On Sun, Dec 29, 2019 at 10:05:41PM -0500, Kurt Feltenberger wrote:
> 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?

The smallest bodies reasonably qualifying for "planet killer" title
would be on the order of 15 km across.  You can easily get ten million
city killers from an object that size.  To get almost all of it down
to "burn up in atmosphere" size would require a way to very reliably
split it into more than tens of billions of fragments.

But you don't want to.  The energy released in the impacts into the
atmosphere would still be just as large, only now distributed into the
atmosphere (where fragile organisms live and breathe) instead of
mostly deep into rock.  A planet killer has more than 10^24 J of
impact energy.  Releasing this minimum into the atmosphere over half
the planet through a period of a couple of days, the average power
output dumped into the air would be tens of kilowatts per square
metre.  That's enough to heat almost all of the atmosphere to 300 C
even after accounting for heat radiated back out into space and
absorbed into ground and water.  It would bake nearly anything that
wasn't grilled by direct radiant heat from the fireballs.