[Snail]

Are you sure that website is accurate? I entered the statistics of Pluto (density, size) moving at the speed of a comet and yet it says that Earth loses negligible mass.

[Flipside]

Not really, no, Jupiter is one hell of a bigger target, with one hell of a bigger gravity well, Earth's been hit time and time again by large comets, meteorites and all sorts of junk over several millennia. It's still here, and so is life. Even the moon, which is 1/3 the size of Earth, has no atmosphere, water or molten core to absorb the impact, and despite the fact it has been hit by even bigger meteorites more frequently, is still there.

It's not impossible, look at one of the moons of Jupiter for a really bad hit, but then, that was an object caught in a Gravity well something like 9000 times stronger than ours.

But we are disregarding the probability of the impact AND the thread creator asked for the worst-case scenario. 

Damn this thread grows too fast.

Well, the worst case scenario is that the Earth fractures through the middle and splits in two and we all end up starring in an alien version of the Final Fantasy movie

[Retsof]

I never got how all those "earth fractured" thing worked.  You end up having all these little "islands" floaing aroung in one big atmosphere.  I'm pretty dang sure that's a stretch even for fiction.

[Flipside]

Transfer of momentum between a moving and (relatively) stationary object could cause dispersion, but it would have to be one heck of a junction of vectors, after all, the moon was most likely caused by Mars bouncing off of us early in the creation of the Solar System, and the Earth survived that impact despite having millions of tons of mantle scraped off of it.

[watsisname]

Worst case scenario?  We're discussing comet impacts, and they have a limit to their destructability because they are only so big and are only so dense.  Most comets are between 1 and 10km across, and wikipedia says the largest discovered is predicted to be ~40km, so let's go with 40km.  At 51km/s, that's an energy of over 10 billion megatons.
The fireball alone would be >600km in radius (yikes), and the airblast would be lethal even 5000km away.  But still, nowhere near enough to "fracture the earth".  You'd need one hell of a big impact to fracture a planetary body, and no such impact is known to have occured since the formation of the solar system.

[Flipside]

As I said, one hell of a junction of vectors, Earth already has some big cracks in it, it's not just a question of how big, or how fast, but where it hits.

And the worst case scenario doesn't have to apply to 'averages', that's what worst case means. The largest discovered doesn't mean the largest there is. For all we know there could be a 200-300km Comet with our name on it, and an orbital period in tens or even hundreds of thousands of years which is so far beyond the orbit of Pluto that we don't even know it exists at the moment.

That's a 'worst case' scenario

Edit: Oh, and there is evidence of shattered moons in Saturns' ring system, though not much evidence of shattered planets, unless the Myths of 'Vulcan' are true:

http://en.wikipedia.org/wiki/Transit_of_Vulcan

Which, in truth, a massively doubt, but I've always found it an enchanting thought

[Flipside]

That website isn't entirely accurate, apparently a 1000km diameter iron object striking the Earth at around 72km/s would cause:

    The Earth is not strongly disturbed by the impact and loses negligible mass.
    25.89 percent of the Earth is melted

Which seems to contradict itself.

[Uchuujinsan]

Not really - if something is melted, it still retains its mass.
The earth only looses mass that gets thrown into orbit or reaches escape velocity. Also dont forget that the impact mass also gets added to the earth

[watsisname]

The thing is that if you want to discuss an impact capable of disrupting the Earth, you're talking about an impact that isn't known to have occurred for billions of years.  This simulation is more suited for impacts that occur in our solar systems present state, meaning things smaller than a hundred kilometers across.  Also it completely ignores climatic effects, which renders its use for the discussion of long-term effects to our ecosystem somewhat moot.

Yeah, we can toss figures of impact energy around and debate how hard an effect that has on life in the longrun, but how much physical data can we use to back that up?  AFAIK the best known impact-created extinction event was at the K-T boundary, the details of which are still not fully understood.

[Flipside]

Well, as I stated earlier, something the size of Mars wasn't even enough to totally disrupt the Earth (though, in fairness, it was probably molten back then and therefore a great deal more malleable).

Though, in truth, I was partly doing it to prove a point. One of the reasons I love this place is because when I said that the odds of doing permanent and irrevocable damage to the Earth were low, people disagreed, so instead I said that it could do permanent and irrevocable damage to the Earth and people disagreed

Edit: The Earths molten core is like a Shock Absorber, the odds of it getting actually disrupted are miniscule, even the odds of wiping out all life on the planet are miniscule, I'm pretty much certain of that

[watsisname]

Yes, I think we can agree that life would survive a comet impact.  The original question was what would it be like for the survivors?

[Flipside]

Dark, and probably quite short

[Maniax]

Some fascinating thoughts here, thanks guys!

Wanderer, do you know what would be required in order to bring about the scenario you suggest here?

In worst case... asteroid/comet impact energetic enough could heat the material to vaporize the rock formations into more or gaseous mixture.. Which would scroch, boil and burn pretty much everything on the surface of the planet. Pretty much end game for everything with possible exception of the life formed around black smokers in the deep sea though large enough impact would erase those too.

[Bobboau]

WORST case? I don't think we've covered worst case, because that would include things like rogue Jovian planets/brown dwarfs/black holes plowing through us out of the blue.

[watsisname]

A planetary "collision" with a black hole...  that would be interesting.

[Excalibur]

Well, there's that 2km - 60km/s stuff that crashes into Jupiter:

Sucks to be people within 500km, but wouldn't cause the destruction of the Earth.

[attachment deleted by Tolwyn]

[BloodEagle]

Well, there's that 2km - 60km/s stuff that crashes into Jupiter:

Sucks to be people within 500km, but wouldn't cause the destruction of the Earth.

Isn't Jupiter a gas giant?

[Excalibur]

If an Earth crashed into Earth at 12km/s and angle 15 degrees, you may have some survivors on the opposite side....

[TrashMan]

Doubtful. Methinks an impact of SUCH magnitude would leave Earth without atmosphere

[Snail]

If an Earth crashed into Earth at 12km/s and angle 15 degrees, you may have some survivors on the opposite side....

Okay, that website is definitely filled with bull****.