Assume a comet impacts the moon

[onejayhawk]

What happens to us?

The standard doomsday scenario is that a massive meteor or comet cracks the Earth's crust. Suppose instead that something hits the moon hard enough for it to fracture.

Clearly, a large mass of matter will be scattered as debris from the kinetic strike. Some of that debris will have a decaying orbit and eventually encounter the atmosphere. How big do they have to be to be worrisome? How much reaction time would be available, assuming a large enough chunk is on a decaying orbit? Would blowing it up with nukes be a good or a bad thing?

There will be much bigger chunks. Will they tend to stay together due to gravitational attraction or will they become an asteroid field and new, smaller moons? How much would climate be affected?

J

 

[Lexicus]

I believe there have already been plenty of examples of moon rocks blown free by impacts and then entering the Earth's atmosphere. They are called lunar meteorites.

 

[red_elk]

IIRC comet cores are usually quite small to cause global scale consequences even if they hit the Earth. One of hypotheses for the cause of Tunguska event was a comet impact.
The doomsday scenario involves climate change after impact of large asteroid, like ~10 km in diameter or so. If asteroid of this size hits the Moon, I think it would not cause serious troubles to us.

 

[Michkov]

Have you read Seveneves? Pretty much starts with the moon breaking apart in the first paragraph followed by 500 pages of dealing with the consequences.

 

[Lexicus]

If the moon itself is entirely pulverized I would expect effects on earth to be much worse than the Endor Holocaust.

 

[onejayhawk]

IIRC comet cores are usually quite small to cause global scale consequences even if they hit the Earth. One of hypotheses for the cause of Tunguska event was a comet impact.
The doomsday scenario involves climate change after impact of large asteroid, like ~10 km in diameter or so. If asteroid of this size hits the Moon, I think it would not cause serious troubles to us.

Comet cores are quite massive compared to most meteors. Otherwise, all the volatile compounds would have boiled away ages ago. Also, comets are generally much faster moving than rocky bodies. Some comets are larger than 100 km. Regardless, the bollide has to hit the moon hard enough to break the moon into pieces.

J