous asteroid impact. The impact site is believed to be in the Gulf of Mexico, off Yucatan, and to have caused the extinction of the dinosaurs, along with 60% of all other species. This crater is more than 100 miles in diameter.

So what is causing these periodic episodes? One theory hypothesises that as our solar system moves around the galaxy within its galactic spiral arm, it bobs up and down the plane of the spiral arm in a 30 million year cycle. Thus every 30 million years it passes through the densest region of the arm where the stars are packed closely together. Either this region is heavily populated by comets and asteroids or those within our solar system are tugged out of their normal harmless orbit and sent hurtling towards us.

Another theory, the Nemesis theory, proposes that our sun has a companion star, Nemesis, that every 26 - 30 million years comes close to the Earth and causes comets or asteroids to crash into us. See Our Nemesis?

So what happens when the Earth is hit by a large chunk of rock? A one kilometre asteroid striking the Earth at a typical speed of 25 to 30 kilometres per second would have a devastating effect. On impact the enormous kinetic energy of the body will instantaneously be dumped into target rock in an explosion equivalent to 300,000 megatons of TNT - the largest man-made nuclear weapon had a yield of 60 megatons. Flash and blast from the impact will destroy an area the size of Belgium. A 20 kilometre wide crater will be excavated in seconds, and debris will be ejected into sub orbital trajectories. This debris will later re-enter the atmosphere like a massive meteor shower all over the planet creating an intense global heat pulse, raising fires that will destroy a significant proportion of the biomass. The ozone layer will be obliterated. Major volcanism and seismic activity will occur as the shock wave from the impact ripples through the planet. Intense acid rain resulting from the ionisation of the air as the impactor entered the atmosphere, and large quantities of pyrotoxins produced by global fires will fall world-wide.

In addition to these effects, an impact at sea will produce a significant tsunami, capable of travelling considerable distances, and possessing enormous energy. Such surges will pose a substantial threat to low lying coastal areas. An impact in the Atlantic Ocean by a 1 kilometre asteroid will create a deep water wave 10 to 15 metres high. When it hits the continental shelf of Europe and North America, travelling at 600 kilometres per hour, it will run up a wave height of between 300 and 800 metres, depending on coastal topography.

All of this will cause a global environmental and humanitarian disaster of extreme severity, but the main threat to life will be the vast amount of dust and debris injected into the upper atmosphere, combined with smoke from the firestorms. The surface of the Earth will be shrouded in darkness and it is this that will pose the greatest threat to the global ecosystem as photosynthesis stops, food chains collapse and cold and starvation set in. After a year, or perhaps two, the atmosphere will clear, but the Earth's albedo will be higher due to snow and ice, and it will reflect more of the Sun's radiation, leading to a runaway feedback situation, possibly leading to a new ice age.

All that from just a 1 kilometre wide asteroid! Statistically, we are hit by an asteroid of this size, not every 30 million years, but every 100,000 years.

At midnight GMT on August 10th 1998, asteroid ML14 crossed the orbit of the Earth at the exact point the latter had occupied only 18 hours earlier. Had ML14 reached that point at 06.00 the previous morning, an area the size of France would have been totally devastated by 06.05. By 08.00 most of the world's vegetation would have been in flames. By late October 30-40% of the human race would be dead or dying. ML14 has a diameter of 2 kilometres.

In 1908 an impactor detonated some 5 kilometres above the ground in the Tunguska region of Siberia which yielded an estimated energy equivalent to 20 megatons of TNT. A Tunguska sized impact over London would destroy everything within the M25. The impactor would have been in the size range of 50-100 metres and the statistical time-scale for such impacts is between 50 and 100 years.

It is not a question of if we will be hit by a mass extinction sized asteroid or comet, but when. It is going to happen. Whether or not the human race will survive the impact remains to be seen.

What do I think?

Sitting here, in front of my computer, on a pleasant summer's afternoon, it seems improbable that I am contemplating the human race coming to an end. However, it could. We could be hit tomorrow, we wouldn't necessarily even see it coming. We didn't even see asteroid ML14 until it was going away from us.

It doesn't have to be an impact either, a nearby supernova would destroy all life.

One chance we have is that if we are given another twenty years or so, we could develop a missile defense system and use a nuclear blast to nudge the asteroid off course, providing we spot it in time. Plans are in place for such a defense system, but you know how it is with budget cuts.

I can imagine there are lots of people thinking it could never happen. Why not? Did you not see comet Schoemaker-Levy smash into Jupiter in 1994? Had it hit Earth instead, we would not be here, not any of us. Some people think that such a terrible event simply could not happen, simply because it is so terrible. Perhaps they believe that someone is watching over us. Take a look around, do you think someone is watching over us? See Are all religions false?

As I said in Is there a reason for our existence? I do not think we are here to serve a purpose, or that anyone is looking out for us, (God for example), but that we just happen to be here, oddly enough because of the asteroid that destroyed the dinosaurs. Next time it could just as easily be us. There are no guarantees. We're nothing special in the great scheme of things.

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