[thstone]

Haha!

PCA does not specify the exact materials allowed as ballast.

When I was younger, I worked in the missile industry designing guidance systems, and we always used tungsten to balance the missile airframe.

Other more dense materials all have major drawbacks: Platinum is much more expensive, Iridium is fairly rare, and Osmium is poisonous when exposed to air.

A 4" cube of tungsten weighs 41lbs!