QUESTION:
Would it be possible to see the effects of a Kuiper-belt object striking 
the atmosphere of Neptune?

ANSWER from Alex Storrs on June 10, 1996:
I figure the energy of a 1 km dia. iceball falling onto Neptune would be 
about 2.9E17 erg. Neptune reflects about 4.5e22 erg/s from the Sun, so 
in order to see the impact flash all the energy would have to be 
dissipated in about 10^-5 sec. It took more like 10 min. for the 
majority of the SL9 impacts on Jupiter to dissipate the energy (impact 
and fallback).

Would the impact "scar" be visible? This is more promising. The SL9 
impacts left large brown clouds on Jupiter for several weeks. The 
problem is that Neptune's disk is so much smaller, esp. as viewed from 
the Earth. The only way to see such "scars" would be with HST, and we're 
only looking at Neptune once or twice a year. Thus there's a good chance 
we'll miss it.

A groundbased thermal infrared monitoring program is probably the best 
bet for detecting KB impacts on Neptune. The SL9 "scars" were very small 
particles in the upper atmosphere, and tended to be much warmer than the 
underlying clouds. Thus getting a good 30/60/90 micron brightness for 
Neptune every night would be a good way to see impacts-- if there's a 
jump then something might have happened.