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| Near Earth Objects
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What is a Near Earth Object?
A Near Earth Object is a fragment of rock and ice hurtling through space that passes dangerously close to Earth. It can be an asteroid or a comet, vast or tiny. These constantly bombard the Earth, every day. Shooting stars are caused by asteroids burning up in Earth's atmosphere. This is what destroys most of these, but occasionally one is large enough or moving at such a terrifying speed that it collides with the Earth's surface, with massive destructive potential.
What damage could they cause?
A NEO does not need to be large to devastate. One the size of a small garage would annihilate a large city. One big enough to leave a 10km crater, still nowhere near the size of the biggest (there is a 300km crater on Earth), would have the destructive force of every one of the world's 10,000 nuclear warhead combined. The one that wiped out the dinosaurs was several kilometres across.
When have they hit before?
The most famous example of the sheer power of NEO's was the extinction of the dinosaurs. To do this a large meteor, several kilometres across, would be required. Comets imprinted the face on the moon, which are clearly visible from 90,000 kilometres away. Earth will have sustained many more collisions than this, but erosion here is much greater, masking the destruction caused by these in the past.
What would happen during a large collision?
In 1994 a large comet hit Jupiter. The Hubble telescope recorded the impact with terrifying accuracy. As it entered the atmosphere of Jupiter temperatures reached 20,000 degrees. The impact sent a fireball thousands of kilometres into space.
The damage of such an impact on Earth could kill all life. Huge earthquakes would streak around the globe at thousands of kilometres an hour, destroying all in their path. It would also trigger volcanic eruptions all over the world. A huge dust cloud would be sent into the atmosphere. Gasses released into the atmosphere would cause intensely acidic rain and destroy the ozone layer. A vast fireball would be sent into the sky. However devastating these would be, the real danger would be from the dust clouds sent into the atmosphere. The amount of debris released into the upper atmosphere would block out the sun, trigger a nuclear winter and potentially kill all life on Earth.
What about smaller impacts?
In 540AD, chronicles record the impact of a comet. They say that the 'whole sky seemed on fire' and that there were 'battles in the air'. One chillingly reports that 'real blood dropped from the clouds and dreadful mortality ensued'. These ominous words are backed up by evidence. Tree rings show how much a tree grew in each year, indicating the conditions at the time. In the years after 540AD these rings are remarkably close together, indicating stunted growth caused by a dramatically cold period. This was the result of the impact. But this was just its effect on trees - all over the world such an impact would have caused the famine, crop failures and plague outbreaks that occurred at that time. And this was only a small impact.
This may sound like the distant past but in only 1908 an asteroid with a 80m diameter smashed into the forests of Tunguska, Siberia. A fireball exploded into the sky with the force of 200 atomic bombs. The blast was heard 1000km away. Millions of tonnes of dust were blasted into the atmosphere. For 2,000 square kilometres trees were flattened. Fortunately Tunguska is virtually unpopulated but the area destroyed is that of size of a large city - the casualties of such a meteor in an urban area are horrific.
How many are there?
We simply so not know. The largest yet discovered are an awesome 25 kilometres in length. Scientists estimate that there are around 1000 asteroids larger than 2km that pass close to Earth. An impact by any one of these would be an Extinction Level Event. Just as terrifyingly, there are more than a million bigger than 50metres across - each capable of destroying a city.
How likely is an impact?
Impacts on this scale occur alarmingly often. An impact similar to the one at Tunguska occurs approximately once per hundred years. Slightly smaller ones, but easily big enough to devastate a city, occur at least 5 times per century, for example Brazil in 1930. Not all these will hit the Siberian wilderness or Brazilian rainforest - sooner or later one will hit a major urban area. Just as bad, one could fall in the sea and trigger a mega-Tsunami.
Those large enough to cause extinction, 2km in diameter, occur more rarely - approximately once per million years. This sounds to unlikely to be scared about? Scientists estimate that you stand a 1/20,000 chance of being killed by a meteor - much more likely than being winning much on the lottery.
No large NEO yet discovered is on course for earth - but the total world wide effort searching is just 100 people.
How can we stop them?
If we discovered a NEO was on course for Earth we would have only three options. We could try to destroy the asteroid with high yield nuclear weapons. This would be extremely risky as without detailed knowledge of the structure of the NEO, no one could tell how much it would take to destroy it. Also, there is the risk of incompletely destroying the asteroid. This would results in many small NEOs. Not only would these be much harder to stop, but they could also cause far more damage as the whole of the Earth's surface could suffer impacts.
The second option is to try to deflect the NEO. The most essential requirement for this is time, as the deflection required is inversely proportional to the time available. However, only small adjustments to the course of the asteroid or comet would be needed to make it miss Earth. The object could be diverted using nuclear weapons or placing 'mass drivers' on the object which, fuelled by substance on the asteroid, could divert its course.
The third is to try to prepare for the impact. Ground zero - the area of impact - would have to be evacuated. This area could be vast with a large NEO. Nuclear winter would mean that natural food sources would be unavailable for at least a year, the surface of the planet would be inhospitable for an extended period and most infrastructure would be destroyed, delaying a return to civilization.
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