Spaaaaaace Patrol!

I was reading the classic ISLANDS IN THE SKY (a collection of
science fact articles from Analog Magazine)

There was a facinating article by Robert Zubrin entitled
"Colonizing the Outer Solar System"

In it, he postulates that Mars can be helped along in terraforming
by slamming a few ice asteroids into it.
As it turns out, it requires much less energy to re-direct an
asteroid in the outer solar system than it takes to re-direct
one that is in the asteroid belt.  Simple orbital mechanics:
asteroids in the outer solar system move more slowly.  So it requires 
less delta-V to alter the orbit from a circular one to an 
elliptical orbit.

However, if it is possible to beat a sword into a plowshare, it
certainly
is possible to do the reverse.

Therefore, once such technology is in place, it is likely that all 
space faring nations will have large fleets patrolling the asteroids
of the outer solar system, to ensure that no rogue nation tries to
aim one of those dinosaur killers at Earth.  And keeping all the
enemy fleets honest.