[Alneyan]

All construction happens *after* movement (on the Last day of a month), so it will not work this way. If your planet is still alive at the end of the turn, it will have built mines, and you will then be able to launch them before any movement can occur (so if the enemy ship is one sector away from your planet, the mines will be deployed before it gets there).

It is also why you may find it difficult to colonise planets in systems controlled by the enemy; you need to colonise the planet and then build mines for one turn without losing the planet in the process.