Theres lots 100 meter asteroids out there headed towards earth prbit that we cannot see.

They track hundreds if not thousands. But if we cannot see them yet and they are coming to earth they must be alot smaller or even further away.....if we can see the 140m asteroid... and obviously (to some extent) the smaller they are the less we have to worry.

Naturally that ain't 100% correct, they don't even have to hit the earth surface to cause destruction on earth.

I wonder if they have a set size an asteroid has to be before they consider it "potential" to cause damage if there was a collision with earth but then I guess we don't always know the composition of them.... any one know much on that?

I wonder if they have a set size an asteroid has to be before they consider it "potential" to cause damage if there was a collision with earth but then I guess we don't always know the composition of them.... any one know much on that?

Formula for a Sphere(oid) shows this object is 2.75 times more massive than the Tunguska event object, which is currently believed to be only 100m diameter. Advanced computer modeling, with some new theories and applying all known physics, has shown that it takes far less to be destructive than was once thought.

If I remember correctly, It is currently believed that a 30 meter diameter object can destroy an entire metropolis in an "airburst" scenario.

The number of objects of a particular size increases exponentially as the size you are investigating decreases. Based on volume and mass, we would expect a 30m diameter object to be about 37 times as common as a 100m diameter object, if the distributions are inversely proportional to mass.

sirchick: Any asteroid bigger than about 10 meters across can cause major regional damage.

At ~10 meters an asteroid can destroy a city.
At ~30 meters, it can destroy a small country (equivalent to a 15 megaton nuke or so).
At ~100 meters, it can cause low to moderate planet-wide damage.
At ~1000 meters, it can cause severe planet wide damage.
At ~5000 meters, mass extinction occurs.
At ~50000 meters, not much but bacteria survive.
At ~500000 meters the entire crust of the planet and upper mantel liquifies, extinguishing all life.

...the Tunguska event object, which is currently believed to be only 100m diameter.

The most recent simulations indicate that it was between 30 and 50 meters in diameter, depending on the composition.

gopher65 - you have to specify your density/velocity assumptions to make your table valid.

The amount of energy in a strike is mass X velocity squared. The mass of an asteroid of a specific size is determined by its density, which can vary all over the map. Even more important is its velocity, which is essentially determined by its orbit. A slow moving quite massive object could do considerably less damage than a much smaller, more dense, fast moving object. The impact velocities are incredibly variable. A hyperbolic orbit moving in the opposite direction of the earths orbit could impact as fast as 20K kilo/sec. A co-orbital object could hit us at a few hundred kilo/sec or even slower.

So unless you are talking absolute worse case, iron-nickel asteroids traveling at 20,000 kilo/sec, your table is misleading.

Unless the orbit changes by 0.00000001% in the next 28 years, then it's headed for New York!

astroid apophis would have already shattered earth by 2029

angle, and target material

In addition to all the other factors mentioned by others, I was going to add these. Whether it lands over land or water is a significant factor, as well as desert, mountain, etc.

Taking all things into account, the odds of a significant impact happenning in our lifetime are slim. Furthermore, the odds of one that does more damage than an average landfalling hurricane are exceptionally slim. And finally, it's likely that any event like Tunguska would happen in an area uninhabited by people. As with Tunguska, there might not even be anyone to witness it. However, an ocean impact might be amplified by secondary effects, such as land slides along the contintinental shelves, which could cause much more damage through tsunami than the impact would.

gopher65 - you have to specify your density/velocity assumptions to make your table valid.

Those are for dead average velocity and composition.

When you get to the larger ones (10 kilometers) it stops mattering too much, since the damage at that point is already going to cause a near total extinction of life even under the best circumstances.

But for the smaller ones, speed is the big variable (though the other stuff you all listed matters too). Speed varies from a slow 10km/s (if they're in a similar orbit to Earth and catching up from behind when they hit) to a staggering 70km/s if they're coming in from the Oort cloud. 20-40 is normal.