Space is, of course, an ocean. And sometimes your Space Navy needs to block access to a port, a space lane, or even an entire planet. How to do this? Why, Space Mines, of course. These are like their seagoing counterparts, but IN SPAAAAACE!!!
Just like there are several types of Sea Mines, so there are also many types of Space Mines. Most common are proximity mines, that go off when a ship gets too near, and contact mines that go off when they hit a ship's hull. Magnetic mines are attracted to metal hulls. Homing mines are essentially missiles that sit around until they detect a ship's engine or weapon energy signature, then angle themselves at the target and let fly. Some mines are miniature weapons platforms that open fire when ships get too close. Remote-detonated mines can be set off by a waiting vessel when a target ship gets in range. Nuclear mines have nuclear warheads in them. Then there are mines that rob ships of power (dampening fields) instead of just exploding. There are many other variations and combinations.
In real life, there would be some major limitations to this trope. However, since Space Mines seem to be an ubiquitous part of Space Operas, sometimes the savvy writer thinks of the above limitations and writes around them - and real-life applications have even been discussed in military circles.
In order to secure a whole planet, you'd have to mine space three-dimensionally in order to be effective. In fiction this is often not done.
Earth's ocean and sea terrain contains a lot of inlets, natural harbors, bays, straights and other types of terrain that make natural choke-points where the use of mines is a practical way to deny or substantially delay passage to unwanted ships. No such barriers or terrain exists in space to prevent ships from circumnavigating such a barrier. Even protecting a very small moon with a density of one mine every few thousand cubic km would require huge numbers of mines and logistical support to successfully achieve coverage. The same logistical resources would be of better use in improving detection and interception/quick reaction capability. Can be justified if there IS a conveniently narrow pathway to barricade, such as a local entrance to Hyperspace or the Portal Network, or if the object to be surrounded by mines is fairly small, such as an asteroid base.
Laying mines takes time, and the larger area you need to cover, the number of mines you would need increase quadratically. To cover large or even moderate areas could take hundreds of years, even if it only took a few seconds to lay each mine. Justified if the mines have potential to locate and approach, or shoot, their targets from massive range, thus ensuring blockade functionality despite low minefield density, or can be all released in a single spot and relocate and organise autonomously. As for the matter of quantity, this can be explained by having automated manufacturing and minelaying facilities operating over lengthy time periods, or have the mines themselves be self-propagating Von Neumann machines.
Sea mines are deployed under water, greatly complicating the task of detecting and clearing them. There is no such barrier to visibility for Space Mines. Unless there were some sort of mitigating factor (sensory disruption, cloaked mines, etc.) space ships could just pick them off with long range guns/lasers/missiles/decoys/whatever. May be partially justified by the fact that, unlike enemy ships, they can be inert, dormant and undistinguishable from generic space debris until they're close enough to strike.
Everything with mass has gravity. In space, little things that are relatively close to each other tend to clump up — this is how planets and stars are born, and why there are no Asteroid Thickets. The mines would need some way to fight or negate the effects of gravity on each other that also wouldn't run out of fuel.
There are massive space minefields around the Men's Planet (Tarak) that is visited towards the end of Vandread (second season). They are used to reveal First Mate BC as The Mole, since s/he knows the friend-foe codes of the mines that allow Nirvana to pass them.
In Gundam Wing, the area around the space colonies is seeded with mines to prevent any unauthorized travel. Trowa's first assignment upon enlisting with OZ is to help destroy the mines, as part of OZ's "kinder and gentler" facade.
The original series had both the Gyan, which could deploy floating mines from its shield and the MSV model kit Zaku Minelayer, which was just the standard Mook with a bigger backpack that dropped mines.
The Tothian minefield in Galaxy Quest provides a rather dangerous shelter for the Protector when "Taggart" and his crew find themselves outgunned by Sarris. It comes into play again in the finale, when they use it to pull a variant on the Wronski Feint against Sarris' ship: the magnetic mines trail behind the ship, and Sarris runs straight into them. Averts the usual problem with space mines in that the field has enough layers to cover a significant portion of space (enough to look like a nebula from a distance which is why they fly into it in the first place), though still woefully small in the grand scale of things.
Stewart Cowley's Spacecraft: 2000 to 2100 A.D.. During the war with Proxima Centauri, the perimeter of Earth's solar system was seeded with nuclear mines. They would home in on the warp generators of arriving enemy ships and destroy them while they were recharging their power banks after the warp jump.
Honor Harrington has this; it gets around Space Is Big by having the mines basically be the same 10,000-kilometer-range laser warheads found on their missiles. They later come up with a system defense variant of their missile pods, creating missile-firing mines with much greater range. Additionally, mines tend to be placed to cover small areas: the orbits around a planet, a wormhole exit, or (occasionally) quickly laid in the path of an incoming fleet. The visibility issue is avoided in that professional forces (ie. everyone who isn't State Sec) use them as a force multiplier rather than a sole defense: slow down to take potshots at the minefield and you leave yourself wide open for attacks from the system-defense forces.
A number of space mines appear in the Star Wars Expanded Universe, where they are particularly useful when placed along the exit points of hyperspace routes. Besides the usual exploding types, Wraith Squadron encountered a variant that used a gravity well generator to snatch fighters from hyperspace before triggering an ion pulse to disable them.
E. E. “Doc” Smith's Family D'Alembert series - in the seventh book, the heroes have obtained (or so they think) the location where the enemy fleet is going to assemble. The best place in which to deploy for the killing ambush follows as a corollary. It's also seeded with mines. The Fleet avoids annihilation through a stroke of luck.
Pops up in a chapter of Battlefield Earth as a way for the rag-tag human freedom fighters to defend against other invading aliens following the initial liberation of Earth. Where the humans got them, how they managed to whip up a few thousand space mines, or why the Psychlos had the mines sitting in storage somewhere instead of in use, is never explained.
The hypernet gates used for transit between star systems important enough to justify the expense in The Lost Fleet turn out to also be capable of supernova scale detonation. In a sequel series the character discover two other species that also have the gates, are aware of this alternate use, and use it accordingly (read: not in important systems, but as perimeter system defensive mines).
In the pilot of the original Battlestar Galactica, the Galactica must pass through a narrow region of space in a dense nebula that has been mined by the Cylons.
In the Babylon 5 episode "Matters of Honor", we see a Centauri minefield deployed to interdict access to a planet. This minefield actually consists of orbiting weapon platforms that fire on intruding starships instead of merely exploding when they get within range.
Captain Sheridan also used nuclear warheads as mines on at least two occasions, but these were remotely detonated from a starship when enemy vessels were deemed close enough.
"Booby Trap". The Enterprise enters an asteroid belt that contain "acceton assimilators", mines which drain the ship's power, convert it to deadly radiation and beam it at the ship.
"Chain of Command Part II". An Enterprise shuttlecraft is used to lay mines in the McAllister C-5 Nebula, trapping the Cardassian ships hiding inside it. In this case, the ships were already present, so the mines just had to be placed near/on them.
During the Dominion Wars the Defiant mined the entrance to the Bajoran wormhole. At least in this case, the mines were protecting a single, uni-directional portal and were both cloaked and self-replicating to prevent easy removal.
The Klingons established an illegal cloaked minefield in "Sons of Mogh." The mines are dormant and have to be remotely activated in event of war — and would effectively cut DS9 and Bajor off from support from elsewhere in the Alpha Quadrant.
In Star Trek: Enterprise, the Enterprise runs into a Romulan minefield surrounding a planet. The mines are cloaked and scattered everywhere, a rare example of such a field done right.
The Stargate SG-1 episode "The Serpent's Venom" takes place in a space minefield where the mines all lock onto any sign of weapons, which is used by the Go'a'uld as a neutral meeting place. SG-1 has to reprogram a mine to attack one of the Go'a'uld ships at the meeting, in order to instigate a conflict.
Star Fleet Battles. Ships can "roll a mine out a hatch" and leave it to blow up a ship pursuing them. Mines can be set to accept only certain sizes of ships as targets. Major space installations often had minefield belts protecting them. Some of the mine types available:
Command mines can be ordered to detonate or to activate/deactivate themselves.
Chained mines detonate when other mines explode.
Transporter bombs can be beamed into position.
Captor mines can fire weapons at targets.
Iron Crown Enterprises' Cyberspace game had Orbital Mines for use against enemy spacecraft in orbit. Some had onboard computers and could make their own piloting/targeting decisions, others had to be operated by remote control.
AKVs in Transhuman Space are not exactly mines, but similar enough: they are AI-controlled missiles that can float in ambush until a craft flies near and then attack.
Battlefleet Gothic has a large number of scenarios in which the defender can buy and deploy defence systems ranging from purely in-system patrol vessels and armed space stations through to this trope, with Orbital Mines (homing, slow moving, relatively myopic target acquisition) and Deadfall Torpedo Salvoes (ship-killing torpedoes, faster moving than mines, with slightly better target acquisition, but functionally non-homing once their drives come up). It's also possible to refit a carrier into a mine-laying role, swapping attack craft squadrons for the capacity to drop a pair of mines.
Wing Commander has Porcupine Space Mines (proximity, limited homing); there were also turret mines (miniature laser platforms), high explosive contact mines and viral mines (broadcast virus-infested transmissions to infect and shut down ship computer systems).
The Babylon Project: In three levels of the Earth-Brakiri War, you have to navigate through a Centauri minefield. Unlike traditional contact or proximity mines, these mines shoot at you (see the Babylon 5 example above).
There's an online Flash game called Space Minefield.
Also one called Space Mines.
And a company called Cogmed created a flash game called Space Mine Patrol to demonstrate working memory.
The first few seconds of Sector X in Star Fox 64 has a cloud of mines you have to go through.
Homeworld has Minelaying Corvettes. The mines themselves are proximity-triggered homing mines, solving the problem of mining in 3D space.
Freelancer has a few minefields, which have their own haunting ambience theme whenever the player is near or within one. These minefields are less like a field of mines and more like a thick, spherical meshwork of explosives, which kill any intruding spacecraftnote that aren't using cheatsvery quickly.
Sword of the Stars has Minelayer sections in both destroyer and cruiser sizes. There are various warheads but all are proximity-triggered homers. There's also a Leap version that trades some power for much faster movement as an anti-Point Defense measure, as well as a Gravity mine that doesn't hurt but pulls ships to itself and an Implosion version that combines said Gravity effect with an actual blast. They can be very effective, especially against the AI. There's even an upgrade to the Complex Ordnance Launcher that allows you to launch a minefield from afar.
The Artemis System Net from Master of Orion 2: Battle At Antares is a massive minefield that surrounds the system it's built at. It has a chance of damaging or destroying enemy warships that attack the system depending on the size class.
This is the special weapon (proximity, limited homing) of the Defiant-class in Star Trek: Armada.
And becomes a relatively common aft weapon in Star Trek Online, with a variety of different payloads (including tractor beams). However, because mine spreads are stationary relative to the local map, and have such a limited homing range, their best use is possibly taking down other mines and incoming heavy torpedoes
Also possible in Sins of a Solar Empire, mixing two of the above excuses with 2D Space, and direct lines of approach to separate gravity wells.
Several varieties of Space Mines in X3 Terran Conflict. SQUASH Mines are your standard high explosive mines, Ion Mines target ship shields, Tracker Mines... track the enemy, and Matter/Antimatter Mines are like SQUASH mines but with more boom. In theory. In practice only the names are different. They all do the same type and amount of damage. One of the most effective uses of the mines is to load your ship with them, get a ton of enemies to chase you, drop all the mines, then order one mine to self-destruct. Big bada boom.
Stage 1 of R-Type Leo has space mines that form a laser grid with other mines.
Aera in Vega Strike has Porcupine "mines". It's a sluggish (about 0.5 g in any direction) Attack Drone with an autotracking gun and 100 shield-piercing shells to pelt enemies in range. It explodes if approached, but weaker than any missiles proper. In terms of volume and compatible mounts it's a "medium missile", which allows Aera escort fighters to carry a dozen of these pests, in addition to six mounts of the same weapon with much more ammo, rocket pods (in assault variant) and normal missiles.
Stars! minefields attack enemy ships traveling above certain warp speed and thus prevent a sudden invasion — an important part of the game. Ships equipped with Ramscoop engines suffer more damage; those that aren't tend to rely on fragile fuel transports, and since mines, like missiles, aren't fully negated by Deflector Shields, unarmored ships are killed easily. Specialization in mines is a primary racial trait, "Space Demolition". These guys have all 3 mine types, minelayer hulls, lay mines on the run and are much more capable both of surviving mine attacks and using their own mines offensively. They also use minefields as detection arrays — if used correctly, this makes sneaking up on them almost impossible: move fast and get blown up, or move slowly and be detected early. Or run into normal and heavy mines, have surviving ships stopped by speed traps and detected — unable to repair without blown up transports or even run away due to still being stuck in the middle of 3 overlapping fields. Conversely, War Mongers don't have such defensive weapons at all.
In Another Century's Episode (the original), one mission (loosely based on Gundam Wing, mentioned above) has you clearing out a minefield so Relena Peacecraft can make a goodwill visit to the colonies.
In VGA Planets ships with torpedo tubes can lay mines. Efficiency depends on Tech Level. It can be expensive in terms of restocking ammo, though mines that aren't swept can be scooped by another ship or even another allied player's ship, so it sometimes allows to reload empty torpedo tubes in the middle of nothing. Both laying and sweeping minefields are fleet orders, i.e. ships have to stand there and not do anything else other than defending themselves. Crystals race also have web mines — these stop enemy ships at random, so the fleets break apart and steadily drain fuel, so the victime can get stuck for good.
Mines are used to defend a Void gate in Infinite Space, but the invading fleet cuts right through them with a massive missile barrage. The Oort Interception System around the prison planet Skantzoura is made up of mines that shoot anything that comes close like the Babylon 5 example.
In Ten Minute Space Strategy, orbital minefields are one of the special traits your empire can develop which speeds up the destruction of invading enemy fighters on your planets.
Space Pirates and Zombies also the player to drop mines as a weapon. What you're really dropping is a cloaked dispenser that will scatter a limited area with cloaked mines that are attracted to your hull. One type of side-mission involves entering minefields to recover cargo though.
While most science fiction applications of Space Mines are ridiculous in practical application (see the fridge logic page), it might be feasible to set up Space Mines in certain high-traffic orbitals, e.g. geosynchronous orbit. Other targets would be those used in an orbital insertion after launch from a major ground-based spaceport. You could also position them at Lagrange points due to their use in the Interplanetary Transport Network.
Space mines have actually been considered in real life studies as possible space weapons utilizing current tech. Examples:
Currently, space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion and collision fragments. To date there have been several known and suspected impact events, and several satellites have been destroyed.
As the chance of collision is a function of the number of objects in space, there is a critical density where the creation of new debris occurs faster than the various natural forces remove these objects from orbit. Beyond this point a runaway chain reaction can occur that reduces all objects in orbit to debris in a period of years or months. This possibility is known as the "Kessler syndrome", and there is debate as to whether or not this critical density has already been reached in certain orbital bands.