Total kinetic energy (presuming it were all directly converted to heat) of a 1 km diameter, 3000 kg/m³ asteroid impacting at 25 km/sec would be around 490 exajoules, around 3.2% of the daily solar bolometric energy of ~14970 exajoules. It’s the same kinetic energy all in one lump or broken up into a bunch of small parts, so presuming that is the starting object, it isn’t terribly significant in the grand scheme of things and I don’t know that there would be any long term climate change due to the heat from impact. If it did impact to the surface there’s a possibility of an impact winter occurring which may decrease the average temperature for several years or decades.

I don’t think a broken up object would impact over a period of several days, either. If the object is broken up enough so the parts are no longer gravitationally bound to each other, a very small change in velocity of any of those parts - a few cm/sec - would be enough to cause a complete miss of the planet, at least on that particular revolution.

The term “planet killer” can mean a few different things. If you are thinking of something large enough to completely shatter a planet - larger than the impact which created Earth’s moon, for example - nuking it ahead of time will have no effect on it. If you mean something that would normally cause a mass extinction event (like the K-T event) that’s also probably too much for any explosive to deal with but more creative means, such as perhaps spinning up the asteroid or comet using local water as propellant and solar energy as a power source to the point where it falls apart might produce approximately that effect. If there’s a “all the free kinetic energy you want” M-Drive available, even in test, that would solve that problem nicely.

Generally when talking about using nuclear weapons on an asteroid, it’s not to destroy it but to deflect it. The principle is that a nuclear detonation above the surface by 100 to 400 meters or so cause a part of the top layer of the surface to heat and eject off at greater than escape velocity, causing a small but high enough thrust for a deflection if done long enough ahead of time. That’s for the 150-250 or so meter diameter asteroids, which wouldn’t be extinction events anyway.

If this has been discussed here before, it would be interesting reading.

On Dec 29, 2019, at 7:05 PM, Kurt Feltenberger <xxxxxx@thepaw.org> wrote:

Folks,

I'm trying to develop some backstory for a world that has had a societal collapse from several different threat vectors.  The world is very much Earthlike; consider it being similar size, hydro, etc. when the event happens.  It's just on the cusp of developing the J-Drive, Fusion, and M-Drive, so some of these things are already operating in some form of testbed.

A comet or meteor is discovered that will hit the planet and naturally after initial panic is over, they decide to try and destroy it with nuclear weapons.  The plan works...to a point.  The single object is destroyed, but now it's created a swam of smaller objects that will pass by/impact the planet over a two day period.

I recall reading a discussion on this list many years ago (pre 9/11/01?) about something similar and the effects that all the smaller objects would have.  IIRC, even if they didn't impact and cause significant damage, they would burn up in the atmosphere and raise the average temperature, the debris would linger in the atmosphere and potentially help cause a greenhouse effect, and I'm not sure what all else.

What would happen if only a few city killer sized objects made it through, but the majority of a large "planet killer" sized object burned up in the atmosphere over a couple days?

--
Kurt Feltenberger
xxxxxx@thepaw.org/xxxxxx@yahoo.com
“Before today, I was scared to live, after today, I'm scared I'm not living enough." - Me

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