[cogitator]

It would appear that (at a minimum) a "totally blown up" asteroid/comet of 2x the crater size would be blown into so many small fragments that the remains would get heated and destroyed.

However, you have the "Armageddon" movie problem; you have to insure that the object is totally blown up. If you just make a crater in the object; well, then you have an object of similar size with a large hole in it.

[Robert A Cook PE]

Granted: However, a workable solution (protecting from solids up to (maybe) 600 feet across)) protects against everything smaller as well.

Now, what if a 900-foot object approaches? Well, the smaller solution "might" make 4-400-4000-40,000-400,000-4,000,000 surviving little pieces. And many/most/almost all of the smaller pieces would still get absorbed over a large area by the atmosphere.

The smaller the object, the more likely it is to occur.

Since a "global-busting" asteroid/comet is rare (every 250-million years on average) we should not postpone a do-able solution for "small" (city-busting) impacts trying to get a perfect solution for every impact.

Not the government would delay and study forever trying to get a perfect solution (since "studies" are IS their research money!!!!!!!!) but they should get a workable solution going.

THEN, and only then, improve it to larger and larger sizes, each larger size less and less likely.