From Gineipaedia, the Legend of Galactic Heroes wiki
Space mines (Japanese: 宇宙機雷) were small unmanned explosive devices that, upon detecting an enemy vessel, would propel themselves toward it and detonate upon reaching a certain proximity. A single mine could destroy an Imperial battleship. (LOGH: 'Bloodshed in Space')
For station keeping, mines were equipped with low output ion thrusters.
The warheads used by both sides were nuclear fusion warheads. Exact warhead yields were classified but were estimated by civilian experts as being on the order of megatons.
To prevent the blocking of navigation routes after battle, both the Alliance and Empire agreed in 670 UC (361 IC / 3470 CE) to impose a time limit on mines, after which the warhead would automatically be inactivated. The maximum time was 1000 hours, though in practice the timers were more commonly set on the order of tens of hours. (Data Book: Mechanic & Seiyū Encyclopaedia, p. 185)
Production sketches for space mines appear in the extra features of the DVD box set.