On Fri, Nov 28, 2014 at 11:51:01AM -0500, Kurt Feltenberger wrote:
> Where I'm having trouble is coming up with a ballpark for their
> mass. I'm thinking with the numerous solar systems to mine, that
> tungsten would be the best option...but I am open to suggestions.

Tungsten would be an excellent option.  It's very dense and has a very
high melting point.

There's a rough sort of lower bound on mass, determined by the
thickness of the atmosphere.  The projectile needs sectional density
greater than that of the atmosphere it's passing through.  If it's
made of tungsten passing through Earth-like atmosphere, that it would
be ideal if it were on the order of a metre long, or longer.  The
width should also be at least about 1/20th of the length.  Putting
those together, the minimum useful mass would be about 50 kg.

Arbitrarily larger masses are possible, depending upon how much death
and destruction they have to cause and the capabilities of the
launcher.

- Tim