Throwing Rocks at the Earth
Copyright (c) 1996 by Dean_Randle@mhs.dia.govt.nz

Here is an extract from an article at NASA about the affects of NEO (Near 
Earth Objects) and their impact on the earth. The affects are scary and I 
presume their energy yield estimates are accurate. Note the frequency of 
impacts of various sizes that they list.

...
We conclude that, for impact energies below about 10^4 Mts (i.e.
impact frequencies less than one in 6x10^4 years, corresponding to
comets and asteroids with diameters smaller than about 400 m and
650 m respectively), blast damage, earthquakes and fires should be
important on a scale of 10^4 or 10^5 km2, which corresponds to the
area damaged in many natural disasters of recent history. However,
tsunami could be more damaging, flooding a kilometer of coastal
plane over entire ocean basins. In the energy range of 10^4 to 10^5
Mts (intervals up to 3x10^5 yrs; comets and asteroids with sizes
up to 800 m and 1.5 km respectively) water vapor injections and
ozone loss become significant on the global scale. If the submicrometer
dust injection fraction from the pulverized target material is much
higher than is presently thought to be most likely, then dust
injection could also be important in this energy range. This energy
range is a conservative lower limit where damage might occur beyond
the experience of human history. The energy range from 10^5 to 10^6
Mts (intervals up to 2x10^6 years; comets and asteroid up to 1.8
and 3 km diameter) is transitional between regional and global
effects. The dust lifted in this energy range, the sulfur released
from within impacting asteroids, and the soot from fires started
by comets can produce climatologically significant global optical
depths on the order of 10. Moreover, the ejecta plumes of these
impacts may produce enough NO to destroy the ozone shield. Between
10^6 and 10^7 Mts (intervals up to 1.5x10^7 years; comet and asteroid
diameters up to 4 and 6.5 km respectively) dust and sulfate levels
would be high enough to reduce light levels below those necessary
for photosynthesis. Ballistic ejecta reentering the atmosphere as
shooting stars would set fires over regions exceeding 10^7 km2,
and the resulting smoke will reduce light levels even further. At
energies beyond 10^7 Mts, blast and earthquake damage reach the
regional scale (10^6 km2). Tsunami cresting to 100 m and flooding
20 km could sweep the coastal zones of one of the world's ocean
basins. Fires would be set globally. Light levels may drop so low
from the smoke, dust and sulfate that vision is not possible. At
energies approaching 10^9 Mts the ocean surface waters may be
acidified globally by sulfur from the interiors of comets and
asteroids. The Cretaceous-Tertiary impact in particular involved
evaporite substrates that very likely generated a dense widespread
sulfate aerosol layer with consequent climatic effects. The
combination of all of these physical effects would surely represent
a devastating stress on the global biosphere.
...

It seems likely to me that a detection and prevention regime for such 
catastrophes would be part of the charter for founding terradyne and that 
the nations would require stringent safety protocols, inspection, etc. to 
be in place for the weapons systems needed to move or destroy such an 
object. 

The detection systems would almost certainly be set up to provide 
continous live feed of information to the scientists of the world and 
thus provide a fairly efficient means of keeping an eye on Terradyne 
activities (they look in all directions at once, all the time, and are 
capable of detecting an object only a couple of hundred meters across and 
with a low albedo at distances of 2x10^8km (3 months warning of an 
impact, aproximately the distance from the earth to the sun). Such a 
system could easily track man sized and larger objects in Earth orbit 
(although they would be looking the other way and probably in a high 
orbit themselves) but could keep a close watch on the Moon, mars, la 
grange points etc.

One other article noted that, if a big comet was approaching (2+km 
across) and an attempt to destroy it failed, the resulting simultaneous 
impact by several fragments would be more devastating than a single large 
impact.

Sorry if this post is too long, thought it might be interesting to the 
group.
-----------+-----------------------+ What goes around, comes around,
Dean Randle|randled@mhs.dia.govt.nz| and usually catches you in the
-----------+-----------------------+ back of the head. - anon