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
warhead combined. The one that wiped out the dinosaurs was several kilometres
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 240,000 miles 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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