http://smallcomets.physics.uiowa.edu/

[...] things like that. Playing up the risks and all, well past the point of reason. Not that you should turn your back on them — if nothing else, they could come in handy down the road. But then again, maybe [...]

So far, I understand the best plan to divert a potential impactor is a gravitational tractor. That involves putting a spacecraft into the same trajectory as the asteroid, just a short distance from it, and affecting its orbit just enough to make it miss us.

The trick is to find enough funding to have the system all developed, tested, and ready to go when we need it. Then, assuming a few years' warning, we could divert the thing in time.

Bruce

A) we can't track them cause we can't see them.

B) if we spot one and it's due to collide with the Earth, then we're hooped anyways.

Am I right?

Comets, especially long period comets with orbits of 200 years or more, are very large, come through our inner solar system infrequently, and would be almost impossible to stop.

Asteroids have smaller orbits, can be tracked, and possible scenarioes can be figured out for the intervention of their impacts with Planet Earth.

As evidenced by the flattened trees and trees that were stripped of their branches in the Tunguska Event, a comet that was big enough to not burn up in the atmosphere could still produce enough destructive energy upon impact to vaporize itself.

Comets tend to be less of a problem because they are rarer and every one around tends to be discovered quickly – actually there's an ongoing contest among observers, amateur and professional to be the first to discover new ones. A couple of months back, a minor comet 17pHolmes blew a gasket and brightened up by a massive amount, going from virtually invisible in a telescope to being visible to someone without a telescope. The gas ball surrounding the comet grew to bigger than the diameter of the sun and the apparent size of it grew to be bigger than the apparent size of the moon. What's more – it's done that trick in the past.

When you see a meteor shower like the orionides, perseids etc, remember these are cause by dust particles left along the orbit of a comet.

Offhand, it would seem that even small tiny chunk of a comet would be potentially dangerous to people were it to wind up arriving here on earth in a populated area. A substantial chunk could be quite dangerous to civilization.

Years ago I was traveling SR-395 (backside of the Sierra's) from Reno to Los Angeles at night. Was mesmerized by a bright streaking light that continued getting bright (knowing full well what it was at the moment)… it continued getting brighter, as it slowed, lighting the night, as if it were day, and finally burned out (pooof)… all went dark! No explosion, only it was "lights out"… followed by "did you see that?"

So what causes em' to explode, what would a foreign object be made of to create another Tunguska?

I guess that is true that the kinetic energy is what matters, as well. I suppose regardless of whether or not it burns up in the atmosphere that energy is still going to have to go somewhere, no matter what the size of the object.

Also, what is the critical size needed for an asteriod, meteorite, or comet to be able to get through the atmosphere? If that is known, then any object that might get through the atmosphere can be broken up (with the application of nukes I'd imagine).