A few related questions here. The level of technology is that of humans capable of solar system exploration and colonization, with sub-light drives for long journeys within the solar system and nuclear-powered rockets for short range flight. Weapons are limited to missiles, light lasers and maybe rail guns, but rare. Anyway onward to my question.

First off, if a spaceship, let's call it spaceship X fired a missile at spaceship Y, and spaceship Y had shields or heavy armor, I would assume the blast would knock spaceship Y in the direction the missile was heading.

What possible way (does not have to be super realistic), can spaceship Y avoid this, aside from mounting 360-degree mini rockets on the hull?

Second, for spaceship X, if the missile was launched from a tube built into the ship as opposed to dropping it and firing, would spaceship X experience some sort of recoil?

How can these effects be mitigated or negated?

  • $\begingroup$ Maybe mount a 360 degree mini turret like the B-29 (sorry for lack of imagination). $\endgroup$ May 9, 2018 at 1:16
  • $\begingroup$ Similarly, do you mean lasers or plasma cannons? lasers don't show up in space :P which means no cool star wars effects $\endgroup$ May 9, 2018 at 1:28
  • $\begingroup$ ok, maybe plasma beams instead of lasers, but that was just to set the context of the technology level. $\endgroup$
    – Arkhaine
    May 9, 2018 at 2:34
  • $\begingroup$ either you hit gentlemanly or very very hard so that once connected you have a black hole ;D $\endgroup$
    – user6760
    May 9, 2018 at 3:42
  • $\begingroup$ Those "360 degree mini-rockets" are already a requirement for spacecraft. See en.wikipedia.org/wiki/Reaction_control_system $\endgroup$
    – UIDAlexD
    May 9, 2018 at 18:39

6 Answers 6


Let's deal with the question of recoil first because it's simpler.

Newton's third law applies, even more so in space because there is no pesky gravity or wind resistance throwing off the effects. So, if you shoot a missile, a bullet, or anything else out of the ship, there will be recoil. This is actually critical to your spaceship's function because (when you get right down to it) the propulsion system is just a sophisticated recoil system - you're shooting out a propellant at high speed to push your ship in the preferred direction.

As for 'dropping' your missile; you can't. No gravity. What you CAN do is push it out of the ship gently, then let its own guidance and propulsion systems activate. This will massively minimize recoil effects, but won't eliminate them to a mathematical zero.

Now; let's get into point defence.

Yes, for the reasons described above, the missile WILL push you in a direction on impact, but that's not the primary reason for wanting to avoid it hitting you in the first place. whatever the payload is, it's dangerous to your ship otherwise the enemy wouldn't be investing the energy and cost to launch it at you in the first place. HOW it's dangerous isn't material to your question; let's just say that none of us want that missile to hit and we want ways to stop it.

Modern warships and some modern military transport planes (including Air Force One) have a range of what we call point defence systems for getting rid of missiles. They range from starburst style hot chaff systems designed to get the missile to detonate before it reaches the plane, to 'metal storm' solutions that fire a massive volley of (essentially) ball bearings into the path of an incoming missile to (again) get it to detonate before reaching a ship, to turrets and guns designed to shoot down the incoming ordnance. One can also consider drones (both UAV and subs) to get in the path of torpedoes and other heavy ordnance so that the primary target is protected.

Your spaceship is no different, except that it has to consider a 3-dimensional theatre instead of a 2-dimensional theatre. Your best bet is to have turrets strategically positioned around your ship, and a small supply of kamikaze style drones that are programmed to fly directly into the path of incoming missiles as a contact target, protecting the primary ship.

To address Joe Bloggs' comment...

It has been pointed out that the point defence system may only be part of your solution because the overall inertia directed at your ship may not change much, just be spread over a wider area based on missile debris (Thanks Joe). Two your point defence system should, therefore, be designed to prioritize payload destruction, meaning that the 'dangerous' part of the missile (the warhead) is inactive by the time the missile strikes, and you're dealing with just the raw impact, which your armour would ideally keep out.

Of course, that means missiles would evolve to keep the payload as protected as possible, meaning that the other strategy would be for your drones to be of sufficient mass to knock the missile off course. If that was their sole function (especially if the payload is designed NOT to detonate until near the primary target) then you'd employ the 'double tap' of knocking the missile off course with your drone, then taking it out with point defence turrets.

Ultimately, you want the primary ship protected, so your only choices are to destroy the missiles before they reach you, or provide another target. Turrets and drones give you options that cover both strategies and with the addition of the above strategy, also deal with the 'mass driver' style missile system.

  • $\begingroup$ Thank you for your answer. What if there is no way to avoid it, is there no way to dissipate or redirect the momentum? A possible scenario could be that spaceship Y would be blown towards an incoming large object such as an asteroid or a larger ship for example. $\endgroup$
    – Arkhaine
    May 9, 2018 at 5:08
  • $\begingroup$ @Arkhaine the simple answer is to make your ship as heavy as possible. It already has a forward momentum, so all the missile is going to do is alter that momentum, not change it completely. The heavier the ship is, the less change in velocity. But, ideally you use your maneuvering thrusters to compensate if you can, and if it's a combat ship, it should already have impressive thrusters for combat agility. $\endgroup$
    – Tim B II
    May 9, 2018 at 5:37
  • 2
    $\begingroup$ Other fun one is that in space even if you get the missile to prematurely detonate the wreckage will still carry on towards you. If the missile has enough time to get up to speed and get close then even point defence might not help too much. $\endgroup$
    – Joe Bloggs
    May 9, 2018 at 5:47
  • $\begingroup$ @JoeBloggs good point, and I've updated my answer to reflect this concern. $\endgroup$
    – Tim B II
    May 9, 2018 at 5:57
  • $\begingroup$ You can launch a missile without moving the firing ship one bit. Observe how the military does it now: Man-portable anti-tank missiles. The missile lights in a tube but the exhaust (or in a few cases an inert reaction mass so it can be fired without a lot of open space behind) goes out the back, the missile goes out the front. The soldier stays put. $\endgroup$ May 10, 2018 at 3:23

The Kzinti lesson: "A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive." And your ships have nuclear reaction drives.


Already proposed here: antimissile weapons similar to what aircraft carriers use. Nothing wrong with that. The AEGIS system would work even better in space than it does at sea because there is no air resistance. https://en.wikipedia.org/wiki/Aegis_Combat_System

But you can do something in space that is not easy to do on land, and that makes it cool. I propose that your nuclear engines could be used as antimissile defenses. Your ship can wheel around using maneuvering thrusters (my favorite part of the TV series The Expanse is the maneuvers like this) and fire their nuclear engines at the oncoming missile. These are reaction drives and expel reaction mass probably at enormous velocities. You could have a typical spray propulsion but when you need the defensive property, screw down the nozzle and get a stream. Leave the stream on and swing it around with your maneuvering thrusters. How cool would that be?? Remember - that stream is going to push you forward too so have someone keep an eye out for what is in front of you.

mighty blaster firemans nozzle

Stream mode asks a lot of the magnetic confinement nozzle so not how you want to use your engines day to day. But you already have the nuclear engine and a stream of massive, 0.1c speed particles is excellent for dissuading incoming unwelcomes of any sort (including attacking Kzinti warships!)

Re equal and opposite reaction: a bullet pushes you back because the bullet pushes against the gun to depart. Same for a railgun. These are flying bits of metal (and excellent space weapons). But a rocket has its own propellant. Consider rocket tubes - flimsy little frames sticking up from a Humvee or a cylinder on some dude's shoulder. Rockets will push back minimally on the vehicle of origin because they push back on the propellant they throw behind them.


The question about recoil has been answered more than sufficiently, so I will look at the effects of missile impact.

Given that interplanetary velocities are ridiculously fast, you will discover that the common experiences on Earth are not good guides to what happens in space. Consider that the American Space Shuttles suffer damage to their tiles in orbit and on a few occasions had to have their armoured windows replaced because the orbiter had struck cops of paint that flaked off from spent boosters or other equipment in orbit. This gram sized fleck, too small to show on optical or radar searches, was moving at 7 Km/sec meaning it had a massive amount of kinetic energy. And 7 Km/sec is Earth's orbital velocity; you need to be moving much faster to travel in interplanetary space.

Space weaponry is likely to take advantage of this kinetic energy, and either direct the entire missile to strike the target, or release submunitions to create a "cone" of penetrators to catch a maneuvering vehicle.

The second effect is normal rules of physics no longer apply with hypervelocity impacts, and the impactor and target will behave more like fluids. The spaceship won't be "knocked off course", but more likely be split in two or more pieces, or a shimmering ball of plasma rapidly expanding into space. If that isn't dramatic enough, nuclear warheads can be used to drive weapons effects, ranging from pellets moving in a tight cone at 100km/sec to nuclear HEAT and EFP warheads driving metal into the target at 3% c.

Of course, using nuclear warheads to drive clouds of pellets at incoming missiles would be an effective counter to protect the ship, which would then lead to a calculus as to how many missiles you should carry as opposed to using the same payload to carry anti missiles.....

  • $\begingroup$ If I understand correctly, sort of a nuclear powered shotgun? By the (sort of) same thinking, can armor-piercing bullets fired at high velocity work against the missle, and would the recoil be less for the defending ship? $\endgroup$
    – Arkhaine
    May 9, 2018 at 5:11
  • $\begingroup$ I’m not sure even nuclear shotgun anti-missiles would be an effective counter to a hypervelocity missile unless it was a fair way from the ship, but if the missiles serve both as offence and defence You’d have a sort of pitched battle midway between two ships trying desperately to avoid hypervelocity wreckage. Which would be cool. $\endgroup$
    – Joe Bloggs
    May 9, 2018 at 5:51

The recoil a ship experience will depend heavily on how large your space ship is and how much force the missile is capable of impacting on the spaceships structure. A lot of energy is used up deforming and melting a spaceships hull which doesn't leave that much energy left over to love the entire spaceship.

In addition to that, your spaceships shouldn't only have engines/drives that only work in a single direction. This is space and you should have enough small engines on the spaceship to move it in all directions to allow better deceleration and turning ( If you only have engines in one location, you will be applying a force through the entirety of your spaceship to get it to turn. Think of it like turning a shopping trolley from one end by yourself, compared to having someone else pushing the front to help you control it), as well as lower the amount of stress your spaceship is under when its accelerating.

You would only have to cover 3 axis ( X,Y,Z ) to be able to move a part omni-direcitonally, but the actual number would again depend on the size of your spaceship and how much flexibility you want it to have.

  • $\begingroup$ Melting and vaporizing a hull segment with lasers will indeed impart a change in momentum as matter is ablated away from the ship. It could also cause explosive decompression which can be pretty forceful. Check out en.wikipedia.org/wiki/Laser_propulsion $\endgroup$
    – abestrange
    May 9, 2018 at 1:54
  • $\begingroup$ @abestrange I never considered lasers or rail guns because while they are mentioned they aren't part of the question asked. $\endgroup$
    – Shadowzee
    May 9, 2018 at 2:16
  • $\begingroup$ Thanks for your answer. basically if the missile is faster than the ship can maneuver to get out of its way and if there are no more anti-missile systems, then the ship will have to take the hit. Any ideas on last ditch protection against this? From the answers above, more armor would mean the ship would be slow to turn or move away anyway so was hoping there could be other ways. Can lasers be good enough in space to do so for example? $\endgroup$
    – Arkhaine
    May 9, 2018 at 5:20
  • $\begingroup$ @Arkhaine Lasers could work to prematurely destroy the missiles, but there would be work arounds for it as well. You can't really have a fool proof defense system. You could just have them travelling at an incredibly fast speed, that way projectiles can't really hit it, or have them spinning in a non-uniform pattern so that its harder to lock onto a specific section. $\endgroup$
    – Shadowzee
    May 9, 2018 at 6:35

The blast would knock the ship in a direction away from the blast. Does the missile have to impact the ship, or can it detonate close to the ship like today's air to air and surface to air missiles? If it must impact the ship to detonate, then the missile's vector is also a factor, yes, but probably not as much as the blast itself. The point of impact also makes a difference.

To avoid this, a ship could deploy missiles that take down other missiles, or chaff, or Phalanx CWIS-style projectile systems, or lasers, something else to throw off the enemy missile's guidance system, or some kind of magnetic or gravitic shielding, or maybe tiny mobile black holes, or something to absorb and harvest the energy so it doesn't cause damage. The possibilities are endless.

A missile launched from a tube using photochemical propulsion will probably have recoil. If the tube is hollow at both ends, it will have less recoil. If the missile is propelled by gravity, no recoil but acceleration would be very low.


To answer your second question first:

The momentum imparted on spaceship X firing a missile would be equal and opposite to the momentum of the fired missile when it leaves the launch tubes.

If the mass of the launched missile was fairly small compared to the mass of the spaceship X, and its velocity relatively low, it probably won't have a huge effect on our spaceship X.

The faster the missile and heavier the missile is, the greater the change in momentum of spaceship X because:

$$\text{Momentum} = \text{Mass} \times \text{Velocity}$$

You can mitigate this by launching the missiles at a fairly low velocity and accelerating them after they have left spaceship X.

This will leave hot trails of exhaust vapor that will make it easier for spaceship Y to detect and then mitigate, however.

Firing a railgun will impart all of the momentum of the fired round back onto spaceship X, but will be much harder to detect on spaceship Y. (Assuming the railgun fires a decent % of the speed of light($\%\times\text{c}$)

For question 1, avoiding being hit will be a much bigger deal than course corrections.

If you have viable sub-light propulsion, then you can pretty quickly recover from any course changes caused my momentum imparted by impacts. This can be done by simply rotating your ship in the direction you wish to accelerate and use your main engines.

Something to consider is that the direction of the imparted momentum from a missile could be pretty much any direction, depending on where the missile blows up in relation to the ship.

Imagine the missile passing the ship and blowing up behind it rather than slamming straight into spaceship Y.

Realistically if something hits you hard enough to change your velocity significantly, you will probably be blown to pieces, or at best have major holes blown through your ship.

The more armor your ship has, the more mass it has. That means it will resist the force imparted on your ship by any incoming projectiles, and receive a smaller change in velocity accordingly.

Avoiding being hit at all will be much more important to survival, so you want the smallest possible surface area exposed to your enemy. You want to have point thrusters all along you ship for emergency evasive maneuvers. Finally you will want to have point defenses to try and shoot down any incoming missiles or fighters.

Also note that having heavy armor will slow down your ability to react to incoming salvos, and probably won't be able to save you from a direct hit anyways.

  • $\begingroup$ It's kind of an aesthetic edit--not really necessary--but I put your MathJax into display mode and straightened the text. Feel free to rollback if you need to: It's more of a structural/easier to read/center edit. $\endgroup$ May 9, 2018 at 17:49

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