Space mines are not floating bombs that lie around waiting for ships to strike them. Instead they are automated weapon platforms, equipped with powerful one-use engines (since they are unmanned they can achieve accelerations that would kill humans), a fuel tank (they burn without though of deceleration or recovery), and some form of offensive weapon. Mines are linked together in vast grids (or minefields if you like) using powerful comms and advanced computer systems.

Upon detecting an enemy ship, a number of mines will be "released" by the grid and start accelerating against the intruder. When they achieve optimum weapon range they will start their attack runs and continue shooting until destroyed, run out of ammo, or sail off into the darkness of space.

Some mines are indeed "explosive", so-called "contact mines. These mine carry powerful thermonuclear warheads and will completely obliterate any target they hit or detonate close to. Being nuclear these mines are no longer in use. Other mines have powerful PPCs, autocannon (gauss in the SL era) or missile packs.

A typical mine will hang dormant in space, either zipping around in orbit around a planet, parked in a pirate jump point (where gravity balances out, thus removing the need for using thrust to keep station), or at a standard jump point (where they use solar sails to keep in position, much like a recharging jumpship).

The risk of "striking" a mine depends on how many there are per volume of space. Low orbit around a planet is "small" and here the risk can be very great. Some smaller pirate points near important worlds have also been heavily mined (Luthien's L4 point, for example). Mining a standard jump point is possible, but required thousands of mines even for a very sparse coverage.

Mines are little used at the dawn of the new millennium. While cheaper than spaceships, they still eat premium industrial capacity, and do not last forever (they need servicing and eventual replacement). But mostly it's a question of lost computer technology and less powerful weapon systems. Without SLDF computer tech the mine grids aren't what they used to be. And without nuclear power the mines are nowhere near as potent as they used to be. On the other hand, there are no warships anymore, so it kind of balances out.