I don't think we could do this at all, even theoretically.
If we took all the proven petroleum reserves in the world, and magically converted them into kinetic energy with 100% efficiency— With zero overhead for transportation, launch, agriculture, or obeying conservation of momentum— That still wouldn't be enough to launch even a single planet killer. At most you could crater a small country, but not kill a civilization:
So let's say you do nuclear pulse propulsion like Project Orion. You've still got Tsiolkovsky's rocket equation to deal with. Assuming a speculative fusion bomb ISP of 75,000s, you would need a rocket with… over 50 orders of magnitude more mass than the entire observable universe, in order to accelerate a single proton to 0.9c:
Light sails will be huge, obvious/visible, and slow. Beamed power will run into issues with diffraction.
In fact, reaching 0.9c while you're still in the solar system plainly implies maintaining multiple hundreds of g's of acceleration over many dozen astronomical units of distance. That doesn't seem feasible at all. It's wildly beyond not only our best existing ion drives, but probably also any remotely feasible existing concept for space propulsion.
If we took all the proven petroleum reserves in the world, and magically converted them into kinetic energy with 100% efficiency— With zero overhead for transportation, launch, agriculture, or obeying conservation of momentum— That still wouldn't be enough to launch even a single planet killer. At most you could crater a small country, but not kill a civilization:
https://www.wolframalpha.com/input?i=2+trillion+barrels+of+o...
So let's say you do nuclear pulse propulsion like Project Orion. You've still got Tsiolkovsky's rocket equation to deal with. Assuming a speculative fusion bomb ISP of 75,000s, you would need a rocket with… over 50 orders of magnitude more mass than the entire observable universe, in order to accelerate a single proton to 0.9c:
https://www.wolframalpha.com/input?i=e%5E%280.9c%2F%2875000s...
Light sails will be huge, obvious/visible, and slow. Beamed power will run into issues with diffraction.
In fact, reaching 0.9c while you're still in the solar system plainly implies maintaining multiple hundreds of g's of acceleration over many dozen astronomical units of distance. That doesn't seem feasible at all. It's wildly beyond not only our best existing ion drives, but probably also any remotely feasible existing concept for space propulsion.