How big would an asteroid have to be before the resulting fallout destroyed all life?

Size is not the only factor. Size just means its dimensions but there are other factors which need to be taken into account, for example:-

- Mass

- Angle of impact

- What the asteroid is made of (e.g. mainly ice, rock or iron)

- The location of the impact (land, ocean, soft rock, hard rock, etc)

- Its velocity

Therefore it is impossible to give a minimum size. However, this impact simulator from Imperial College, London lets you play around with the variables and see what effect you would get>>> http://www.purdue.edu/impactearth/

Yes it has been calculated. (and no, I don't remember the value)

And size, by itself, is not enough to guarantee a total kill.

Killing off ALL life is a big task.

The gigantic asteroid that killed off the dinosaurs did a lot of damage to life... but it did not end it.

A big enough asteroid (say over 24 km across or 15 miles), regardless of speed or angle, would probably carry enough kinetic energy to cook the entire crust (and boil off all oceans, and turn the atmosphere to an oven, and fill the air with soot and dust and...), thereby doing a LOT of damage to ANY life.

As the asteroid comes to a sudden stop, all its kinetic energy is transformed to heat. THAT is the real problem from something that size. That is why the biggest damage from the dinosaur-killer was the world-wide fires that followed (the mechanical damage was pretty well restricted to the hundreds of kilometres around the impact point).

Wiping off the humans would probably require something around 10 km. Something like that would probably do away with most (but not all) life. If the speed, angle, point of impact, etc., happens to be "just right" (or, in this case, just wrong). To give you an idea of the size: when the front end touches Earth's surface, the other end is still at the altitude where transatlantic passenger flights operate.

No matter the size of the asteroid, it would not wipe out all life.

It would take a body the size of Mars hitting the earth to completely destroy it. This would be the end of life as we think of it, and certainly humans. Anything less than total destruction of the planet would not destroy us humans, we are just too stubborn to die out.

If the planet was completely destroyed, and fragments of earth flew out in all directions, then the extremophiles would still survive, cast adrift into the void in their states of suspended animation. Eventually, it is likely that some of these pieces of earth would crash land on other bodies in the solar system. If it hit a place like Mars or Europa, both of which could theoretically support life as we know it, life could re-volve all over again. We would see another biosphere develop, and maybe even a new sentient species arise. Perhaps then some member of this species would create a search engine website, that would include many other services, including a user-driven question and answer service, and vuala, this question gets asked all over again.

There's many factors - the composition of the asteroid, where it may impact, speed, angle of descent, etc...

A rock 6 miles wide destroyed about 90% of the larger animals 65 million years ago; it's *believed* to have been a mostly rocky meteor. One that was mostly iron/nickle would cause a lot of damage - but because of it's density, it may be *less* damaging to the planet as whole, given the same speed and mass. A comet of the same speed and mass may be far more damaging, since it will begin exploding *prior* to impacting the ground (or ocean), and continue to release energy *after* impact.

For argument's sake, probably 4 to 6 miles across would be a very damaging rock, wiping out much of the world's population. Less than this size will still be *very* damaging, but (perhaps) not on a global scale. Larger than 6 miles, and we'll see a repeat of the dinosaur-extinction event. Man may not be wiped out (we're pretty smart... we can figure out how to keep a few of us alive, I'd bet...) but we will be severely reduced in number.

About the size of the Isle of Man would do it.

Whether it fall straight, at an angle, in the sea or on land.

The general speed is around 40,000mph.

It would break clean through the Earth's crust and into the mantle, the shock would reverberate all around the surface of the Earth.

Mega tsunamis, Mega volcanoes and massive earthquakes would crack the crust open at the fault lines.

The fall out from the ejecta would be worldwide.

How big would a truck have to be to completely flatten a Pinto in a collision?

What? Depends on how fast its going? How about where it hits? How fast is the Pinto going? Gosh.

ALL life??

Have to be pretty darn big.

There's life, even in deep ocean trenches y'know.

It depends on other things like: composition, mass the place it hits.

About the size of Oprah Winfrey.