[Carthego delenda est]

“…imagine the kind of damage a microasteroid would do to a spacecraft…”

Imagine a spacecraft built with materials (yet to be “had” on earth) impenetrable from micro-asteroids, or anything else.

[DoodleBob]

It's not about penetration. It's about physics. When near light speed, a collision with an object as tiny as 10-14 grams would have an impact energy close to 10,000 megajoules.