Mark, at 50 to 70 km/sec a 100 kg warhead won't even be *noticed*. The
kinetic energy of the impact of a 1 ton missile at those velocities is
equivalent to a *small* nuclear warhead.
Ever see a picture of the Barringer Meteor Crater in Arizona? Your
missiles could make a crater like that. It was made by a body that
probably massed 100 times as much, but it was only traveling about
10 km/sec. So your missile would have the same energy if it was
going only *10* times as fast (E= m*v^2/2)
The kinetic energy of the missile is equal to converting 1/36 of a gram
of matter to energy. (assuming a 1 metric ton missile at 70 km/sec).
And if the missile is attacking from the front, the energy depends not
on the square of the missile's velocity, but on the square of the
*sum* of the velocities!)
To put it simply, you *can't* armor against kinetic energy weapons.
Not when the velocities are tens to *hundreds* of km/sec.