A spaceship is a very expensive thing to own and maintain. But what's more, this is a spaceship that can sustain high-g burns for long stretches of time, like in The Expanse verse. So this means that if it accelerates in one direction at about a month, it reaches a significant portion of the speed of light. And since it's now a very massive thing going relativistic, Very Bad Things will happen if a planet happens to be in its way. As in "continental devastation and up from there" levels of bad. And even without that, a spaceship with such an engine is a weapon even if it doesn't try to ram a planet - if your target is another spaceship or station you can just fly up close, turn around and melt it with your exhaust plume.

Given all that, without any additional measures, no sane government will allow individuals to own such weapons of mass destruction, even if those individuals would be rich enough to afford to own a spaceship. Every spaceship would be state-owned (Or at the very least corporation-owned), and every spaceship that would deviate from its carefully preplanned course without warning should be annihilated on the spot in case it was hijacked to become a suicidal RKKV.

Can there be something that would allow classic private space opera spaceships to be plausible despite the aforementioned consequences of them not requiring years to cross interplanetary space?

  • 35
    $\begingroup$ It is not clear what you think is the difference between an asset owned by a person and an asset owned by a corporation. For example, in real life, there is this corporation named SpaceX which owns quite a few rockets. As it happens, SpaceX is a private corporation owned by a certain Mr. Elon Musk. In a very real sense, Mr. Musk owns a fleet of rockets. Yes, those rockets could wreck havoc on unsuspecting targets. No, that is very unlikely to happen. (And anyway, who owns the nuclear ballistic missile submarine is irrelevant. What counts is who is in command of that submarine.) $\endgroup$
    – AlexP
    Jan 30, 2022 at 3:34
  • 6
    $\begingroup$ @AlexP Usually corporations, especially large ones, have many regulations and checks in place regarding corporate property, they require it to function. I'm sure the CEO of BP can't just one day show up on one of their oil hypertankers and take it out for a spin without making lots and lots of arrangements and agreements with lots of company's departments beforehand. $\endgroup$ Jan 30, 2022 at 4:04
  • 24
    $\begingroup$ The CEO cannot, but the captain of the hypertanker can. That is my point. It is irrelevant who owns the ship: what counts is who commands the ship. $\endgroup$
    – AlexP
    Jan 30, 2022 at 4:15
  • 5
    $\begingroup$ I would point out that even relativistic spaceships require years to cross interplanetary space. If you want classic space opera, you're going to need some form of FTL (to which you can place whatever constraints you like on to prevent weaponization if that's your desire.) $\endgroup$
    – Gene
    Jan 30, 2022 at 7:34
  • 5
    $\begingroup$ fortunately, sustained high-g burns are about as much science fantasy as FTL. the energy requirements are just too high, no matter how efficient your engines are. $\endgroup$
    – ths
    Jan 30, 2022 at 12:21

19 Answers 19


We already manage this today, with airplanes.

Airplanes can cause immense damage to places if they ram into them, as 9/11 proved. We prevent them from causing damage in other ways.

  1. Near countries you have defined routes you need to fly. Planets and stations would have similar ideas. They'd ping you at a certain distance and tell you to lock in x route if you wanted to come in. If you instead burnt to move at relativistic speeds, they'd shoot you down. If you just fly in at really slow speeds they'd watch you like a hawk, but not shoot you down.

  2. Everything is tracked. We have radio waves and other stuff scanning everything for rogue planes. Space is even easier to track people in, and at relativistic speeds you generate so much heat you'll be spotted from a solar system away.

  3. The world is big. Most planes that crash crash somewhere harmless, because most of the planet is sea, and there's nothing human there. Space is even bigger. If someone does a burn to your planet and you blast them just do a small course correction and they'll fly off into space to never be heard of again.

None of these issues mean individuals shouldn't own vehicles. It's not like terrorists can't make corporations. Every vehicle would be carefully monitored.

  • 12
    $\begingroup$ There are major problems with shooting down a spaceship effectively though, once it's on a collision course the debris stays on that course, even if that debris is in fact a cloud of ionised vapour it's potentially deadly. Otherwise yeah. $\endgroup$
    – Ash
    Jan 31, 2022 at 1:42
  • 4
    $\begingroup$ @Ash For space stations, certainly. For Earth no. If it's broken in tiny pieces the atmosphere will burn everything that remains before it hits the ground, and will spread the energy of the impact over several billion cubic meters of air. Negligible effects anyway. $\endgroup$
    – Rekesoft
    Jan 31, 2022 at 10:44
  • 1
    $\begingroup$ To get to 2% of light speed takes a week at 1g acceleration. That's a lot of time for a spaceship to move away, or fire massive laser bursts at an enemy. And yeah, small fragments are no issue for a planet. Against a spacestation you need enough mass that a spaceship can't batter it away, or an engine to keep the weapons on target. $\endgroup$
    – Nepene Nep
    Jan 31, 2022 at 12:02
  • 3
    $\begingroup$ @Rekesoft For space station you shoot the ship so it can't change its course any more and then you change the space station's course a bit to avoid being hit. $\endgroup$
    – Jan Hudec
    Jan 31, 2022 at 13:19
  • $\begingroup$ @Rekesoft I can think of a few things I wouldn't want entering the atmosphere in any great quantity but you're right that direct fragment impacts are a negligible threat to an Earthlike world. $\endgroup$
    – Ash
    Feb 1, 2022 at 5:15

Lets break this down a bit.

So this means that if it accelerates in one direction at about a month, it reaches a significant portion of the speed of light

There are a number of assumptions here. Being able to sustain high-G burns is one thing, but reaching a significant percentage of the speed of light is very definitely another.

The first problem is the tyranny of the rocket equation, and the second is propulsive efficiency.

The first tells us that in order to reach very high speeds without needing implausible amount of fuel and reaction mass, you need a rocket with a very high exhaust velocity, and the second tells us that the efficiency of a rocket travelling below its own exhaust velocity is significantly diminished.

What can we work out from this? Well, if you want to be travelling at relativistic speeds, you're going to need antimatter, and you're going to need a lot of it. Fusion just isn't energy-dense enough, and fusion rockets don't have nearly enough exhaust velocity.

This means that the problem you have is not just one of relativistic projectiles whizzing into your inhabited worlds, but the idea that you would allow anyone to put enough antimatter in one place to crack a hole in the crust of Earth.

Here's the solution to your problem: don't let anyone wander around with hundreds of tonnes of antimatter.

Antimatter production is very hard, requiring large amounts of energy, and the only way to do it economically is using huge solar arrays in the inner solar system. It cannot be produced in the required volumes surreptitiously. That's where security and state controls will be. Literally no-one is going to want to be hanging around next to large quantities of the stuff, because the risks are just too high. Nobody really needs an in-system ship that can reach relativistic speeds, and so no-one will ever be issued with enough antimatter to get them to those speeds.

Hell, with any sensible system, people would get either micro or even nano grams of antimatter or none at all. Now there's no danger of anyone sustaining multiple-G burns for a month, because they'll be out of fuel in a couple of days at most.

a significant portion of the speed of light

Space is big, but we know well enough that it isn't empty. Space in our solar system is so not-empty that you can see its not-emptiness with the naked eye, if conditions are right.

You may be trucking along at several tenths of the speed of light cackling about how much of a hole you're going to leave on Mars, or whatever, and there's a reasonable chance you're going to hit a sand grain and it is going to make a very ugly hole in the front of your ship, which will probably already be partially stripped away by impacts of dust and gas.

It is dangerous to the ship to try and fly around interplanetary space at these speeds. Consider that there may not be any viable protection against this kind of damage, and it might even be fatal. Also consider that any interception technique can really very easily use your own velocity against you. Speaking of which,

sustain high-g burns for long stretches of time... it accelerates in one direction at about a month

Now, if you've followed my guidelines above, no-one will ever be able to travel at relativistic speed within a solar system, because it would be stupid and dangerous and probably obscenely expensive as well.

But what if they tried?

Well, if the aforementioned rocket has an exhaust speed that's a significant chunk of lightspeed (and it needs to) and it is accelerating at several gravities, then you're going to be able to see its exhaust jet from the Oort cloud. It needs an engine power of ~45 megawatts per kilo to manage a single measly G with an exhaust velocity of .3c (the effective exhaust velocity of a beam-core antimatter rocket). A thousand tonne ship would therefore need a thrust power of a little under half a petawatt. But wait! Antimatter rockets of this kind release something like 60% of their annihilation energy as gamma rays, so your total power is going to be more like 1.6PW.

This will be obvious to everyone, everywhere in the solar system with a line of sight. And lightspeed being what it is, it'll be visible long before you get up to relativistic velocities.

And you know what will happen next.

Project Rho mentions the phrase "loudness, lawyers and lasers" and what you could expect for braking space traffic control laws. If you ignore the demands, and the legal threats, then what happens next is that you will be promptly destroyed. There are a whole range of ways to do this, but suffice to say that if you can accelerate your ship to silly speeds then there will be purpose-built relativistic interception systems that will reduce you to a cloud of plasma before you can say "I wonder what's generating all those gamma rays"

To summarise, then:

  • You won't be able to get to up dangerous speeds, even if you wanted to.
  • Even if you could, everyone will notice well in advance of any collision, and you'll be vaporised.

Now, there are various ways of getting around a solar system promptly that don't necessarily require everyone to drive relativistic kinetic kill vehicles and carry planet-cracking amounts of antimatter. Feel free to ask a separate question about them, because this answer is already too long.

  • 1
    $\begingroup$ Yes: gubberment control of antimatter fuel. But instead of "tonnes" I would have had it "tonnnes". $\endgroup$
    – Willk
    Jan 30, 2022 at 17:41
  • 1
    $\begingroup$ To be fair, anyone trying to crash into a planet is on a suicide terrorist mission anyway. They probably wouldn't be too worried about their own safety along the way, as long as their impactor can reach its destination. $\endgroup$
    – Drake P
    Jan 30, 2022 at 20:21
  • 1
    $\begingroup$ Also do you mean to say "the efficiency of a rocket travelling above its own exhaust velocity is significantly diminished"? $\endgroup$
    – Drake P
    Jan 30, 2022 at 20:23
  • 2
    $\begingroup$ @DrakeP they'd be worried about the ship surviving the flight, vs. getting wasted by a fleck of grit. And the efficiency of a rocket travelling much above its $v_e$ is reduced, but not as severely as when it is travelling much below it. Take a look at this chart from the propulsive efficiency page I linked above. $\endgroup$ Jan 30, 2022 at 22:05
  • 1
    $\begingroup$ @terdon the problem is that photons don't have an awful lot of momentum, so in order to make a heavy ship go fast without waiting a hundred years and accelerating over trillons of kilometres you need a colossal lightsail and an extremely large laser (or laser array) to drive it. Neither are subtle things, and their construction is very difficult to hide. The sail will also occlude a lot of stars (making it spottable by routine visible light and near-IR astronomy) and when in operation it will warm right up (making it visible in IR, too). $\endgroup$ Jan 31, 2022 at 18:27

Control on the ownership won't do anything in the direction of mitigating the possible damages caused by a wrongly controlled spaceship.

Let's make a comparison with general aviation: a commercial airplane goes fast enough and has enough fuel on board to make some serious damage when impacting somewhere. Do you need some examples?

  • The airplanes which were hijacked on 9/11 were not individually owned, yet they caused enormous damages.

  • The airplane involved in the Germanwings flight 9525 was not individually owned, yet it caused substantial causalties.

  • The airplane involved in Malaysian Airlines flight 370 was not individually owned, yet it disappeared with all the lives of the passengers

Unless you have a way to enforce state control also on piloting such a ship (and good luck managing the anti-inertials or the no-turn-leftist), the control of such devices will always be in the physical hands of 1 person.

Drawing a parallel again with general aviation, one can increase safety and preventive measures to reduce the chances that whatever might go wrong ends up in going badly wrong, but as long as you need a human to give commands to it, that human will be the point of failure for incidents, not the owner.


No, it won't be safe..

Maintaining law in deep space: dream on

Forget police in open space, it won't happen. You can't enter a space ship and arrest the crew. When a mad man or a terrorist in a space ship wants to crash a planet, he can attempt that. The ship can approach from anywhere, at some point in time.. Earth needs early warning system and it has to act very quickly. There is no way any government can prevent the approach.


The mass of the asteroid that killed the dinosaurs was 1,400 megaton, arriving arriving at 30 km/sec. Suppose, the ship comes in at 1/3 speed of light, which is 100.000 km/sec. Kinetic energy is

enter image description here

Square speed !! Fillin the numbers, you'd get 25,000,000 times the impact energy, when the ship would have the asteroid mass. That is improbable.. Divide this by 25,000,000 and you'll get the same energy.. with a mass of 1400/25 is 56 tons for the ship. That seems a lot, but a space shuttle fully loaded is 2000 tons.

A space ship impacting on earth with 1/3 speed of light will be a mass extinction event.

It will be an expensive gamble

Near speed of light velocities are quite unsafe, there is a lot of particles, dust and small asteroids in stellar space.

The perpetrator would be very stupid going full speed anywhere near a planet, with crew inside the ship. A collision, even a small collision, can destroy the ship. The ship needs to be abandoned well before impact. Whether the ship will actually reach the planet surface is uncertain. The ship is much smaller than the asteroid, a relevant part burns up in the upper atmosphere.. it could disintegrate before it reaches Earth surface. But the real mad man will accept that risk. They will loose their ship anyway.

Let Hubble save us

The planet is highly developed, with thousands of satellites, space stations, space military.. The space ship will travel fast, it will approach in a straight line. This makes the path of the ship very predictable. Suppose the ship is detected near Saturn coming in, there will be 1.2 billion divided by 100,000 km/h is 12,000 seconds to act. That is over 3 hours. Earth could attempt to block the incoming ship, by maneuvering satellites and space stations in its path. There's an old telescope named Hubble, size of a bus, weighing 24 tons. That will stop the ship ! The explosion will affect the stratosphere, lots of debris will reach the surface, causing havoc.. a complete hemisphere of Ozon protection could disappear. Nasty, but not deadly, or extinction level.

More advanced defense

The above scenario is current technology level. When a mad man can build (or buy) a ship like this, Earth's defense will be more advanced than it is now. Defense against incoming, unmanned space ships will be similar to current defense systems against missiles. Suppose the terrorists will leave the ship when it's traveling 100,000 km/sec past L2. From that point, Earth will have 10-12 seconds to intercept. This could be easily feasible, with advanced space defense technology.


A smart wall around inhabited planets, space stations, etc. Obviously at just a high G burn it takes quite some time to get to relativistic speeds; and once you do, it is nearly impossible to steer because the high-G is no match for the built up momentum. So we are traveling in a virtually straight line, like it or not. And that makes our attack projectile wonderfully predictable, far ahead of time.

That same high G burn with an AI can be a part of a million large steel sacrificial shields (iron is embarrassingly common in space). These sacrificial shields each have an AI controller as an independent member of a swarming AI (like bees or ants). They are defensive, and position themselves with shifting coverage a million miles from the planet/station, with many able to quickly fly into the path of any approaching relativistic object.

So the object must plow through thousands of these thick steel shields long before it reaches the planet, or if it is a station, giving the station the time to move itself.

In the meantime, the shields themselves can be tasked with reproducing themselves; a few hundred square yards of focused sunlight can make a great smelting forge to refine asteroids into materials, to build new sacrificial shields. These can orbit the sun; like the asteroid belt, and vigilantly monitor the surrounding space. Give them some advanced telescopic detection equipment so they can detect such approaching relativistic objects months in advance.

The Obvious Alternative: Ditch the Planet.

I think it was the "Belters" in The Expanse. After a three or four generations, everybody lives in space, nobody is homesick for the open planet, wildlife, "nature", etc. The notion of fixed buildings that cannot move seems stupid to them; sitting duck targets. Like arming yourself with a knife against a cannon.

They are all perfectly content living in their low-gravity rotating ships; that is how they were born and raised for generations, and they just don't share their ancestors love of "Earth"; just like I don't share the Roman citizen's love of watching gladiators fight to the death.

Nobody ever feels the urge to have an open sky overhead or open plain before them. That feels dangerous and uncertain to them, they like walls and ceilings, they hate the open randomness of uncontrolled "nature." Bugs? Who the hell wants bugs? Ditto for wildlife, who's stupid idea is that? Watching them behind glass in a zoo is one thing, but you don't just walk into a cage with them.

Earth is quickly just their forgotten cradle; they are hunter-gatherers once more, searching for resources, gold and platinum and other relatively rare metals they require to build new ships and expand their swarm. Because as long as we are mortal, love, sex, kids and family will always be a big part of people's lives.

Besides, they can get plenty of planty nature on their giant (movable) hydroponic garden ships. Flowers, trees and all. Because that is still a pretty efficient way to convert CO2, H2O, organic waste and sunlight into tasty food and oxygen.

With sufficient warning of some incoming relativistic projectile, the swarm of ships all just disconnect and step aside, open a hole in the path and let the bullet pass, then regroup and reconnect after it passes. Got a lot of relativistic bullets? Disperse! Nobody gets hit.


Defence Networks

High value systems all have a defensive sensor & interceptor matrix surrounding and permeating them. Anything coming in too hot would find that a small interceptor has been placed into their path. The network is designed to defend against aggression from other governmental entities, but they're more than happy to blow up individuals that get any crazy ideas too.


Navigations limitations

Onboard navigations computers are mandatory for most ships in most settings, mostly because humans are generally too dumb or too prone to mistakes for manual navigations to be permitted(the usual in-setting reason). All you need to do is to make Suicide Tactics & Relativistic Instant Kill Logics Prevention so integral to navigations programming that it will be fundamentally impossible to point your ship directly at an object and accelerate toward it at Relativistic Speeds without the entire system literally preventing you from doing so.

Could even have navigation systems be supplied by a governmental entity, allowing you to enforce the fact that most if not all nav systems have prevention measures in place, with those who are exceptions being either governmental in origin anyway(allowing you to control it and sanction scorched earth policies for hopeless situations(essentially nuke-equivalents in space), or saction self-destruct protocols in case of take-over), or entirely reprogrammed from the ground up by a non-governmental entity(which most people would rather not do for in-setting reasons)

Now, this will drastically affect flight times, because STRIKLP will prevent you from fully making use of a ship's potential speeds. Not to worry though, you can still go at RS if you won't be at RS when you arrive at your destination, allowing the ship to slow down to safer speeds as it approaches its destination. This will also make docking safer, because you can't go fast enough for the ship to impact with the dock mechanisms due to your max speed always being an amount equal to how quickly your automatic break systems can slow you down over a period of time.

The only time the nav systems will fail with this is if something unexpected happens such as an asteroid or another ship suddenly coming into the flight path of the ship, which is going to be a risk ship owners will have to take if they don't want to fly along routes that are the equivalent of space 'highways' where everyone travels in the same direction at more or less the same speeds.


Here's one more idea:

Already with airplanes they are pretty damn difficult to control. Pilot training is pretty lengthy and expensive. It only gets worse for spaceships. Currently we rely on large ground crews and careful monitoring of EVEYRTHING by many radars and sensors just to get it into orbit safely.

So how do you get privately owned soap-opera spaceships that can be safely piloted AT ALL then?

The answer - advanced AI. Basically people won't be in control of the spaceship AT ALL. They'll just tell the AI where they want to go and it will take them there.

And the AI systems are where all the safety measures are. An AI is very difficult and expensive to develop, so it will be only done by large corporations, which themselves will have heavy regulatory oversight.

Of course, one might just try to disable or modify the AI and take the direct control of the ship. Which will then trigger additional safety systems. If you disable those too, then the next level of safety systems are external - upon approach to a planet (or anything of value, really), a communication channel is established and the AI needs to authorize itself. If it doesn't, then Red Alert is triggered with all the consequences. There are alternative protocols for when you have a malfunctioning transmitter, for example, but they all involve flying very slowly. If a spacecraft is approaching fast and silent - it's KABOOM TIME!

  • 1
    $\begingroup$ ... and Boeing will be there to design them. I recommend bunkers ... deep bunkers. $\endgroup$ Jan 31, 2022 at 13:19
  • $\begingroup$ @MikeSerfas - Deep SPACE bunkers. Somewhere far, far away from any noteworthy locations, like planets. $\endgroup$
    – Vilx-
    Jan 31, 2022 at 15:25

I just realized another issue with this premise: the spaceship is actually the smallest one of your problems.

Energy doesn't come from nowhere. As noted in other answers, if you can get your ship to come in close to lightspeed, it will deliver enough energy to wipe out the planet.

So where did this energy come from? Obviously the spaceship has some sort of energy source that has enough energy in it to do this kind of devastation. So ditch the ship and just blow up your power source if you want to do incalculable damage.

  • $\begingroup$ No. The starship isn't a source of energy but it can be an energy storage device. You can't blow up the power supply because the energy isn't there. Consider the previously-mentioned laser-pumped lightsail. Solar panels connected to a laser, nothing that could possibly go boom. It can push the lightsail to relativistic velocity and that hits with an incredible boom. $\endgroup$ Feb 1, 2022 at 1:12
  • $\begingroup$ @LorenPechtel - I don't think that's the kind of spaceship we're talking about here. OP wanted a "soap-opera spaceship" so basically your Millenium Falcons and interstellar family minivans. $\endgroup$
    – Vilx-
    Feb 1, 2022 at 7:42


Currently computers already dictate a lot in normal transportation. Different profiles in your car allow certain amounts of acceleration, help apply breaks, ABS and more. This is true for many years on all cars, not just the hyper modern ones where you can play angry birds while the car stays in the lane.

A spaceship is a little bit more complex. Just to navigate a 3d space correctly you need more subtle maneuvers in many directions than a human can provide. Computers are already taking over these tasks today and this is expected to be fully done with computers. The interstellar medium is vastly empty, but efficient and safe travel still relies on on time minute adjustments. You need computers to calculate and execute this.

In the end all navigation will be done by computers. If you want to use them at all, you need to accept the limitations build in the computers so you can't ram into a planet or something. You'll simply be locked out from such maneuvers.

Computers will most likely write their own code at that point, making (digital) hacking something that can only be done by computers. Writing your own code also means changing code on the fly. Any attempt to break it will be detected and thwarted, requiring a processor that is stronger than the ship to have a reasonable (within a lifetime) chance to succeed.

The effort and funds required are so high it'll deter most. Social and moral wven more. The chance of someone wanting to do it long enough, having the means and lucky enough to break through the security will be low enough to be acceptable.


Psychology may be the better answer, restrict the direct control rather than ownership etc.... Ownership doesn't matter if the crew can be trusted not to use the ship as a WMD. Screen those who get to crew spaceships, especially your officer class, starting when they're toddlers. Ongoing evaluation and conditioning of potential candidates then continues for a couple of decades before they ever set foot on the deck on their potentially deadly new home. Biometric control locks mean only professional spacers carefully conditioned to take their own lives rather than endanger others can run spaceships. Think the Suk school doctors from Dune only ships' pilots instead of physicians.

No system is ever going to be perfect though, the odd crewman is still going to be subject to an unpredictable psychotic episode or have a warped pathology that goes unnoticed until it is too late. Hijacking by sufficiently sophisticated agencies cannot be ruled out regardless of the safety precautions taken either.


Vigilance ... and manners ... and cheap aerographite spheres

Planets have more resources than individuals. They use these resources in what you might say is a substantial extension to the asteroid defense modern space forces dream about.

  • A network of gravitational wave observatories detect the movement and acceleration of all masses in the system. No heavy fast-moving ship goes unnoticed, period.
  • A network of neutrino observatories monitors nuclear reactions in the system. No large mass of radioactive material goes unnoticed. Even ten miles below ground, a nuclear weapon (or propulsion mechanism) is as visible as if it were out on your front yard.
  • A network of listening posts is embedded in the heliopause, monitoring faint disturbances in the electromagnetic vibrations of the stationary solar wind. Any large vessel or astronomical object will be heard at the moment it penetrates the system.
  • A network of visual observatories watches the maneuvers of ordinary space traffic.

The manners of space flight may include any number of customs regarding shipping lanes, but there is a more fundamental rule: No spacecraft shall take on a trajectory that cannot be stopped, nor any trajectory which can rapidly be changed to become unstoppable. There are countless spheres of aerographite arranged in negatively refractive structures, standing guard all over the system, as part of a collaborative federated network of self-defense, which can use the Sun's rays to accelerate quickly and for free, which can change their trajectory merely by expanding or constricting their metamaterial networks a little in one direction or another. They will softly pound any uncontrolled or wrongly controlled spacecraft away from an undesired (i.e. unstoppable) trajectory. The catch is that they will start doing this as soon as the ship starts such a trajectory, wherever that is. Another catch is that the operator risks receiving a bill for the spheres if and when he tries to reenter the world of legitimate commerce.

If it only takes a moment to make a ship crash into a major station or ecological surface, then that is an undesired trajectory. However, if the ship merely crosses such a crash course, with hours to go during which it can be battered into a different course, then there is no need to initiate such measures (yet). We're not looking so much at the delta-vee here (the amount of propellant needed) as the delta-a (the average intensity of acceleration needed to decide whether a collision occurs). However, the network will inevitably involve some human decision making, since a large stockpile of antimatter could evade all the precautions above, and human decisions would need to be made by its risks.

  • $\begingroup$ Unless the technology improves massively, neutrino detectors are effectively useless for detecting ships, let alone nukes. By the time enough neutrinos have been detected to build up a sensor track, a high speed source will have either impacted hours or days ago or passed by harmlessly. $\endgroup$ Feb 1, 2022 at 1:12

Exotic Space-Drive

Reaction drives are so last century.

The new hotness is Space-Drives. A fantastical new technology which allows high-efficiency inertia-less motion by < technobabble >Leveraging exotic matter's interactions with spacetime curvature< /technobabble >

The catch is that this has a strong negative interaction with gravity.
Essentially, gravity acts as a braking force on your fundamental engine components (all that degenerate/exotic matter does interesting extra-dimensional things that don't act intuitively in our normal space)

No problem in open space between planets, where the solar system is topologically fairly flat, but when you get close to a planet, the Drive experiences something analogous to drag.

Deep gravity wells are basically a mudpit. The drive can't gain "traction" and will generally be slowed to a halt. The faster you hit the gravity-well, the harder the braking force and the more energy you'll need to pump into the system to overcome it.

If you want to operate within the hill-sphere of a planet (for earth, that's about 1.5 million km across, which is many times further than the distance to the moon) you'll need a lot more power to keep moving, and this power rises more or less exponentially the closer you get to a planet.

Conventional reaction-drives are needed to lift things into high orbit, and within the hill-sphere, the Space Drive is very low efficiency. Not really a problem with the huge resources of nuclear fusion and/or anti-matter reactors behind it, but you can't get much acceleration out of it close to home.

Once you're beyond the moon though, drive-efficiency shoots up and you can reach serious fractions of C quite quickly, allowing a privately owned spacecraft to reach other planets in a matter of hours.
Slowing down and reaching orbit at the other end is as simple as "grav-braking". Allowing the exotic matter used in operating the drive to act as a Space-Brake and capture the ship into a high orbit where conventional thrusters can take over

Of course, the drive itself has a major impact on fuel-consumption. The braking effect can't be turned off. So the typical practice for larger craft would be to use shuttle-type craft for low-orbit/landing-craft and keep the main ship in high orbit. Smaller ships (with less exotic matter to slow them down) would be able to retain most of their efficiency as long as they have the power-generation to operate the drive near a planet. So there's a sweet-spot of power-to-weight where a ship can operate reasonably effectively near planets, puttering around between lagrange space-stations.

A spacecraft using a Space Drive is functionally travelling "uphill" to get to a planet or indeed towards the inner solar system at all. And so the use of a starship as a relativistic Kinetic-kill weapon is impractical in the extreme.

You could feasibly use a starship in open space to act as a launching platform for a dumb RKKV. Simply accelerating up to substantial fractions of the speed of light and dropping the projectile before either decelerating or allowing the hill-sphere of the solar-system/planet to slow you down.

But then you're a bright shining energetic beacon doing something incredibly obvious and recognisably an attack. And so your projectile can in principle be detected and intercepted by any other starship (they merely need to match velocities, clamp onto the projectile and then stay that way, using their engines as a brake as the projectile enters the target hill-sphere)

TLDR: The upshot is that a spacecraft attempting to perform a RKKV Suicide attack hits the planet's hill-sphere and the interaction between its exotic-matter space-drive and the planet's gravitational hill-sphere acts a bit like a bullet-proof vest, or perhaps oobleck. Robbing the spacecraft of its relative velocity proportionally and ensuring a spacecraft can never achieve the velocities required to be dangerous to a planet while near said planet.

  • $\begingroup$ There are a couple other implicit problems with this space-drive I hadn't gone into. First and foremost, the constant braking-effect of having a large body of degenerate matter means that any spacecraft with a space-drive is inherently incapable of a ballistic orbit. They must constantly/regularly use their drive to maintain orbital altitude when near a planet or slowly lose velocity and find themselves losing altitude. This also applies to spacecraft in lower solar orbit. A spacecraft that isn't under power will crash into whatever hill-sphere-object they're currently orbiting eventually. $\endgroup$
    – Ruadhan
    Jan 31, 2022 at 14:03
  • $\begingroup$ Every space-drive equipped starship is living on borrowed time, the exotic matter requires constant effort to prevent it migrating to planets and moons. Likewise a planet or moon with a large amount of the exotic matter on it will eventually lose velocity relative to its own parent body and be dragged down into the sun. Ultimately, space-drive exotic-matter is a serious long-term problem for the society that uses it. Akin to nuclear waste. The only saving grace is that it's self-disposing. Just put it in a solar orbit and wait.. $\endgroup$
    – Ruadhan
    Jan 31, 2022 at 14:08
  • $\begingroup$ An interesting plot-hook might be for a dastardly organisation to create an exotic-matter breeder-reactor on the moon which simply releases the matter to drop into the moon's core.. They keep this up for a few decades, producing kilotons of the stuff and eventually people notice that the moon's trajectory is slowing, and the tides are shifting, and before too very long the moon is going to crash down on earth and there's nothing anyone can do to stop it.. Or if you prefer something your heroes can stop, the villains have created a lot of the matter and plan to launch it as a blob at a planet. $\endgroup$
    – Ruadhan
    Jan 31, 2022 at 14:19
  • $\begingroup$ This is a good answer. Relativistic space travel is just not going to happen in any serious hard SF setting without such drive. $\endgroup$
    – alamar
    Jan 31, 2022 at 16:26
  • $\begingroup$ Thinking further on what it might look like for a spacecraft travelling at relativistic velocities impacting a planetary hill-sphere.. The principle forces are all acting on the ultra-dense exotic matter that acts as the core of the engine. So very rapidly, the engine gets torn apart by a ring-shaped blob of super-dense material, the whole ship disintegrates, and you get a much lower velocity conventional shotgun-blast of material. Could still be very dangerous. $\endgroup$
    – Ruadhan
    Jan 31, 2022 at 17:10

No spaceships can go at relativistic speeds

Spaceships themselves cannot reach relativistic speeds. However, there is a network of "jump gates" that lets them dock to the jump gate, which ferries them at the relativistic speed needed. Also, spacecraft use "conventional" reaction drives, like fusion. Also, there are delta-v limits in place, prohibiting spacecraft with above a certain amount of delta-v from being owned unless a permit is had(only well-checked, rich individuals or corporations can own one). Also, spacecraft are mainly launched by the space station, which has a fleet of support "bugs" that help maneuver spacecraft and are a service. Also, delta-v is locked, and only in emergencies, the extra delta-v can be accessed.


Let's define "significant portion of the speed of light" to be $0.01c$. Why? Well, if I do that, I can hand-wave away a bunch of the relativistic behaviors that I have to worry about at higher speeds. Also, it turns out that a 1g burn gets you to 1% of the speed of light in about 340 days.

Why 1g? Well, obviously a 10g ship could get to those speeds in a month, but that could be something solvable. Sustained 10g may simply be something that you ban (perhaps limiting fuel capacity). 1g is, after all, what they sustained in the Expanse.

First off, how far do you have to go? a one-way trip at 1g of 340 days, using $x=\frac 1 2 a t^2$ shows us a travel distance of $4\cdot10^{12} \text{km}$. Now, consider good old Pluto. Its orbit is roughly $6 \cdot 10^{9}\text{km}$. So you're looking at traveling from a distance 1000x times further than Pluto. It's actually a good chunk of the way to the Oort cloud. You have to travel pretty far.

I point this out because we don't have to worry about "insider threats." Anyone remotely close to being in a reasonable shipping lane isn't going to be able to get to these speeds, even if their ship is capable of it. And this means we can look outward.

In the expanse, the governments all had sensor networks. They can look for things. If an angry $0.01c$ freighter is something they are concerned with, they'll be looking for it. And it turns out objects traveling at these speeds are hot. I tried to find out how hot, but it was hard to find numbers. Most of the interesting numbers were for faster speeds... but they were worried about melting the nosecone. So I'd expect that the object is shedding IR radiation at a rather high rate. You're not stealthy at those speeds. So the government is going to be ready for you.

And they don't really need to stop you. You're not a giant asteroid whose fragmented components will cause planetwide panic. You're a small freighter. If they do something to disrupt you, its unlikely you'll be able to keep targeting wherever you're going. And disrupting you will not be that hard. At $0.01c$, you are the bullet. They just need to put something in your way and you will be a sad panda.

(Incidentally, the linked article suggests that about 50% of the speed of light is the limit for a custom designed craft. Faster than that, and the radiation caused by plowing through interstellar hydrogen kills the crew. The effects would be worse in the thicker "atmosphere" of the solar system).

Also interesting is that, in near-light space travel, thrust is far less important than ISP. The expanse engines have an ISP of about 1.1 million seconds, which corresponds to an exhaust velocity of 11,000km/s. By the rocket equation we can see that if you want to get to 0.01c, you can only have a payload mass that's about 3% the whole rocket. If you want to get faster, its gets harder.

Also interesting, the [Rocinante][4] apparently had about 10 hours of fuel at 5g. A 5g 340 day trip is going to be a lot of fuel!

[4] http://toughsf.blogspot.com/2019/10/the-expanses-epstein-drive.html?m=1#:~:text=The%20250%20ton%20Rocinante%20needs,30.75%20tons%20of%20fusion%20fuel.

  • $\begingroup$ Just a math fix, you're off by almost exactly one decimal place. Accelerating at 1G, you will be going 0.1 c after 3057234.93779 seconds (35.4 days), not 340 days. $\endgroup$
    – Vogon Poet
    Mar 14, 2022 at 15:03

Space Caltrops

Lots of other answers here have lots of science-y answers for how to solve this problem, but in a "classic space opera" as mentioned in the question, science can be hand-waved in order to make the story more fun.

Assuming you can get a ship to light speed, and assuming you can do so without being detected, how would you defend against it?

Space Caltrops

Caltrops are sharp bits of metal which prevent people or animals from walking (quickly) through an area. You have to walk slowly and carefully to avoid getting a nail in your foot.

Space caltrops can be small, ball-bearing sized objects which are put into (random, intentionally unknown) orbit somewhere well outside of the region you want to protect, perhaps at a similar distance to the Kuiper Belt. Any ship which tries to enter the star system at relativistic speeds will most1 certainly crash into one of the space caltrops. The damage to the ship from hitting even a small object would be catastrophic as described in other answers and would annihilate the ship. Since the annihilation happens at Kuiper belt-equivalent orbit, your planets wouldn't be at risk of harm from any resulting energy or solid matter fallout.

Assuming your planetary system follows a traditional model of being mostly-planar, you would only need enough space caltrops to form a deep enough dish to cover the eccentricities of your system, which for most systems would be far less than a complete sphere. Your system is safe from attack from "above" and "below" because anyone piloting a relativistic vessel won't be able to do more than minor course corrections, so they would not be able to plot an elaborate2 course and attack from the "top" or "bottom" of your planetary system3.

Finally, to allow wanted vessels through, you could either allow them to enter slowly and safely push through your curtain of caltrops, or you could define some tunnel-like entry point which could be navigated at safe speeds.

1Plot device, someone invents a ship which can safely calculate the route of the caltrops and does destroy a planet, causing mass panic.

2Hopefully nobody flies out of the galactic plane and comes at you from a different direction! Maybe that's not possible due to lack of fuel, space dragons, etc.

3Assuming your system itself isn't eccentric with respect to the plane of the galaxy, otherwise you would need to build a full sphere!


Non-Newtonian Drives can be Relatively Safe

The problem with most planet killer ships is that they rely on Newtonian Physics, but actually reaching near light speeds using Newtonian Physics is next to impossible due to the sheer amount of energy required and devastating result of hitting tiny specs of dust at near light speeds.

Instead, many space operas overcome this problem by creating engines that rely on manipulating space-time instead of kinetic energy. Look at the equation E=MC^2. Current technology only knows how to manipulate E (energy) and M (mass) while leaving C (speed of light) a constant, but physics will work very differently for a future civilization that can manipulate M and C instead.

If you have an engine that could manipulate M and C while leaving E alone, you could make a ship travel much faster without adding any actual kinetic energy to the system. So if you have a 10,000 kg private yacht flying along at 3km/sec, and you were drop its mass to 0.1 g and raise its "speed of light" to 10,000C, your ship would then be moving at 0.1C relative to normal matter. However, since the kinetic energy has not changed, if such a ship were to ram a planet, it would only result in a 45GJ impact... which would give you a blast radius of about 150 meters. Enough to level a few city blocks perhaps, but not nearly enough to threaten anyone on a planetary scale... Besides, good luck aiming a 0.1C ship accurately enough to hit your target. More likely than not your terrorist ship would just hit the Ocean, wilderness, or maybe wipe out some farm land.

This related answer goes into more detail about how matter interacting at different Speeds of Light would work.

Another way of manipulating spacetime would be to use wormholes to take shortcuts through space. These work along the same principle because your ship never exceeds normal Newtonian speeds to cover greater distances.


Edit: removed speculation about FTL

No, there is nothing that could offset the risk sufficiently.

Spaceships impacting a planet at speeds close to the speed of light (say, 0.9c) would obliterate the whole planet. As in, the planet would mostly cease to exist as a roughly sphere-shaped rock. Each such ship is capable of killing billions of people and destroying the infrastructure of a whole planet.

A planet dwelling civilization must require every last bit of risk reduction. In that regard, allowing privately owned/controlled ships is absurdly risky.

Although this could answer the Fermi Paradox: at some point, every civilization finds a way to easily reach relativistic speeds for their space ships. And without fail someone takes a drunken joy ride and blows up their home planet.

  • $\begingroup$ It's not about if relativistic speeds are required or not, ANY space opera spaceship can accelerate to relativistic speeds, because they're hella fast without FTL as well. $\endgroup$ Jan 30, 2022 at 11:27

You would want multiple layers of defense. In other words: "All of the above" referring to the other posts.

Note that there are two kinds of space ships, planet hoppers and STAR ships. Planet hoppers don't have the fuel or engines to do a lot of damage. Mind you, they can still recreate 9/11, but since they don't go at relativistic speed they are easier to defend against.

Star ships is where the danger is.

First rule of star ship travel, a computer flies the ship. The computer makes sure everything is safe. This computer is NOT running an AI. It is a running fairly stupid program that is dedicated to avoid collisions. AIs can not be trusted any more than other sapients.

Hacking this computer might be possible, but will get you permanently grounded.

Second rule of star ship travel is nobody aims straight at a planet, space station or other important fixed object. You aim slightly to the side. Yes, this adds some weeks to your travel time, but on the plus side you won't be shot down.

Aiming straight at a planet is a war crime. It doesn't matter if you are in control and avoid the collision. The act of aiming is in itself enough to put you in an early grave.

To get into interstellar space, even as a passenger, you have to pass psychological screening. If a shrink says that you are the type who might blow up a planet to watch the pretty sparks, you are grounded for life.

Even with all this in place, we still want a physical defense. As other have said, just putting any small object in the path of the ship will turn it into a cloud of debris. You want to do this far enough away from the planet that most of this cloud misses it.

By the way, you want to have well established shipping lanes that are kept clean of random small objects.

Detection can be a problem, but in soft SF you can just say "long range scanners" and get away with it.

Detectors and defenses are likely to be much better at old well established planets like Earth than at newer colonies.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .