Nah, just strap engines and a guidance computer onto a few big rocks in the asteroid belt, time them to hit around the same time and wait. No need for a massive ship or operation.
If that were possible...
a) It'd be very slow unless you used hundreds of (expensive) engines.
b) The enemy would have an ample window to intercept the lumbering rock with either missiles or sustained fire from energy weapons.
c) You also would need to get the massive engine cargo there somehow (not to mention install it all), which means...
--- 1) The cargo ships could be intercepted as well.
--- 2) The enemy would have a lot of time to disrupt your plan.
So I don't think it would be such a good idea unless you're hitting a frontier colony with minimal military presence. And even if it were the last colony of a largely defeated alien race, simply invading the planet would take far less time and expense. And probably be more effective across the board.
I know this is from the previous page, but this reminds me of a sci-fi book by David Weber called "The Shiva Option". Basically, the humans and their allies are fighting the Bugs, a hive-mind race that fits the typical horde mentality: fast, expendable ships that use kamikaze tactics basially by default. Normally, the humans use fighter craft armed with anti-matter missiles to bomb hive worlds into molten rock, though casualties are typically high thanks to orbital platforms.
At one point, the humans decide on a different tactic: attaching engines to several large rocks and hurling them towards the planet. Attaching the engines takes months, the asteroids are horribly slow (at first), and the Bugs repeatedly try to destroy or divert the rocks, forcing the allied fleet to defend them and taking massive casualties in the process.
While the asteroids worked (all but glassing the planet and destroying the orbital defenses in the process), it would have been faster and far cheaper to just bomb the planet like normal. The allies never try that tactic again.
What's interesting is that in many ways, "The Shiva Option" is harder sci-fi than Aurora. In system travel is slow, FTL can only be obtained using jump points that must be searched for, communications and sensors are limited by light speed, and ships must move thru a jump point one at a time or risk "intersecting". In fact, one of the main reasons I started playing Aurora is because it reminded me of this book.
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