You don't need to be traveling faster than the speed of light to encounter some pretty major problems.

At just 30% the speed of light (90 Mm/s) a spaceship of mass 4 x 10^5 kg (about the mass of the ISS) colliding with a planet would impart 1.736 x 10^21 Joules of energy (Zettajoules) accounting for relativistic kinetic energy.

That's a lot of energy, approximately the same as a 1km asteroid colliding with earth or the total energy of a large hurricane (although the second one doesn't sound as impressive). That's a 20km (diameter) crater!

Actually, this isn't really that bad. The effects would at best be regional, although it could be devastating for a city if the impact was aimed well. However, 0.3c isn't very practical for transportation. What happens at 0.7c?

With a craft of the same mass (still rather impractical, but maybe it's more of a one-seater), an impact would release 1.439 x 10^22 Joules, the equivalent of 3 teratons of TNT. In case you're wondering, this much energy would easily destroy any Earth-sized planet.

So the question is, how does a planet defend against such a threat? In a developed civilization, crafts traveling at such speed would likely become a normal form of transportation, at least within solar systems. Even undeveloped planets are at risk from malicious alien simulations. And considering how much damage drunk drivers do in cars traveling less than 100 mph, imagine how bad of a problem drunk flyers could be.

Possible solutions:

  1. An interplanetary tracking network and a system of lasers designed to shoot down problem ships.

  2. Complete automation of spaceflight. You punch everything into a computer, it takes you there. No manual flying option available. However, this doesn't defend against malicious attacks.

  3. Strict licensing and regulation would cut down on dangerous drivers, but there is still such a huge risk of mistakes with catastrophic consequences.

  4. Some sort of force-field that absorbs the energy from these impacts. This seems like the most practical but most advanced solution.

What other options are there? I'm looking for realistic solutions, as the premise of this question is realistic (no FTL). Also, feel free to critique or modify the four that I proposed.

  • $\begingroup$ Planetary shields are common in SF as are orbital fortresses, interceptors, sensors. Licensing is a little silly, personally no offense intended. Automation and complexity are always interesting though. If you want to deal with the problem children then you can expand. If not, then you can safely ignore it. Let me know, which direction you are headed. $\endgroup$
    – dcy665
    Jun 3, 2017 at 23:23
  • 2
    $\begingroup$ Generally, you just need to turn a hit into a near-miss. Requires detection of inbound objects coupled with a method of slightly changing their trajectory before any final course corrections are made. You can burn off their engines with energy weapons, or use a kinetic interceptor to bump them. Either might be fatal to people inside the object, which introduces moral tradeoffs. $\endgroup$
    – user535733
    Jun 3, 2017 at 23:49
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    $\begingroup$ @NathanielD.Hoffman problem children re rich kids stealing daddy's space yacht and/or terrorists. If those are interesting plot points. If they aren't, ignore it. The modern terrorist has easy access to transport. There is nothing that suggests the very, very expensive and resource intensive space travel will allow the same. $\endgroup$
    – dcy665
    Jun 3, 2017 at 23:53
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    $\begingroup$ You are describing a relativistic kinetic kill vehicle. It's come up a few times in other questions. Your first few paragraphs introduce the idea from scratch rather than referring to it. That's what makes me think that the question is not considering what's already been discussed. $\endgroup$
    – JDługosz
    Jun 4, 2017 at 9:12
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    $\begingroup$ "In case you're wondering, this much energy would easily destroy any Earth-sized planet." - no, it's not. en.wikipedia.org/wiki/Gravitational_binding_energy - Earth 2.24 x $10^{32}$ J $\endgroup$
    – MolbOrg
    Jun 4, 2017 at 15:41

6 Answers 6


If you looking for a realistic premise, then you should first define what is used to accelerate the craft to the speeds.

At an acceleration rate of 100g, the craft will achieve the 0.7c speed after covering the distance of about 150 a.u. To slow down it also requires about the same distance. (for 0.3c the distance 27 a.u). 10 g acceleration and the distances will be 10 times longer, 1g and the distances will be 100 times longer.

Pluto, a former planet, is about 40 a.u. away from the Sun.

Top speed at 1 a.u. acceleration distance at 100 g acceleration rate will end with resulting speed of 17'000 km/s or 0.0577c. and that speed is way enough to travel between major celestial bodies of our star system.

Regulation is not a help in the situation, same as automation of the flight. Everything can be violated and broken.

However shooting down as the first option isn't smart too, instead it should be - inform, rescue, deflect, destroy.

A transporting ship(it trasport humans) most likely isn't build to penetrate defense compared to a thing which is specially designed to do so. However, it can be used as destruction device and be designed for that, and the moment you should clarify for yourself in your setting.

But it is expected that automated systems, no humans on board, can accelerate faster than those with humans, so it is possible to send automated rescue mission to a ship on a collision course with goals to rescue people, slow down the ship, change the ship course etc.

For interstellar travel, it depends on what do you use as mean to accelerate the ship to interstellar velocities. having the fuel and reactive mass on a board of the ship might be not the most practical way to do so, thus there might be external means to accelerate and deaccelerate craft.

So, prior of thinking about collision problem and drunk drivers you should determine different use cases and parameters for your ships.

Option 1 is ok - if we mean to inform, rescue, deflect, destroy.
Option 4 is ok - if we mean field of impactors which will disperse the ship's energy or deflect it from collision course or a system which is used to accelerate-deaccelerate ships.

  • $\begingroup$ If you plan to rescue the ship, you have to match speeds with them. If you just plan to deflect them, smashing into them is all that it takes. $\endgroup$ Jun 5, 2017 at 4:09
  • $\begingroup$ @BobTheAverage yes, both statements are correct. I would say gently smashing, basically no smashing at all to prevent producing debris flying in the direction you do not want them to fly, so basically space tug if it is possible it requires to change speed vector a little. It is not fair to suggest the answer as it is how to wage a war with a network of drones in the solar system, but such system of drones could be used for all 4 tasks of option 1 and option 4. It can be used for multiple purposes including OP's situation. $\endgroup$
    – MolbOrg
    Jun 5, 2017 at 5:42

Summary: Your problem is a doozy. No solution is perfect. Murphy's law will get everyone killed. Greed, and laziness are dangerous. Well funded malice is almost impossible to defend against.

Note: I will use Earth and other objects in our Solar System as a short hand. Adjust the details to fit the setting.

Solution 1: Lasers

Lasers are problematic. If a craft is speeding on an intercept course with Earth, and you shoot it with a laser you now have a piece of wreckage on an intercept course with Earth. The laser does not actually change the trajectory of the ship. A mega-powerful laser could possibly explode the thing into a million pieces that would then scatter and expand like buckshot. This would make a lot of smaller craters, which is almost as bad.

Switching from lasers to giant bullets or missiles might be more effective, but it brings up a ton of problems. Your bullets would have to be as big as the craft they are knocking out of the way, and you would have to launch them way early, because they move slower than lasers.

For any solution, your early detection system would have to be incredible. You have to be able to spot objects that are tiny on an astronomical scale while they are at least 10AU away, and when you first see an approaching craft traveling at 0.3c, it is already 30% of the way to you. Any solution that knocks dangerous craft out of the sky is problematic for many reasons. On the other hand, it is the future. I assume you have some great technology.

Another early detection problem is that it would be nearly impossible to tell the difference between a ship that would impact Earth and one that would stop next to Earth. They are on nearly the same trajectory.

I think it can be assumed that the people running these defense systems are grim and humorless. They would be required to blow up people if there was any chance they were a terrorist, or if they were off course headed towards Earth. Zero tolerance for mistakes or malfunctions. Plenty of innocent ships would get killed.

Solution 2: Completely automated space flight.

Completely automating things helps, but machines aren't perfect. Machines designed, installed, and maintained by humans are prone to error.

Human error or equipment breaking down will cause screw ups. Not too long ago, Air France Flight 447 crashed after a air speed sensor iced over. The autopilot disconnected when it sensed an equipment malfunction and the human pilots reacted wrong. If your relativistic flight becomes as common place as air travel, things will go wrong. Murphy's law is about human nature, not technology. You can make the pilots machines, but the technicians who install the machines might still be bozos, or the engineers who designed it might have missed something, or an unexpected solar flare pops up, or any of the other 1000 things that no one has thought of yet.

Solution 3: Strict Licensing and Regulation

Regulation is not a perfect solution. If you put super strict regulations on modern airlines, you would have an entire industry looking for ways to cut corners. The airlines are in a bitter cost cutting war right now. Some of them would try to evade the regulators, because that could be cheaper than following the regulations. If you sent an army of regulators, some of them would take bribes. If you sent an army of unbribable regulators, the industry would buy lobbyists. A small number of very rich, highly motivated transportation executives could inadvertently destroy your fictional Earth.

Nuclear power presents a good counter-example to air travel. The potential for disaster is there, but it so far hasn't happened outside of Russia. Everything went wrong in Fukashima, but the damage was contained with minimal loss of life. There are fewer people involved in nuclear power than the airline industry, the profit margins are not as thin, and the government regulation is a lot stricter. Nuclear power spends a lot of time and money on a system of layered fail safes to keep giant disasters at bay.

These safe guards are expensive and time consuming. We have them because the accidents we did have made us believe that nuclear power is scary stuff. If the Earth had a "minor" accident in your universe, it would be more plausible for people to take extreme precautions. Maybe the kind of minor accident that just destroy's a couple of states, not a continent. If there are no near misses, people eventually let their guard down. Conversely, if the public is terrified of something, no amount of lobbying can relax those regulations.

It also helps that there are relatively few nuclear power plants. We live in an age of air travel where airlines run thousands of flight per day while trying to cut costs to the bone. If you are imagining an equivalent system of relativistic rockets, Earth is doomed. If there are a handful of giant flights per week, we might do ok.

A lot of my discussion has been under the assumption that you would have private industry running the show, and government regulating it from outside. It could be a government run program in the first place. Then all the concerns about lobbying and regulation get a lot weirder. If it is a government program that is making a ton of money on ticket sales, you get to have all of the problems with greedy private industry AND all the problems with hidebound government bureaucracy.

Solution 4: Force fields

Handwavium powered force fields can solve any problem ever. I assume yours will combine nano-technology with applied phlebotinum. I don't think there is a hard science way to do this. I could be wrong.

New Solution: Inbound speed limits

If you are headed towards Earth and closer than Jupiter you have to limit your speed way down. This makes the giant bullets proposed as an alternative to the lasers far more effective. This also gives defense stations a little more warning for malicious attacks. When someone doesn't slow down after passing Jupiter, blow them up.

New Solution: Be VERY VERY nice to anyone powerful outside the system.

If an enemy government outside the system wants to destroy Earth they can. Simply launch a field of gravel at Earth at relativistic speeds. It could be hard to detect, and even harder to shoot down. The lasers or massive bullets would take out one pebble each. The only defense here is your handwavium powered force fields, and I imagine they can be overwhelmed. The pebbles would slam into the Earth's atmosphere so hard they would cause fusion explosions. That's what happens when something travels at relativistic speeds in an atmosphere.

New Solution: Gravel Launchers and Diplomacy

Build your own gravel launchers and make it clear to the space Soviets, that you will destroy them if your own planet gets destroyed. Mutually Assured Destruction.

New Solution: Off planet terminals

All relativistic travel must slow to a stop at the L4 and L5 Earth Sun Lagrange points. These are stable orbital points about 1AU from Earth. They are a nice place to park an orbiting bus station. The bus station is much smaller than a planet, therefore it is less likely to get hit on accident. It also doesn't have 10 billion people living on it. Any ship seen not heading for a bus station, will be destroyed. The distance from Earth means that it is much easier to tell if a ship is heading towards them or towards Earth. Therefore you have more time to deal with rogue ships.

  • $\begingroup$ Lasers are a better solution than you think. " This would make a lot of smaller craters, which is almost as bad." Not so, as long as the intercept is far enough away that the debris field can expand to much greater than the diameter of the Earth. Then most of the debris will miss. Also, the debris will (arguably) have a wide range of velocities, which will produce a large time spread in impacts, which will allow time for secondary intercepts. Or had you assumed any lasers must be one-shots? $\endgroup$ Jun 4, 2017 at 16:03
  • $\begingroup$ @WhatRoughBeast What makes you think the craft will break into an EXPANDING cloud of small pieces? You are thinking of sci-fi laser explosions. Lasers are great at delivering a lot of heat to a target, but terrible at delivering any change in momentum. $\endgroup$ Jun 5, 2017 at 4:07

If it's going fast enough to be a problem, apply another dose of whatever got it going that fast in the first place, in a less dangerous direction.

Odds are your joyrider's not going to be particularly stealthy (and if they're trying, out come the military-grade countermeasures), so sending out a stellar tugboat-equivalent (or police car-equivalent) to provide course corrections should be economically viable provided the ratio of joyriders to sane drivers is sufficiently low. Depending on how much acceleration your Hypothetical Sublight Propulsion provides (see MolbOrg's answer), tugboat intercepts may be started anywhere from seconds to months in advance of the potential problem.

Also, it should probably be explicitly noted that the tugboats in question are likely to be stationed in orbits around the star where they can perform their friendlier intercepts with a minimum of delta-v, rather than, say, in orbits around the planet (or worse, on the planet's surface).


I'm thinking very strict "space traffic control" would be required within solar systems. Much like our current air traffic control systems here on Earth...

Pilots would be required to send in flight plans and follow commands from space traffic controllers. With crafts moving this fast, following pre-approved flight plans would be essential, you're simply going too fast to see and avoid other crafts.

On approach to a planet or high traffic area the pilots would be in close communication with traffic controllers​ that would issue orders on how to safely navigate the area or when and where to land/dock.

With all this pre-planning and communication it becomes a bit easier to spot errant crafts. This is the fun part...

Speed limits and air traffic control orders are strictly enforced. Any unidentified craft or craft that deviates from it's flight path can and will be fired upon.

First think flying a large jet, not driving a car. Then think military air base rules, not commercial airport rules.


To be honest, if you're concerned about this type of stuff, you would also have to consider the problem that any ship traveling at 0.7c would long be destroyed by collision with space dust or other debris before even hitting the planet (unless their shields are strong enough to tank explosions on the level of nuclear bombs with ease).

But if you still want to go down this route then:

  1. Having the ships be in some type of pseudoscientific phased state which will allow it to push aside low mass objects and be "knocked out" of the state by any high mass ones

  2. Bypassing commercial high speed travel altogether in favor of wormholes or some type of spatial bridging technology. And maybe have some other type of super expensive FTL/super-fast travel tech only available on the rarest ships


A commercial, long term use spacecraft would likely be built in space and rely on some form of deflective shield to protect it and it's cargo from space debris. Being built in space would reduce cost and allow it's construction to be far lighter than planet side build vessels. This would also mean that they aren't built for reentry and could be designed to break-up and disintegrate before colliding with a planet. The cargo, be it travelers or objects, would need to be transferred to either orbiting stations or space elevators where it could be safely transported planet side. This still leaves man made structures in space vulnerable to collision, but those can be rebuilt in a way ecosystems can not.


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