NOTE: This question has been reworded in response to an "on hold" designation.

In this post there is a description of an alien species: (You are murderous alien pirates, how do you get onboard a victim's spaceship?)

How can a boarding party or individual from this species be killed or a boarding action repelled?

World Notes:

Currently, small arms and handheld weapons are mostly projectile based (though the tech-level of such weapons doesn't need to be limited to our own). There are no phasers or handheld plasma guns. Powered/exoskeletal armor is plausible (but non-military folks won't have it).

Ships have a robust (space opera style impossible) artificial gravity (and the equivalent of your Star Trek "inertial compensator") that negates the effects of the ship's motion through space on it's contents and provides a consistent gravity throughout the ship. The artificial gravity can be turned off, but there are safeguards to prevent turning off the device that protects the contents of the ship from the acceleration forces etc. Ships also have a means of recycling air, but the air reserves themselves are finite, so anytime loss of air can be avoided is ideal (though the lesser of two evils compared to being murdered by scary aliens).

It is common practice on both merchant and warshsips for everyone to don space suits in any battle, rescue, or unusual situation (though the rule is followed more closely aboard warships); if this rule has been followed likely everyone on the boarded ship is wearing one; the aliens may or may not have them, but can survived exposed to vacuum for a decent period of time, though not indefinitely.

Warships may have built in security systems, armored bulkheads, the works, but trade ships aren't necessarily going to have such expensive or sophisticated measures, unless the owners are rich. They will more likely rely on inexpensive, a few of the most effective, and/or jury-rigged defenses. Warships may also carry a small unit of marines.

Computer technology is advanced and autonomous function is easily possible given appropriate programming, but self-aware or super intelligent AIs are not. The computer can't make decisions it hasn't been previously programmed or designed to make.

The aliens are very clever. Any defenses which are easily avoided or outsmarted will become ineffective very quickly. Some of them may possess one or more pieces of metal armor that is equivalent to the sci fi battle-steel variety and extremely difficult to bend or break. They can also hold their breath for a comparatively long period of time. Long enough to find where Joe is hiding and get him? Maybe, depending on how big the ship is (and other factors). At least if they have to hold their breath they can't sniff out Joe's location...

Additional Note: Some answers are in response to the original post, which was voted "too story based" and therefore subsequently edited to rephrase the question and provide a better idea of what kind of sci fi setting is being used (since I was initially too vague). Because my original question was worded poorly, answers had to guess on this front, and to anyone whose response was invalidated as a result of my attempt to fix the question, I apologize.

  • $\begingroup$ How long can they hold their breath? @MarielS $\endgroup$ Commented Mar 27, 2019 at 20:59
  • $\begingroup$ @CelestialDragonEmperor I'm not positive about this yet, but for the sake of allowing imagination let's use an extremely liberal estimate and say maybe an hour or two? $\endgroup$
    – MarielS
    Commented Mar 27, 2019 at 21:12
  • $\begingroup$ that's impressive. $\endgroup$ Commented Mar 27, 2019 at 22:17
  • $\begingroup$ @CelestialDragonEmperor extremely liberal, as I said ;) Probably not likely, but hey, I wanna give you guys some room to dream XD $\endgroup$
    – MarielS
    Commented Mar 27, 2019 at 22:24
  • 1
    $\begingroup$ Considering the context this is probably not the answer you are looking for so its a comment instead: space ships are tightly controlled pieces of equipment that need to keep everything in order or you suffer environmental problems, and you are controlling all of that. While I personally am of the "if you fight in space, everyone wears a space suit for the event of being sucked in space" variety of thinkers, almost 100% of the space battles you see inside ships has little to no space suits for the crew. So just playing with the air, sucking it out or replacing it locally, should do fine. $\endgroup$
    – Demigan
    Commented Mar 28, 2019 at 0:27

7 Answers 7


Comprehensive Guide for dealing with boarders

I encountered this issue building my own setting a while back. The tech baseline for this answer is relatively hard sifi, I'm gonna mention alternatives for softer settings though. The ship I'll discuss has no magic gravity system, and is designed for linear thrust and in some sections spin "pseudo gravity". Power is provided by a fusion and backup fission generators, antimatter is availiable in decent quantities. The main propulsion system is a torch drive, capable of acceleration up to 30 G. Shipboard manufacturing is advanced, not on a Star Treck replicator level (even though that would make any tactic involving manufacturing systems easyer), but given the raw material, a blueprint and a thermodynamically reasonable ammount of time there's little that can't be produce. AI is very advanced and handels lots of things on the ship. Nanotechnologie is advanced too, biologically and mechanically. You need to consider if a given tactic fits with your settings technologie.

gas them

This category includes anything involving atmospheric manipulation. These methods only work if the boarders where stupid enougth to leave space suits behind.

  • depressurize the ship

This is pretty self explanatory.

  • poisonous gas

If you expect trouble beeing able to introduce gassed to pacify people on board is reasonable. There is a wide range from nice to nasty. THC. If the fighting force is stoned there will be little fighting. Knockout gas. Self explanatory. Chlorine or Fluorine gasses. Those are the nastiest available, they should be bad for any kind of life form. They might damage the ship slightly or could be illegal in many systems. Could be explained as part of a decontamination system. There are numerous other options CO, CO2, safrin, zyclon b,...

  • mess with the atmospere

Again based on human biologie , yet any species should have its limits here. This is the poisonous gas version for civilian vessels. Removing oxygen from the atmosphere is a sure bet. Stopping CO2 removal will be slow, but effective. Overpressure is another way to do this. O2 turns lethal at 1 bar partial pressure, nitrogen at 4 bar and CO2 at 0,005 bar. Of cause overpressure itself is not pleasant. On that note, spacing, rapid depressurisation, and beeing sucked into space is a grat way to kill someone too. To make this more effective just overpressurise the ship. At 20 bar pressure spacing is 20 times more fun! Needless to say this works against spacesuit users, but doesn't kill them.

evil uncle Newton

This category is based arround killing and harassing with velocity and acceleration. Everything here depends on how and how fast your ship can accelerate, how people brace themselves against acceleration (crash couches, inertial dampeners, nanotechnology and or augmentation), how gravity can be produced in the setting (only pseudo gravity or magic gravity) and how much ship and boarding equippment design seeks to counter the dangers from these approaches.

  • increase "gravity"

Accelerate as hard as the ship can, spin the spin-sections as hard as possible, turn the gravity magic up or even repulsive, while the crew is savely braced. Everything depends on the limits of these technologies. And on inertial dampeners. Turn them off. Better yet, don't use them in your universe. They are a stupid technology that removes tension from stories. Anyway your boarders ahould be well done after 10 min of 30 G. Or better 10 min of irregular, short 30 G thrust spikes followed by swift counter acceleration. That will bounce them like a flummy between flour and ceiling. You might have to repaint after that though.

  • spin the ship up arround an axis it wasn't supposed to spin arround

This was already mentioned in the comments. It won't kill tbe boarders, but it will buy time.

defense systems

This category is about pre installed weapon systems. It might not be fitting for civilian vessels.

  • pre installed anti boarding

Have autonomous or steerable weapons in flour, ceiling and walls. These could include but are not limited to machine guns, low yield railguns, lasers, mines, claymores, blades, spring loaded spikes, flame throwers, acid launcers and rocket launchers. They will obviously damage the ship, but as you know their yields you can disallow them to fire at certain vectors, where they would take out vital systems. So damage can be held managable.

  • defense mode

Have a software enabling the override of standard security measures. This software allows you to rapidly open and close doors or airlocks for spacing and crushing boarders. Makes blowing up pipes possible. Have elevators fly out of the ship.

  • turn main weapons on the ship

Some of the low yield ship to ship weapons could be turned inwards. Look out that you don't destroy vital systems and this can be very lethal. Railguns or gettlings seem best suited. No matter how good the alien armor is, a slug moving at 5% the speed of light will pierce it.

thermal management

Heat management is incredibly important on space vessels and criminally underused in sifi.

  • cooling fluid horror

Different cooling fluids will be used to cool different parts of the vessel. Drowning boarders by flooding overtaken sections might be possible. Freezing them with liquid helium (needed to cool superconductors and stealth hulls) is quite mean. Boiling them with the liquid metal or salt from the drive and reactor cooling system has it's benefits as well.

manufactured menace

Remember how I mentioned production capabilites? All sorts of mean stuff can come from there. The anti bording weapons I mentioned could be quickly produced and placed for example.

  • gray goo

This depends on the nanotechnology of your setting. A flood of all consuming quicksilver might be on the table. Or invisible flyers, carring millions if micro warheads into the boardes lungs or collecting on their armor for a major strike. Just be careful to get these back under controll.

  • bio weapons

How about a virus ramping up paranoia and aggression in the borders? One that allows you to take over their motoric functios? Or one that just kills them? Nah, that would be boring.

  • hunter kill drones. lots of them

Start producing robots. And don't stop. Your aliens sound like formidable fighters. But quality can be beaten by quantity. If a great knight gets surrounded by 30 peasants with pitchforks, he is dead.


  • sacrifice boarded sections

This works better if it is done early. Desinfect the wound before the infection spreads. Overload all system in the area or shot one of your own missiles at the breaching pod and breached section.

  • evacuate

This means loosing the ship, but I'll still mention it. Fleeing in a shuttle is the most obvious option. If cryogenics are a thing, decapitate the crew, freeze the heads and put them on a high burn missile on a course back to you allies. If minduploading is a thing you can evacuate via transmission or play ghost in the machine until a chace for retribution arises. If your aliens are all so bloodthirsty that they all join the boarding operation, counter board their ship. Set up hinderances onboard the old one and attempt a full force counterattack. The aliens might be cought off guard, considering their self image. Since you are technologically superior the enemy's computers will surrender quicker to your hacking than your computers will. Then blow up your own ship.

  • set loose something worse

In the book Revelation Space Ilia Volyova sets free captain Brannigan, who is infected by a super-intrusive and aggressive nanotechnology virus, the melding plague, to regain controll of her ship from the alien software entity Sunstealer. This of cause requires you to have some sort of lovecraftian horror up your sleeve. And then you need to deal with said horror. Watch out that the cure doesn't become worse than the disease.

  • psychological warfare

If you got augmented reality or some form of none invasive memory manipulation, play withbthe boarders to have them kill themselves.

  • hostage situation

Implant the crew with antimatter bombs. Then call the aliens and explain the situation. If any of them are injured we all will die together. Can even be held back if the aliens take hostages to threaten to blow up their ship. Can be done with the main reactor too. But will the xenos belive the threat? Are they willing to learn how real it is?

So this was the defense part. From the aliens perspective trying to hack the ship would be crucial. Wearing full space suits with self repair capability in addition to armor is required. Some form of mobile acceleration bracing would help. Your aliens anatomy could be beneficial there. Shooting holes through everything and advancing in hard vacuum would help against a lot of shenanigans. Using decoys as a front line helps the invaders to figure out how many traps there are. But the defenders will adapt to this tactic soon. Coming in from as many angels as possible will break the defense effored, especially if it improvised. Fully disabling the outside weapons and systems of the ship before bording helps, as well as slagging the enemys drive to prevent funny acceleration games. Using scanners to figure out whats behind the walls will work against traps. Rushing production hubs will be essential. Rail guns could be used to depressurise the enemy ship and take out stratigic targets.

There are numerous tactics there. The aliens challenge is to figure out what to do first. For example taking out all hull weapon and checking for traps costs time. But they want the production hubs offline as soon as possible. Additionally their honor code could slow them down too. After all isn't it cheating to surgically destroy strategic targets, which whould have to be taken at a high blood price otherwise?

As for my weapon of choice, it would be an inwards pointed ship to ship rail gun. My preferred conventional weapon would be a flamethrower, either modyfied to throw plasma or normal napalm with some thermite equivalent as a special sauce in it. A flamethrower does need less aiming than a shotgun and there is a good chance that your alien hesitates to charge through a corridor filld with an inferno. A few claymores would be nice as well.

  • 3
    $\begingroup$ You sir, are even more twisted than I am. Well done. Also, cool profile pic Skully. $\endgroup$
    – Nyakouai
    Commented Mar 28, 2019 at 14:46
  • $\begingroup$ I really like this answer, and I think it counters everything I thought of when describing how to attack. I'm not sure though that it would be considered cheating to destroy strategic targets and avoid the higher losses; after all, we don't consider it cheating to bring advanced technology (a gun) on a hunting trip or to minimize risk to ourselves when we hunt for sport. From the hunter's perspective, I would gain the most satisfaction from a perfectly executed plan in which those strategic targets were destroyed, especially if it minimized the risk to my own hunters. $\endgroup$ Commented Mar 28, 2019 at 15:54
  • $\begingroup$ Wonderfully devious and wicked. @MrSpudtastic the hunters prefer to kill things up close and personal, but ultimately the measure of an excellent hunt is killing the most prey in the most fun way while losing the least number of hunters. So, while they aren't going to orbitally bombard a planet (where's the sport in THAT?) they're not above adapting and using advanced tech to avoid traps etc., they would likely just see this as playing by the prey's rules. But you get extra points if you can get up close and chop a bunch of hiding Joes into pieces with your sword, claws, whatever. $\endgroup$
    – MarielS
    Commented Mar 28, 2019 at 17:31
  • $\begingroup$ "Quantity has a quality all its own" $\endgroup$ Commented Nov 25, 2020 at 22:28

Horrors, the fearsome Bobs boarded our poor Joeys ship!

Quick thinking, how do I survive?

Read this long manual, and avoid to die during the reading.

Trust your most powerful and oldest instinct

Flee like a mad Joe. Those are Bobs, for crying out loud. Hide, and make a dash for any exit opportunity you can take. Remember, Bobs have a form of heat detection and a sharp sense of smell.

You will need hermetic caches, lined with some heat-insulator. Also known as a panic-room, a smuggler cache or a really top of the shelf closet. If it doubles as an emergency pod, you're saved.

You're hidden, but you're stuck?

Thankfully, you recently purchased the Nyakouai's RPSS™ (Really Powerful Security System). While you're crying panicking relaxing in your cache, the ship will replace oxygen (or whatever Joeys breath) by a powerful neurotoxin. Of course, this can go only so far, since Bobs will quickly learn to hold their breath inside a ship. But depending of their endurance, it could force them to withdraw more quickly.

Last time I pitched for the Bobs, I mentionned a flamethrower. Why did I say neurotoxins this time? Well, fire is a hell of a hazard in space (pun intended). It will damage equipement, it will smell like seared Bobs, it ruins your leather seat...

Neurotoxins, on the other hand, can easily be flushed by the airlock, as well as the purple-flushed corpses of Bobs. No amount of armor is going to protect them from an airborne threat. Bobs start wearing rebreather? Buy the RPSS v2™ and switch to a liquidized neurotoxin, than infect the target upon contact on bare skin. Now Bobs are forced to wear hazmat-suits, and it should be harder to chase you while looking like a Tchernobyl rubber duck.

Get away

Now, I'm a Bob. I know my target is hidden somewhere in the ship, triggering traps all around me, and I can't protect from all of them. I'm not going to board that ship. I'm going to harpoon it, and haul it back to our Bob base, where we will disassemble the ship and the crew. Slowly.

As a Joe, you will need a safety measure. Perhaps something like a big red emergency button that makes your ship jump to the nearest Joey's Bob Eradication Facility (or any kind of militarized base able to deal appropriately with the aliens) where your ship will be safely sanitized.

Alternatively, a reasonnably responsive IA can handle the trap-triggering, road-mapping, get-away-ing while you're safely hiding in your closet. Having access to sensors in the ship would allow IA to mitigate damage, shut fire-doors and overall improve your defenses measures efficiency.

You're a stupid unfortunate Joe that was hanging out near the airlock when Bobs boarded.

Drat. You kinda had it coming, to be honest. But fear not, cause your captain recently splurged on shining news flamethrower. Of course, you're gonna burn some vents in the ship. Maybe the captain will flail you for ruining his/her/their fine leather seats (a shame, truly). But no amount of armor is gonna protect a Bob from the scorching hot plasma/flammable liquid very much aflame/nasty thing you're going to spray it with.

Fire has a big edge over ballistic weapons, beside the obvious psychological impact. It burns the cells. As a self proclaimed expert on Bobs physiology, they manage to heal bullet wounds and slash because their organisms can produce at an incredibly fast rate new cells to stitch shut the wounds. So their regeneration prowesses are probably directly related to the amount of damages to heal. With a flamethrower, there will be cells to replace all around. Most efficient way to degrade a Bob body in a short amount of time.

(And I hope they can't hunt you very well with their faces melted off).

If I were a Bob, I would let the airlock open, to prevent any kind of airborne threat to get my way (neurotoxins or flames or whatever). But it will severely limit my time on board until I developp a rebreather and some other apparatus to extend my survivability inside this mad-Joe-death-trap.

But are you safe?

Wait a second. Bobs are shapeshifters. Horror. What if Mathilda, the cute Joe in charge of space maps, is secretly a Bob that replaced the original?

All Joe are now wearing ID chips. This chip is personnal and state where you should be working. And if your chip say that Mathilda real name is Robert and that Robert shouldn't be here... Scorch it. Better safe and single than sorry.

Edit: On top of personnal ID chip, doors to critical sectors in ship are operated using a DNA scanner. A DNA scanner that requires a hand with a pulse to work (if you have no IA on board. Joeys do have a pulse, right?). Okay, Bobs will learn to drag on prisonner alive as a key, but it will delay their progression. They can't, of course, bring a prisonner from a former raid since his/her/their ID chip won't be listed as a ship member's ID.

If you're feeling extra fancy, and you're operating a very large ship, automated drones are tasked to open all the caches and check the people inside are safe and who they are. Everybody isn't released until safety is ensured. (Just pray your drones aren't rebellious - that's another story). Of course, the drones incinerate anyone with a faulty chip.


With Joe now hiding like (comparison redacted) and Bobs racing to disable the ship before it retreat to a safe location, the game is now for one side to hold the invaders back long enough to escape, and for the other to immobilize the ship before preys get away. Sound like a good pitch for an horror story. Oh wait...

  • $\begingroup$ Really enjoyably written $\endgroup$ Commented Mar 28, 2019 at 14:36
  • $\begingroup$ Made me laugh. Well thought out. Maybe a bit more emphasis on clever AIs than I'm willing to spring for in my universe, but a fun solution to invading Bobs. The ID chip concept is a good one. I hadn't thought of that. $\endgroup$
    – MarielS
    Commented Mar 28, 2019 at 17:06
  • $\begingroup$ @MarielS Well, I'm also partial to AI. That tends to oversimplify/overcomplexify things. I put it as an optional, linear improvement. Glad you liked our manual, stay safe from the Bobs. $\endgroup$
    – Nyakouai
    Commented Mar 28, 2019 at 19:02

To avoid unnecessary limitations, let's assume you were aware of the possibility of attack by these predators and took precautions.

OK, so I had a high tech space ship, life support stuff and had to prepare for unwelcomed visitors?

I have plenty of security cameras all around ship, right? (yes, I know in SF this ancient tech gets forgotten)

I would prepare a few nasty surprises for any wannabe pirate:

  • possibility to direct electric current in to a few dozen of key locations, to create trap somewhere between electric fence and... arc welding

  • they use high quality armor? Nice, there is an ancient tech for dealing with that, called HEAT (high-explosive anti-tank warhead). Effectively I'd offer them a few, remotely detonated landmines, but especially for them they would have some anti-tank features.

  • does this ship offer some germ sterilisation lamps? Because it may be improved from wimpish UV to hard gamma radiation. It's neat as it should keep my ship intact, while offer the invader a highly deserved painful death through radiation poisoning. (dodge radiation that would penetrate a few centimetres of lead)

  • I don't want to set whole ship on fire, so just to avoid extra damage I'd pump away all air from infected part. (hope the alien don't mind)

  • Alien presumably is smart and agile to avoid at least some of those traps... So I would sit down in pilot chair and using engines play a nice game called "let's bounce the alien around the walls". (it provides extra satisfaction, if the whole "fight" happens after the alien already received lethal dose of radiation)

  • Life support system should offer me ability to provide people with needed oxygen. In this case needed oxygen means hitting alien with some remotely detonated incinerating weapon and suddenly pump some huge amount of oxygen (like 2 atm of pure oxygen, that's 10 times more than normal). Under such conditions everything starts being highly flammable. (that's a weapon to be used in case of loosing)

Oh by occasion, maybe I'm childish and I don't like to admit defeat. So I would set some Dead Hand autodestruction procedures in case of me being killed or incapacitated.

  • 1
    $\begingroup$ Dead hand autodestruction is a solution that has not been suggested before but is a MAJOR point not to forget in the setting. The Joeys must absolutely prevent the aliens from capturing ships. $\endgroup$
    – Nyakouai
    Commented Mar 28, 2019 at 15:17

Spin me right round baby right round

Depending on the design of the ship, it could be designed for spin to simulate gravity. Now towards the center of the ship the spin is less and right in the center zero gravity.

Upon boarding, you and your crew flee to the center of the ship for safety and the ship spins like a crazy washing machine pinning the boarders to the floor. At this point you use an automated security system to kill them or crew in powered exoskeletons to clean up.


It depends on how large the ship is, but presumably there are going to be a bunch of bulkheads. Seal them off, release a flammable gas inside, and then start a fire with the aliens inside. Once they are thoroughly cooked, the gases can be vented and the fire will be put out. In order to limit damage, you can do this only when the aliens are in corridors. Also, you'd be able to control the temperature of the fire by what gas you use.


Funnel them into appropriately hardened corridors. The alleyways leading toward the bridge and the engine room?



Game over.

Extra credit:

I would like to have either an AA-12 or a UTS-15 (with drum magazine). Preference for the UTS-15, lower recoil, cut down for greater spread.

Or I'd like to be wearing rubber boots and carrying a hose; "Electrify the deck now HAL, 5kV should be fine".

Do you want 'real science' answers?

Seriously, I appreciate you have designed these guys tough. However the ability of real organic structures to deal with the energy of projectile weapons is vastly downplayed by Hollywood. A 10g projectile at 1000m/s delivers 5 kJ over an area of ~0.5 cm2.

If your beasties are 'swoting these away' what are their hands made of?

And noting also this requires a reaction time < 0.01 s at 10m.

I don't care how fast they heal, a couple of head strikes from even a medium calibre weapon at close range, especially if there is no penetration, will turn what is inside the skull to pink (or blue, or green) soup. All that energy has to go somewhere.

BTW, being an apex predator does not require the attributes you postulate.

Joe Human is Earth's apex predator. How did he get there, soft skin, no claws, no teeth to speak of? With his brain and by hunting as a pack.

So, we know they are coming. We are a technologically advanced (space-faring) civilization. Crew bolts to hardened safe-room.

  1. Ceiling mounted auto targeting mini-guns with crossed fields of fire in every alleyway. (cf Altered Carbon)
  2. Metal-storm style weapons with crossed fields of fire in the alleyways (180 rounds in 0.01 s). Use frangible rounds!, or -
  3. Microwave projectors in every alleyway. Hmmm toasty.
  • 1
    $\begingroup$ Thanks, you're the first person (I think) to mention that certain types of projectile weapon might still come in handy. Just because your average handheld gun doesn't hurt them much doesn't mean a well-designed flachette cannon couldn't! Of course, this kind of weapon might also be bad for your ship. A warship could have reinforced bulkheads, but a tradeship might not. $\endgroup$
    – MarielS
    Commented Mar 28, 2019 at 16:21
  • 1
    $\begingroup$ @MarielS, thanks. As one on one combat is so asymmetric, as mentioned elsewhere, area denial weapons are key. Exactly what they might be limited only by imagination and resources. I see some very disturbing imagination in many of these answers and comments. +1 for that. $\endgroup$
    – pHred
    Commented Mar 28, 2019 at 23:17

Throw it out of the air lock.

Ripley killed the Xenomorph this way, Javik always suggests this course of action, and its overall pretty simple. However your aliens seem much tankier then the xenomorph so maybe detaching a large portion of the ship would be necessary. Let them board or lure them to say the cargo hold and then detatch the module (preferably aimed towards a star or something) Another idea would be to lock down the ship and open the air locks. The crew could hide inside safe rooms while the aliens are hopefully suffocating or being dragged into the vacuum of space. Maybe they're clingy and refuse to leave? Well maybe you could rig the vents to start letting out pure oxygen and then ignite it. Both burning them and forcing them into the void. Final idea would be to take down the radiation shielding and just hammer the poor aliens with deadly radiation (if it would actually harm them of course). Once again your crew could hide inside safe rooms. Hope these ideas helped.

  • 1
    $\begingroup$ Ever played FTL? This happens a lot, and the solution is invariably to seal the doors and let the space in. $\endgroup$ Commented Mar 28, 2019 at 8:46

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