I don't think a second strike from asteroids would work. You can't just arbitrarily redirect an asteroid towards Earth whenever you want. You need one that's already going to pass nearby. You might have to wait decades for it to hit, and during that time your enemy can send their own nuke and nudge it away.
But if you could do second strikes with asteroids, that would be a good thing. Second strike capability helps prevent nuclear war. Effective first strikes are what's destabilizing.
Which brings us to your second point. I agree that stationing nuclear warheads in space is a bad idea. But we don't have to do that. With years or decades of advance warning, we can just launch them from Earth.
- "Second strike capability helps prevent nuclear war. Effective first strikes are what's destabilizing."
I don't disagree!
- "You need one that's already going to pass nearby."
There's a very large number of these already, within a small delta-v of Earth. We don't know where they are yet (it was a show-stopping issue with NASA's Constellation (?) program, when they wanted to demonstrate capturing an asteroid, but couldn't find one), but that's going to rapidly change.
- "and during that time your enemy can send their own nuke and nudge it away"
That's pretty useful: that brings it to a point where the defender has to invest an amount of resources comparable to the attacker. That's a win for the economically stronger country. If the difference grows large enough, they can simply overwhelm them with numbers.
That's even before opening the technological possibilities of stealth asteroids.
But if you could do second strikes with asteroids, that would be a good thing. Second strike capability helps prevent nuclear war. Effective first strikes are what's destabilizing.
Which brings us to your second point. I agree that stationing nuclear warheads in space is a bad idea. But we don't have to do that. With years or decades of advance warning, we can just launch them from Earth.