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Any spaceship over several hundred tons is an extremely dangerous weapon in the wrong hands, particularly if it's intended for reentry. Made mostly of metal and possibly shaped to be able to breach an atmosphere without trapping enough air and generating enough friction to destroy itself, it'd be at least dangerous as the worst asteroids that have hit the Earth were it sent plummeting into the surface at full burn. Not only that, but it may well be able to hold its own against an armed assault - it doesn't have to be functional or even in one piece to not destroy a town. Even if it is vaporized or broken up, coming straight down in a planned route, the fireball or resulting explosion would as devastating (if not more) than an atomic bomb.

Clearly, any commercial space-faring civilization has the capacity for one lunatic to try and wipe out as many people as they can with their own death. How would a planet protect itself against such a malicious threat?

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    $\begingroup$ Depends a great deal on the details. Especially the specifics of spacecraft drive systems. For a lot of really powerful drives seen in scifi (Acceleration at hundreds of gravities with high endurance) the answer is that there's no reasonable defence possible and you either accept that, or pretend the problem doesn't exist for the sake of the story. $\endgroup$ – smithkm Sep 17 '14 at 1:40
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    $\begingroup$ Rephrased slightly to avoid profanity (see meta.worldbuilding.stackexchange.com/questions/287/…) $\endgroup$ – Liath Oct 23 '14 at 9:36
  • $\begingroup$ similar question: worldbuilding.stackexchange.com/questions/1260/… $\endgroup$ – Vincent Oct 23 '14 at 15:37
  • $\begingroup$ Shouldn't that link go on the other question? This one is more than two weeks older. $\endgroup$ – kikjezrous Oct 23 '14 at 20:37
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    $\begingroup$ The problem is worse than that... it doesn't to have be suicide crash. Assuming a large, high delta-v ship, all you need to do is use your reaction drive the right (wrong) way and you'll vaporize everything for at least miles behind you, possibly out to hundreds of miles. $\endgroup$ – Dan L Dec 3 '14 at 2:52

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The answer to that is pretty much the same as "What prevents someone from deliberately flying an airliner full of passengers into a skyscraper?" today. It has happened. There may be safeguards and lots of them, but there is a possibility that they will be circumvented.

Given that it is possible but unlikely that a deliberate collision could occur, the most obvious is that commercial spacecraft would be designed to simply not allow a planetary collision without overriding all sorts of things, at which point they would be designed to broadcast a warning (that the ship itself would be programmed to ignore) that the ship's safeguards had been overridden and it was on a planetary collision course. If you knew about that, you could override that too, but I don't expect that it would be made common knowledge.

Planetary traffic control would constantly be checking that it had control (i.e. direct automated control, not just telling the human pilot "Do this") of nearby ships, and if any didn't acknowledge control (i.e. make the exact course change requested by traffic control computers), it would be flagged as out of control to planetary defense. If it was on a collision course, a warning would be issued - if there was time - otherwise it would simply be destroyed. Better a whole lot of small objects entering a planetary atmosphere than one large one. Even if it is full of passengers, it is better that they all die than an Extinction-Level-Event occur on the planet.

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    $\begingroup$ I want to add that if the psychopath accelerates to full speed from a few lightseconds (effectivly out of scanner range) away, he would still be able to simply rush through the planetary defenses, except if you were able to totally vaporize him, which you already mentioned could be devastating on its own if not executed properly. $\endgroup$ – J_F_B_M Sep 17 '14 at 18:06
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    $\begingroup$ @Larethian, as drives become more powerful and speeds become higher, the range of planetary traffic control and defense would have to increase in step. IMO, planetary defense would have to be capable of dealing with two simultaneous threats of this nature at a minimum, regardless of drive capabilities. The consequences of doing otherwise could be apocalyptic. $\endgroup$ – Monty Wild Sep 18 '14 at 6:02
  • $\begingroup$ If the ship is big/fast enough to be an extinction event it's very unlikely you can shoot it down in time. $\endgroup$ – Loren Pechtel Nov 1 '14 at 19:09
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    $\begingroup$ @MontyWild Not really, because in a hot gas cloud, there's nothing keeping it cohesive - the kinetic energy will just become heat, and the heat will dissipate as the volume increases and the pressure drops. If you give it enough time to drop to approximately atmospheric pressure (a few seconds), it becomes essentially harmless. $\endgroup$ – Mike L. Nov 24 '14 at 12:32
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    $\begingroup$ If I remember correctly, in the one of the Space Marines trilogy of Ian Douglas, they annihilate a planet by accelerating several tons of sand to relativistic speed, which destroys all of planetary defense, and then smashing the carrier of such sand into the planet. $\endgroup$ – Duralumin Feb 18 '15 at 16:54
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Short answer: have the author/GM deliberately design the world so that the threat isn't a serious one.

Long answer:

The devil is in the details.

There are essentially three broad categories of safeguard available, the effectiveness of which vary heavily depending on exactly how your world is set up. Unless you tweak the parameters very carefully, they're all likely to have fairly major implications for what your world ends up looking like.

For a lot of standard Sci-Fi worlds - ones with widespread public ownership of starships, FTL travel using highly efficient engines, effective energy shields, and significant technological capabilities available on the black market - the answer essentially boils down to "you can't". An incoming ship is too fast and too durable to stop in time, and the government doesn't have the ability to prevent a psychopath from taking full control of a ship in the first place.

1) Preventing psychopaths from gaining control of the ship in the first place.

There are two halves to this, which would almost certainly be used in concert.

First is making sure that they never get behind the helm of the ships at all. Starship pilot licenses could be heavily regulated, including mandatory psychological testing (probably at regular intervals). Similarly, the ownership and sale of all starships could be regulated, with all spacecraft either directly government-owned, or owned by people who have been thoroughly checked out and determined not to be a threat to public safety.

Second is making sure that if a sick bastard does manage to get ahold of a ship, they can't crash it into anything valuable. Biometric scanners that only unlock the controls for authorized users are one possible tool. Other options are hard-wired safety systems that won't allow the ship to travel above a certain speed within a certain distance from planets, and remote overrides to allow traffic control to take command of any ship that enters their sphere of control. The effectiveness of these is strongly dependant on how good computer/electronic security is in your world - if criminals have the skills and resources to effectively hack and/or hotwire such systems, then other precautions will be necessary.

2) Intercepting the ship short of its target

This one depends heavily on the performance characteristics of your ships and sensor networks. A successful response requires both that the defenders detect the incoming ship early enough to respond, and that they have assets in place that can intercept the ship once they do detect it. Orbital defense platforms are an option but might find it difficult to prevent debris showers from hitting the planet even if they destroy the ship. More likely this would need warships held on permanent alert (if the threat is serious enough, specially-designed interceptors would probably make up a significant fraction of the world's military forces).

Long-range, high-resolution sensor equipment is essential here, and faster-than-light sensor capabilities are highly recommended. In particular, a ship traveling at close to or above light-speed is effectively impossible to intercept without FTL sensors - your first 'warning' arrives at the same time as the incoming ship.

Effective long-range weaponry helps the defenders - without it, they need an actual physical interception with another ship, and that could be difficult. Effective ship defenses help the attacker, as does any mechanism (artificial gravity, force fields, inertial sumps...) that will prevent a ship from being physically deflected from its course by an outside force.

High maximum speeds are more useful for the attacker: they increase damage potential, decrease the time the defenders have available to respond and make the ship a more difficult target to intercept.

High ship acceleration rates favour the defender - or rather, low accelerations favour the attacker (as long as ship operation ranges are significantly higher than maximum sensor ranges). The attacker can spend as long as they like accelerating, while the defenders are scrambling to intercept a ship that is likely already at full speed.

3) Ensuring that there aren't any targets

If neither of the above is feasible, the only remaining option is to ensure that there's nothing available that a suicide run can destroy. Burying everything below the surface of the planet is one option, but only works against relatively slow starships - at sufficiently high speeds, you just can't bury things deep enough.

Otherwise, the remaining option is to abandon planets completely. Move everything vital onto self-propelled space stations, and keep them moving in a random pattern to prevent an attacker from being able to predict where they'll be accurately enough to hit them.

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    $\begingroup$ Ok, that last option is hilarious. $\endgroup$ – Dan L Dec 3 '14 at 2:48
  • $\begingroup$ Well it you read Atomic rockets there is no stealth in space so you won't have a problem spotting an incoming ship. $\endgroup$ – Efialtes Apr 16 '18 at 13:25
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Apart from the comparison to the airliner scenario in Monty Wild’s answer, it might be worth checking out the safeguards we have against somebody launching a nuclear weapon into an inhabited area, since that would be comprably desastrous and has not happened for a long time. That is, if you can get reliable information about this at all.

I think among the important points will be:

  • Have more than one sick bastard be required, i.e., spacecrafts are designed such that steering them requires more than one human by design. (Moreover to avoid people planning to collaborate on this, assign people to missions randomly and never let people work together who know each other.)
  • Take maximum action long before any imminent danger. In case of a spaceship, do not only intiate a preinstalled self-destruct or similar when the ship is on a collision course and short before coliding, but when it takes the first steps to do so, i.e., deactivating the autopilot within a certain distance to earth, taking a collision course while being lightyears away or even entering the solar system with a large ship (which may be unfeasible though). Clearly forbid doing anything like this, so that nobody can complain about being blown into pieces.
  • Inherently delay all actions taken to create time for countermeasures.
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    $\begingroup$ If you're light year(s) away, how is a collision course any different than trying to get to your destination? $\endgroup$ – user3082 Feb 19 '15 at 17:11
  • $\begingroup$ In space, there is hardly any friction, so it’s actually possible to make such precise distinctions between different courses lightyears away. That’s related to the source of this attack possibility: You set the ship on collision course lightyears away and then sabotage the engine and all methods of steering. $\endgroup$ – Wrzlprmft Feb 19 '15 at 17:17
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The solution is have the spacecraft are flown by Artificial intelligence- it could even be a cooperative or a background operation where people appear to fly them but its simply impossible to actively crash the spacecraft to cause mass-damage. Most of our spacecraft are quite automated however the abilities of AI now are not utilised to their limits. A combination of expert system and 3D control and analytics program. The speeds of spacecraft are insane- humans would find it difficult to manage directly anyway. Pilots of aircraft can only manage their planes to avoid collision by having designated separations- only really possible in sonic and subsonic speeds. Control gets more and more difficult the faster you go.

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    $\begingroup$ "Control gets more and more difficult the faster you go." No, control gets more difficult the less time you have to react. Driving a car at highway speeds on a highway isn't terribly difficult once you know how to drive a car. Driving the same car at the same speeds in the middle of a cramped city is a bit more involved. While even our current spacecraft move at non-trivial speeds, they also cover immense distances. $\endgroup$ – a CVn Nov 1 '14 at 13:39
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    $\begingroup$ But now you have to defend against psychopathic hackers with a computer virus that gives AI's suicidal depression. $\endgroup$ – Zan Lynx Dec 11 '14 at 7:03
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    $\begingroup$ You still have to deal with the possibility of the AI going mad by itself. Oh, and don't tell your ship about existentialism! ;-) $\endgroup$ – celtschk Feb 18 '15 at 22:18
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Not a complete solution, but hiding entire planets could be a mitigating strategy.

Imagine aliens greeting us tomorrow with the polite version of "here's what we consider our territory, here are the acceptable means of trade and communication, and any attempts of locating our home planet whithin that territory would be considered an act of war. No offence intended, it's just that your society occasionally produces murderous psycopaths, so it would be dangerous if you knew it"

Many common faster-than-light-travel tropes, e.g. warp drives and wormholes, have the ship just dissappear from normal space than re-appear at the destination, either instantly or some time later. It's not much of a stretch to make it hard to track, especially if you do without the traditional special effect of having the ship sharply accelerate in a particular direction just before dissappearing.

As for the radio bubble, it only travels at light-speed, and would not exist for planets settled after more effective communications are discovered, may be only a thin shell for other planets.

It's not the be-all and end-all of planetary security, but it would be a nice addition if no definite solution is available.

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  • $\begingroup$ Okay. How do you go about hiding an entire freaking planet? $\endgroup$ – kikjezrous Nov 27 '14 at 1:22
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    $\begingroup$ The universe is pretty big. If you don't publish the planet's location and take precautions that your ships can't be just tracked back to it, it's hidden by default. $\endgroup$ – Emilio M Bumachar Nov 27 '14 at 11:04
  • $\begingroup$ But why would you ever want to establish residence on a planet which you'd not be able to find? $\endgroup$ – kikjezrous Nov 27 '14 at 14:47
  • $\begingroup$ Between the radio bubble and the fact that you'll have spacecraft leaving flaming trails and the heat of glowing radiators, how would you even manage to go to a planet without being detected, much less do enough on and around there to make it a target worthy of being blown up? $\endgroup$ – kikjezrous Nov 27 '14 at 14:49
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    $\begingroup$ @kikjezrous: I updated the answer to address the points you raised. $\endgroup$ – Emilio M Bumachar Feb 18 '15 at 13:25
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I think some kind of active countermeasure would be used - missiles armed with advanced expanding foam canisters that adhere to the ship and expand into something like the Mars lander 'air bags' (it wouldn't prevent the impact, just mitigate it). Similarly other missiles could attach directly to the hull and deploy wind breakers to slow the ship down. Such crash landing systems may be built into the ship itself and activated by the navigation computer in the event of a collision being imminent, the firing of retro thrusters would also be a standard countermeasure.

Edit: High speed automated drones could attach to the hull and fire their own thrusters to slow/redirect the ship to a less populated area.

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Like the roman army always kept at a safe distance to the Capital Rome behind the river Rubicon (if my memory serves). And violation to that rule considered active treason without further investigation.

Establish a perimeter that no trespasser allowed inside that zone (friend or foe or any celestial body) by destroying them by any means possible.

Yes you may start interstellar war by destroying a massive alien ship just came by to say hi.

But it's a part of the deal.

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The most effective way would be to have no such dangerous spaceships. Either have no spaceships that are dangerous, or have many design features on the spaceships, and safeguards and precautions such that it would take a lot for someone to use a spaceship as a missile.

Another very good countermeasure would be to have a very healthy society, which detects and heals psychopaths very readily. This would also have many other great benefits, and is a great idea overall.

Another good countermeasure is to have and require entire crews of healthy people to control a ship.

Another is to have technological countermeasures that can detect, divert, disable or destroy such a ship. Depending on the technology available, this might or might not be difficult.

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In hard science fiction, interstellar travel is on the order of a type 2 civilization. And even then, you are sending star wisps, who self-manufacture a civilization at the target, not people.

If you have a planet-bound civilization with a few billion people, heat dissipation limits the energy budget of the land-locked to only a few orders of magnitude higher than our current energy requirements. "Cheap" interplanetary travel requires a lot more energy than that.

So for a mostly planet-bound civilization, only the rich can travel from planet to planet.

Non-planet based civilization are either slow moving and self-contained, or energy-rich like the inter-planetary civilizations (able to import goods from the bottom of the gravity well, be it whatever). While a ship may move extremely fast, it won't change course extremely fast. So your best bet in such a civilization would be to thrust as far as you can outside the solar system, then adjust course on the way back to hit a target (coming in a high velocity). This won't be cheap energy-budget wise.

This all assumes we are still mostly flesh and bone.

If we have uploaded, the physical world could be beyond most of our permission levels. The few interacting with it and controlling high KE spaceships would be heavily audited for safety.

If we go soft sci-fi and start adding near-light sub-light drives that accelerate at 100s of Gs, such a ship very quickly starts having the energy budget equivalent of a Type 1 or 2 civilization all by itself. Living on a planet when you have Type 1/2 civilization energy budgets owned by a few people (a ship) seems relatively ridiculous: the heat dissipation problems alone will make anyone in the planet's atmosphere a pauper.

If we add in warp drive/jump drive and shields and the like, the game changes. One of the reasons why you cannot defend against weapons in a high energy civilization is that chemical bonds are already relatively weak compared to the energy budget of our weapons: shields can change that rule. You could just shield the planet. The energy budget of a single ship would be dwarfed by a sufficiently advanced soft sci-fi planet's energy budget of non-paupers (the heat problem handled by more hand-wavium, like really tall and hot "shield-material" radiators). The ship would bounce off mostly harmlessly.

If shields don't exist, then a ridiculously huge perimeter defence that knocks incoming attacks off course could work, that perimeter getting larger as civilization energy budgets climb (as they also have to defend against relativistic KE attacks from other civilizations, or large parts of their own). Within the perimeter, you are not allowed unless you are run by the planet's trusted "ferry boat"ers. To visit Earth, please park at 3 light years at one of the designated docking stations, and get on board a locally controlled "ferry boat".

Finally, there is no guarantee that any of this would work. For the longest time, attacks were balanced against armor. In the nuclear age, there are no defensive materials that can stand up to a thermonuclear bomb. We don't have weapons that can intercept an ICBM effectively (even a nuclear interceptor). Such a situation could exist. In that case, you'd no more give a private individual a spaceship than today someone is allowed to run a nuclear reactor in their back shed and have hobby h-bomb in the basement.

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By the time such advanced technology is available, I would think all sorts of other advanced tech is around to deal with these issues. Especially since essentially these spaceships behave like asteroids, which we're trying to deal with as is.

First thing that comes to mind from most sci-fi worlds is using a shield. Especially the kind as used on Dune, where the shield blocks fast moving objects. If you surround a planet with such a shield, any kind of suicide run would end with the ship smashing into the planetary shield if it comes in too fast, but regular landing ships can come in regardless.

Slower ships that try to accelerate inside the shields would deal considerably less damage or might even be shot to scrap before they manage to speed up if detected.

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    $\begingroup$ When they ask for the combination, tell them: one, two, three, four, six. $\endgroup$ – Mazura Apr 3 '15 at 1:56
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A partial solution: Multiple types of ships. The big starships that could produce an extinction event are not allowed within a considerable range of a planet--say, a light-hour or even more. Furthermore, anything that's going to bust that zone in the next 24 hours is promptly destroyed. The defense missiles are large kinetic impactors--most of the plasma produced won't hit the planet at all.

Interstellar traffic arrives at a station in the outer part of the star system. Traffic then transfers to shuttles that take it to the general vicinity of the planet where it is transferred yet again to smaller, lower performance craft for the final approach. The basic idea is to keep everything far enough away that the system defenses can take it out if need be.

Alternately, use warp drives for your starships--you move space, not the ship. It doesn't need a drive capable of planet-killing speeds.

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  • $\begingroup$ You do know that warp drives produce planet killing bursts of radiation when they decelerate right? $\endgroup$ – Efialtes Apr 16 '18 at 13:31
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Send up a rag tag team of NASA funded deep sea drillers

Cue Aerosmith. A team of highly trained and equally psychotic unlikely heroes are sent up in a pair of space shuttles to drill a hole into the hull of the spaceship and deposit a nuclear warhead under the armor plating. Hilarity ensues.

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I'm not sure that stopping the ship would be that difficult (depending on total size) - consider the Columbia disaster. Getting a spacecraft to survive re-entry is very difficult, so hitting it with a reasonable-sized missile is going to cause very serious problems for the craft. With a large ship there will likely still be bits that reach the surface, but much smaller and with very little accuracy. As long as it's detected at long range and hit well before it begins re-entry, the casualties should be fairly light.

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Automate ships.

Just like we have autonomonous cars nowadays, all spaceships should be autonomous. You tell them where to go, and they will maneuver there on their own. This is the most ideal way to do thins anyway, since a computer is much better at figuring out the best paths and times to accelerate in order to get the most delta-v out of your fuel.

Also ensure that ships follow the three laws of robotics.

If anyone tries to fly a ship that allows direct human control - brand them as terrorists and blow them out of the skies.

This way, all your psycho can do is ask the ship - nicely - to go to some town or city, and the ship will go there and land gently. If they do want to blow something on the planet with the ship, they will need to hack into the planet's transport infrastructure in the first place. Given a sci-fi scenario, if they are able to do that, they are able to do much worse stuff (i.e.: remotely comandeering nukes), so a rogue ship would be the lesser of your worries.

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  • $\begingroup$ You have "If anyone tries to fly a ship that allows direct human control - brand them as terrorists and blow them out of the skies." Perhaps you forgot something there? Maybe a "...that allows direct human control, and the ship senses they are accelerating too much toward a nearby structure - brand them..." $\endgroup$ – Loduwijk Feb 8 '18 at 18:28
  • $\begingroup$ @Aaron nope. If they even try to take off, blow them away. Human controllers are less efficient and less safe than computers, so if any meatbag tries to fly a ship they likely have bad intentions in mind. If they want to fly dangerously they can pilot their fly-by-stick tincans over Pluto. $\endgroup$ – Renan Feb 8 '18 at 18:37
  • $\begingroup$ Then it does not make sense to provide a ship which has a human control option if you will never, ever allow it to be used. "Me: Nice, this coffee maker has a latte button. Bob: Don't push that button!!! It electrocutes to death whoever presses it as a secondary feature. Me: Then why even bother to include it at all in the first place!?! Bob: In case you want a latte. Me: But... but... ... Bob: Yeah, maybe we ought to rethink that one." $\endgroup$ – Loduwijk Feb 8 '18 at 19:00
  • $\begingroup$ @Aaron if you do find a ship like that it was either smuggled onto the planet or built illegaly. $\endgroup$ – Renan Feb 8 '18 at 19:26
  • $\begingroup$ OH! Ok. When I read through your answer, that part coming on the heels of "all spaceships should be autonomous" led me to believe that "a ship that allows direct human control" was some sort of a trusted ship, like a military or science ship that had a special control mode. $\endgroup$ – Loduwijk Feb 8 '18 at 19:58
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Usual formula to prevent ships from escaping (wich can hold true in the scenario proposed) is to use a tractor beam to stop the ship mid-space and force it to dock into another ship.

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For objects the diameter of what we usually conceive as a spaceship (say, something the size of an aircraft carrier, end on) above about 100-1000 km/s very little of its energy will transfer either to the atmosphere or the crust unless it breaks up or spreads out in some way. This can be understood in terms of the Bragg curve for absorbing energy from heavy ions. A possible mitigation is to enclose the ship in a shield or design them not to break up. A strategy for the terrorist is to strike a glancing blow rather than straight down, to maximize energy transfer and breakup.

Even at .9c, the Lorentz gamma is only 2.3 (http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/ltrans.html will calculate them for you). The classical KE is 9x9=81 times greater x 2.3 = 186 times the energy of .1 c. Or about 200 million times the energy of the actual Pioneer 10.

This is still not enough unless the spacecraft is way more than 500 kg, which of course I assume. The problem is capturing the energy. I work in the field of radiation effects of high energy particles in semiconductors. Most high energy particles have very little to no effect. You have to slow them down so that they are basically stopping in the thin layer of silicon to capture much energy. This is called the Bragg peak, and you can google it.

So the earth and its atmosphere are like that. The atmosphere will catch and stop ordinary meteoroids. Their energy winds up as a bit of heat and light and a sonic boom. It can be destructive near the ground as in Tunguska. The dinosaur event was captured in the crust because its size and frontal area were such that it stopped there, maximizing destruction on the surface of the earth, throwing up debris, etc.

A faster asteroid of the same size would possibly destabilize the earth by interacting with the core. But in this case, the spaceship is far smaller. We just have it going very fast. So it is not going to transfer much energy to the atmosphere. Its time there is too short. You won't "see" it streaking across the sky, it is moving ten thousand times faster than a typical meteor. It'll just punch a hole about its own size in the crust and deposit its energy gradually along its track through the interior of the planet. It will deposit most of its energy right at the end as it stops, just like the heavy ions produced by cyclotrons. If it stops in the core, or anywhere very deep, we'll register a few seismic waves and that's about it.

So I take back what I said about it automatically being an extinction event. It might not even be possible to achieve with "ordinary" size spacecraft because at worst they'd just make an ugly explosion as they rip out the other side. If going fast enough, they'd make a clean hole on both sides and impart relatively little of their energy to the earth, even though their energy is enormous. The hole would fill up with magma which would cool, and unless the crust is already thin there with magma close to the surface, it would not give any trouble. It would take a large diameter starship or a very clever terrorist.

Now, there is some rationale for very large starships, essentially hollowed out asteroids or minor planetoids. These would shield occupants from cosmic rays and any debris they encountered, which will be a serious problem otherwise. Once you are effectively using stars to accelerate the things, it matters little if it is a couple orders of magnitude bigger than the kinds of ships we sail on the ocean. In fact in the 1960s DOD had a plan to send a nuke-propelled town sitting on bedrock on a planetary tour, so ships that size have already been conceived and we have the physics to propel them if we are willing to use it. If such a thing hit the earth at a relativistic speed, it would be catastrophic. Perhaps not of the order of the event that created the moon, because a town is still not so large, but it'd make a very big hole in the crust and almost certainly knock the planet off its axis.


Here is an unrelated analysis inspired by Scott Downey's post "I think some kind of active countermeasure would be used ..." I would have commented, but apparently being a new member I have not the privilege.

This has been looked at for asteroid defense, and you can google that subject. Just blowing up an object still leaves a debris cloud proceeding along approximately the same trajectory, which can be a great deal more dangerous in this case because it spreads the energy over a larger area, helping it to stop (like the Bragg curve) in the crust or atmosphere and do the most damage. Generally, explosions (despite the movie) are the least favored asteroid defense. You have to deflect the thing, preferably without breaking it up into scattershot.

Since asteroids are relatively slow, you can do things like fly along beside it and use the spacecraft's gravity to nudge it off course. Using an array of mirrors to focus sunlight and create a little thrust stream of vaporized material to deflect it has also been proposed. For a faster spaceship, a much more energetic version of either of these would be needed, such as a tractor beam to drag it quickly off course, or fusion lasers to create the surface vapors, but being careful not to break it into pieces.

So, I guess you guys can fashion a pretty wide variety of conditions and related stories by re-sizing the ship. I look forward to reading them!

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  • $\begingroup$ @RobertSchuler, edit your answer instead of providing another one. Now you need to go in and remove one of them. $\endgroup$ – MichaelK Sep 28 '16 at 13:48
  • $\begingroup$ This answer sounds backwards. Smaller pieces tend to vaporize in the atmosphere before reaching ground, where larger pieces (if they don't break up when striking the atmosphere) tend to reach the ground and cause more problems. It sounds looks like you suggested the other way around. What was the reasoning behind that? $\endgroup$ – Loduwijk Feb 8 '18 at 18:51
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Harpoon parachute missiles.

It's a harpoon with an explosive hook. On impact, the harpoon uses the explosive to permanently weld into the skin of the aircraft - the same explosion deploys a parachute. Then quantity. Have fully automatic harpoon launchers and simply have fighter jets assault a large aircraft with these harpoons parachutes. After several thousand, there should be enough parachutes that gunfire could not mitigate the drag of a million mini parachutes.

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    $\begingroup$ The reason you're getting downvoted is simply because that wouldn't change anything. The kinetic energy is still there, and using parachutes (assuming they'd even be strong enough to do anything) wouldn't change the fact that that energy is going to end up in a fireball and a very large amount of dirt and air being moved rapidly out of the way. $\endgroup$ – kikjezrous Apr 7 '15 at 22:41
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    $\begingroup$ Assuming your parachutes attached and were strong enough to survive the wind-blast, compressive heating would burn or melt them away in seconds. $\endgroup$ – Matt Bowyer Sep 26 '16 at 13:26

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