Would we all perish due to excessive heat? Or would that be limited to the area near the impact while the people on the rest of the earth would die from other phenomena such as mega earthquakes, volcanic activities, tsunamis etc.? Does it matter where the impact is – if it landed in Antarctica, would we have massive floods, but if it landed in the middle of the Eurasian continent, would the effect be different?
[Physics] What if an asteroid the size of the moon hits earth
asteroidsearth
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Dr. Phil Plait has written about this extensively. He has a book (Death from the Skies) with a chapter that deals with this.
He has a blog entry about this very subject as well (in addition to a link to one just talking about getting hit by a meteorite). Here is an excerpt:
what are the odds of getting killed by one?
Turns out, they’re a lot higher! Why?
Because you are small and the Earth is big, getting knocked on the noggin by a meteorite is a low odds event. But a big meteorite, say one 100 yards across, doesn’t have to directly fall on top of you to shuffle you off this mortal coil. It could land kilometers away and the blast wave (or the heat) could do you in. And a bigger one can land hundreds of kilometers away and still snuff you out, especially if it hits in the ocean and causes a big tsunami to march over the beaches and coastlines.
However, big asteroids coming in and whacking us are much rarer than small ones; if you go out on a clear night you might see a dozen meteors caused by rocks smaller than a grain of sand, but you could wait 100 million years for a dinosaur-buster. You have to account for that as well. This is a calculation worth doing, because a) a lot of people fret about it, and b) it could in fact mean the end of all life on Earth. That might be worth knowing.
Astronomer Alan Harris has made that calculation. Allowing for the number of Earth-crossing asteroids — the kind that can hit us because their orbits around the Sun intersect ours — as well as how much damage they can do (which depends on their size), he calculated that any person’s lifetime odds of being killed by an asteroid impact are about 1 in 700,000.
One out of seven hundred thousand! That’s still pretty low… and certainly not enough to lie awake at night worrying about it.
But there are a few important things to consider.
1) A big asteroid is rare, but one bigger than about 10 km across would kill everyone, all 6 billion of us. That skews the odds. If one of those hit every 100 million years, then your lifetime odds of dying in an impact is 100 million years divided by 70 years = 1 in 1.5 million.
A small impact might happen 1000x more often (every 100,000 years), but might only kill 1/1000th as many people, so the odds are roughly the same. Weird.
2) We are lousy at understanding low probability events. I know that 1 in 700,000 is a ridiculously low probability, but it’s hard to grasp. As a comparison, you’re more likely to die in a fireworks accident. But what’s funny is, this is a slightly higher chance than being killed by a terrorist! Despite propaganda to the contrary, the odds of any given person being killed by a terrorist attack are incredibly low. While terrorist attacks in the long run are a near certainty, the odds of you getting killed are very low.
It’s like the lottery: someone wins every time (eventually), but chances are it won’t be you.
Worrying about preventing a terrorist attack is a good idea, but (unless you work in a high-risk job) worrying specifically about dying in one is not*.
Incidentally, you have about the same odds as being killed on an amusement park ride. Wheee!
It really all depends on how you look at it. In his blog entry, he does get into how as a species, we actually have the ability to prevent this sort of a disaster with a relatively modest amount of spending and effort. So the odds may actually decrease as we get our act as a species together.
The actual effects of a Gamma-ray Burst on the earth have been thought about a great deal. There is some thought that the Ordovician extinction was caused by a GRB (PDF Paper). This is also detailed in a couple of astrophysical journals.
Gamma-ray bursts (GRBs) directed at Earth from within a few kiloparsecs may have damaged the biosphere, primarily through changes in atmospheric chemistry that admit greatly increased solar UV. However, GRBs are highly variable in spectrum and duration. Recent observations indicate that short (~0.1 s) burst GRBs, which have harder spectra, may be sufficiently abundant at low redshift that they may offer an additional significant effect. A much longer timescale is associated with shock breakout luminosity observed in the soft X-ray (~103 s) and UV (~105 s) emission and radioactive decay gamma-ray line radiation emitted during the light-curve phase of supernovae (~107 s). Here, we generalize our atmospheric computations to include a broad range of peak photon energies and investigate the effect of burst duration while holding total fluence and other parameters constant. The results can be used to estimate the probable impact of various kinds of ionizing events (such as short GRBs, X-ray flashes, and supernovae) on the Earth's atmosphere. We find that the ultimate intensity of atmospheric effects varies only slightly with burst duration from 10-1 to 108 s. Therefore, the effect of many astrophysical events causing atmospheric ionization can be approximated without including time development. Detailed modeling requires specification of the season and latitude of the event. Harder photon spectra produce greater atmospheric effects for spectra with peaks up to about 20 MeV because of greater penetration into the stratosphere.
Gamma-ray bursts (GRBs) are likely to have made a number of significant impacts on the Earth during the last billion years. The gamma radiation from a burst within a few kiloparsecs would quickly deplete much of the Earth's protective ozone layer, allowing an increase in solar UVB radiation reaching the surface. This radiation is harmful to life, damaging DNA and causing sunburn. In addition, NO2 produced in the atmosphere would cause a decrease in visible sunlight reaching the surface and could cause global cooling. Nitric acid rain could stress portions of the biosphere, but the increased nitrate deposition could be helpful to land plants. We have used a two-dimensional atmospheric model to investigate the effects on the Earth's atmosphere of GRBs delivering a range of fluences, at various latitudes, at the equinoxes and solstices, and at different times of day. We have estimated DNA damage levels caused by increased solar UVB radiation, reduction in solar visible light due to NO2 opacity, and deposition of nitrates through rainout of HNO3. For the ``typical'' nearest burst in the last billion years, we find globally averaged ozone depletion up to 38%. Localized depletion reaches as much as 74%. Significant global depletion (at least 10%) persists up to about 7 yr after the burst. Our results depend strongly on time of year and latitude over which the burst occurs. The impact scales with the total fluence of the GRB at the Earth but is insensitive to the time of day of the burst and its duration (1-1000 s). We find DNA damage of up to 16 times the normal annual global average, well above lethal levels for simple life forms such as phytoplankton. The greatest damage occurs at mid- to low latitudes. We find reductions in visible sunlight of a few percent, primarily in the polar regions. Nitrate deposition similar to or slightly greater than that currently caused by lightning is also observed, lasting several years. We discuss how these results support the hypothesis that the Late Ordovician mass extinction may have been initiated by a GRB.
However, the book that I recommend the most is Dr. Phil Plaits Death from the Skies. Not only does he explain it in a manner that is understandable and still has the appropriate detail (i.e. it would be devastating, but the chances are remote). Furthermore, he discusses the actual chances in realistic terms instead of being too nebulous or evasive. I think this book is really the best book for answering your GRB question.
Best Answer
First of all, there are no Moon-sized asteroids in the (sufficiently inner) Solar System. The largest asteroid has radius 450 km which is about 4 times smaller (64 times smaller volume) than the Moon. Moons of planets are not counted as asteroids.
A collision with a Moon-sized object would of course be a terminating catastrophe for the Earth. If you look at the explanation of the Torino scale of such impacts
you will see that the largest diameter even mentioned in the graph is smaller than 10 km, and those already exceed the highest Torino 10 scale which corresponds to the global destruction of the civilization.
You are talking about a collision with an object that is 400 times larger in linear dimensions which means 64 million times larger in volume and mass.
If such an object had the usual velocity, and it would have to have because no similar large objects are "synchronized" with the Earth near its orbit (which is needed for the relative speed to be much lower), the impact energy would be so high that the whole Earth would melt.
This is not something unprecedented. After all, the Moon was (probably) created after a similar collision. The Earth used to be smaller at that time. A large celestial body collided with the Earth. The whole Earth melted and a sufficient piece of the Earth – and perhaps with pieces of the other body – went into space and became the seed of the Moon. The Earth itself ended up larger than before the collision.
Another collision like that would probably create another Moon from the escaping material (most likely, a smaller one than the Moon we already have because the attraction of the now-larger Earth is stronger and keeps most of the material here) while the rest of the Earth would grow even bigger than it is now.
The geological processes would start from scratch. I don't know where and how life could survive. Perhaps some bacteria in our satellites etc. would survive and could return, to speed up the evolution. Just to be sure, no one would survive by hiding in a basement, not even a heavily fortified one.