Let's say there are two planets about to collide. Ignoring how they got there, it seems that in most scenarios basically all life gets annihilated by energy released the collision.

How slow would two planets with plant and animal life with environmental tolerance similar to Earth's have to be moving for their collision to be survivable?

  • 21
    $\begingroup$ The problem is that the minimum collision speed for anything with a planet like the Earth is about 12km/sec, so your scenario is impossible. 12km/sec is the speed needed to get from Earth into space, and something from space falling to Earth acquires that same speed from gravity as it falls. $\endgroup$
    – Mike Scott
    Jul 30 '19 at 5:03
  • 1
    $\begingroup$ 12 km/sec is 43,200 km/hour, or 26,843 miles per hour. Any object which comes from outer space will have at least this minimum speed when colliding with Earth. Big, small, doesn't matter. (Very small objects will dissipate their kinetic energy in the atmosphere. This doesn't work for objects which are not very small.) $\endgroup$
    – AlexP
    Jul 30 '19 at 8:05
  • 7
    $\begingroup$ Even if you used your handwavium anti-gravity machine to place two Earth-sized planets in contact at zero relative velocity, they would immediately collapse into each other. The collapse would not be survivable. $\endgroup$
    – jamesqf
    Jul 30 '19 at 15:11
  • 2
    $\begingroup$ @bob Yes, because the rate of acceleration needs to be on the order of 10^-20 meters-per-second-squared (so that when it multiplies with the planet's mass the newtons of Force are in the "vaguely survivable" range). And it takes 283 billion years to decelerate from 2mph to 0mph at that rate of acceleration. And again, napkin math. I've probably hand waved a whole lot of things away in order to make the calculation, but it should be "within a power of ten or two." … if anyone asks, I did not tell you it was ok to do math like this. $\endgroup$ Jul 30 '19 at 20:31
  • 5
    $\begingroup$ Voted offtopic: This is worldbuilding - You want worldsmashing.stackexchange.com $\endgroup$ Jul 31 '19 at 3:06

Gravity isn't going to let that happen

I'm not the resident orbital mechanics specialist so I don't have specific figures in front of me but as I understand it, there's no way that two planets, both of Earth mass, are going to collide slowly. The reason for that is that they're going to be attracted to each other by gravity. Even if they could collide slowly, the impact is going to ruin everyone's day.

The reasons for this are fairly simple; first of all, the mass that represents the two planets in such close proximity is going to want to form a more sustainable shape than a 'peanut' planet under all but the very most exotic circumstances. That means the two planets over a short amount of time will try to merge with each other to form what will in essence become a bigger sphere. There's a reason planets and satellites of any size are all spheres; it's the shape that maintains uniform equilibrium of gravitational force across the mass.

The next reason is atmosphere. It might sound like the Earth has a very thick atmosphere; hundreds of kilometres high sounds thick to be sure. But, in terms of the scale of the Earth, it's a very thin shell on the outside of the planet. Putting two planets of Earth size in close proximity, even gently, is going to disrupt the gravitational forces causing the atmosphere to cling to the planet and even before that is going to cause major disruption to the weather patterns on each planet. Your atmosphere, assuming a large amount of it doesn't get flung into space by the collision, is going to mix in with all the other mass trying to form the aforementioned bigger sphere.

So; expect drastic (possibly unsurviveable) weather before you're left with not much to breathe, before you're standing on ground suffering the worst earthquakes you can imagine, not to mention the volcanic activity caused by hot liquid rock cores wanting to get to know each other at a rapid rate of knots.

No. It's not surviveable. Your best bet is to get out of dodge before the fun begins.

  • $\begingroup$ Hundreds of kilometers is a bit on the thick side. The Kármán line is at 100 km, and the atmosphere gets awfully thin even before then. I don't have figures in front of me, but I'd say at 30-40 km the atmosphere is so thin as to be for most purposes basically nonexistent. $\endgroup$
    – user
    Jul 30 '19 at 12:58
  • 24
    $\begingroup$ If you gently rested tow planets toegether then let them go just the energy released by the two planets collpasing inot a single sphere will liquify both planets. $\endgroup$
    – John
    Jul 30 '19 at 14:14
  • 6
    $\begingroup$ The separation of "atmosphere" from "crust", "crust" from "mantle" and "mantle" from "core" would break down at the energies involved. $\endgroup$
    – Yakk
    Jul 30 '19 at 15:34
  • 1
    $\begingroup$ @John If two planets the size of the Earth were resting "gently" against each other, then the gravitational force on the point where they meet is $G\frac{m^2}{2r}$ which is 2e32 Newtons, which is just about off the charts. Liquefied, indeed. $\endgroup$
    – kingledion
    Jul 31 '19 at 12:26
  • $\begingroup$ The planets could be in orbit around each other, tidally locked, and (somehow) gradually slowed down so that they approach arbitrarily close, arbitrarily slowly. Their relative velocities could be zero at the moment of impact, and they could remain there, stable, separate but touching. (see my answer far far below) $\endgroup$ Jul 31 '19 at 17:11

Not Survivable.

I am going to ignore gravity's demand of 2 planet masses becoming 1 sphere - Tim B II covered that. And I am ignoring the energy requirements and the resulting lash-back of getting 2 planets close enough together to do 'peanut' that Ryan_L covered.

Even considering that, 'peanut' would not be survivable due to earth is rotating around its own axis. 'Contact' in that case means either

a) Earth and B-Planet have a different spin: one surface grinds into the other, with a velocity differential of probably several hundred km/h. If it doesn't completely dismantle earth's crust, it will at least leave a devastating trail of destruction around the entire earth until the two planets have somehow managed to equalize their spins to rotate around a common center. In the meantime, your atmosphere gets flung everywhere, and dust and tectonic movement and other debris create a nuclear winter of proportions even more epic than the meteor that ended the dinosaurs.

b) Earth and B-Planet have the exact opposite spins so that there is no grinding going on, just touching. However, they still rotate around their own axis, so the touch-point is going to wander around the equator. Not really survivable, because it's kind of smashing everything as it goes along. It displaces water from the oceans, which since the rotation wanders, needs to flood back. If you think 100m high tidal waves, that would be a very conservative estimate. Not to mention that it's going to dig a completely new ocean trench around the equator until the deformation energy slows the two planets' rotation so much that they still

c) Neither Earth nor B-Planet rotate around their own axis, so it really is just a 'touch'. In that case, Earth would have been quite unlivable even beforehand, because 'no rotation' means that one side of Earth is always on the day-side, and one side always the night-side. Even the huge oceans we have probably would not manage to equalize the climate of such an extreme temperature differential. Day-side would get to more than 80°C, night-side would freeze at temperatures lower than -50°C (just have a look at how cold the poles get during polar winter...). There is probably a small area around the Twilight Zone where plants can grow due to the somewhat moderate temperatures. But there probably will be hideous storms (boiling oceans create clouds that condense when it gets colder towards the Twilight Zone), and generally completely inhospitable weather.

d) Insert itself into orbit: B-Planet doesn't just come down and 'touch', but gradually inserts itself into earth's orbit to match velocities and rotate around earth until they are completely tidal-locked (both planets see the same surface of each other). And then they gradually reduce the orbiting height until they touch surfaces. However, this is a long process where two planet-sized masses become each others' moons first. Since our moon is comparatively far away, but already manages to create tides of more than 6m in some places, having a larger planet much closer will enlargen the tides exponentially. It will probably go as far as exerting tidal forces on the tectonic plates themselves, meaning countless earthquakes and volcanic eruptions - Nuclear Winter scenario.

  • 2
    $\begingroup$ Is that mathematician exponentially or journalist exponentially? Since tides are a gravitational effect I'd expect them to behave in an inverse-squre fashion relative to distance. $\endgroup$ Jul 30 '19 at 16:07
  • $\begingroup$ @BlokeDownThePub: The tidal forces are inversely proportional to the cube of the distance. $\endgroup$
    – AlexP
    Jul 30 '19 at 21:19
  • 3
    $\begingroup$ Sorry, this answer makes no sense. "Ignoring gravity's demands" in this case is a bit like ignoring heat and pressure when put inside the Sun, and only being concerned with low oxygen contents and the distance to closest McDonalds. Especially if you end with a king of all understatements: "having a larger planet much closer will ... go as far as exerting tidal forces on the tectonic plates" - no, it will go as far as liquefying both planets and turning them into a single sphere, because that's the very force you were just ignoring. $\endgroup$
    – IMil
    Jul 31 '19 at 2:09
  • 1
    $\begingroup$ @IMil It's comprehensive. If someone really wants to write this, they could write around one problem - how big a problem it is almost doesn't matter. This answer adds more reasons that this is infeasible, which is not a bad thing. $\endgroup$ Jul 31 '19 at 10:55
  • $\begingroup$ @BlueHairedMeerkat I get your point, but still the picture in this answer is too inconsistent to be of any value. The author writes: "[the touch point] displaces water, which needs to flood back ... 100m high tidal waves would be a very conservative estimate" - no, the main problem is that this touch point becomes the common center of gravity of the new system, and all the adjacent oceans turn into multiple-thousand-mile-high waterfalls, and the air from both planets rushes towards the same spot, and both planets liquefy and turn into a single sphere... Sounds familiar for some reason. $\endgroup$
    – IMil
    Jul 31 '19 at 13:55

This scenario results in the sterilization of both planets even if they don't collide in any situation that you could reasonably call a "near miss."

No matter how slowly the planets collide, tidal effects will cause massive heating and disruption of their crusts. These effects come into play long before the planets even touch, and by the time they do touch, they're already molten balls of lava.

Everyone on both planets is dead long before the big show of the collision begins.


Tim B II's answer is really good, but I have another, different reason it can't happen. Let's say you do have some way to slow the planets down so they don't collide at escape velocity. Let's also say your planets are mostly made of some fictional material that actually is strong enough to stay a contact binary planet, a "Peanut planet" as Tim put it. So all we have to be careful about is not wrecking the biosphere.

Whatever method you use to push on the planets to slow down their collision must be sending an obscene amount of energy in the other direction. Think gigantic rocket thrusters. Newton's laws are non-negotiable. The problem is that the exhaust from the rockets on Planet A will cook Planet B. Even if you have them offset so the exhaust doesn't actually hit Planet B, we're talking about just truly insane amounts of energy here. The infra-red light coming off the exhaust would be dangerous as well. Ever stand a little too close to a bonfire? Think of that times a trillion. And what's worse, as the planets get closer together, gravity gets stronger, and the rockets have to push even more.

It is true that only a little over one hemisphere on each planet will be heated directly. But I wouldn't be surprised if this kind of thing is energetic enough to heat the planet all the way through. Even if it's not heated all the way through, this ordeal will probably blast away all the atmosphere on both planets.

So even if you somehow manage to get the planets themselves to collide without merging completely or breaking apart, you're still going to sterilize them.

  • $\begingroup$ What if you don't "push" on the planets to slow them down, but rather "pull" on them from the outer sides? Let's say you attach some sort of super strong spring/bungee rope/rubber band/... to the planets. (I know this would be impossible as well, but this would deal with the issue of the boosters cooking each other, right?) $\endgroup$
    – Teebs
    Jul 30 '19 at 13:16
  • $\begingroup$ @NilsTiebos What is the other end of those bungee cords hooked to? Sooner or later, rockets have to be involved. Something has to apply tension after all. $\endgroup$
    – Ryan_L
    Jul 30 '19 at 15:35
  • 5
    $\begingroup$ Even assuming you can get the planets to "kiss" in a non-catastrophic way and magically become stable, suddenly the center of gravity is going to be near the intersection. All of the water and atmospheres on both planets will collect there. You'd have a single, V shaped ocean wrapped around the contact point, with a flattened U shaped atmosphere resting on top. $\endgroup$
    – aslum
    Jul 30 '19 at 15:36
  • $\begingroup$ What if, instead of huge rocket motors, you keep the planets apart just by having them orbit each other - a binary Klemperer rosette. Then gradually slow them down so they approach each other until they just touch. They don't coalesce, but remain held apart by their remaining orbital velocity. $\endgroup$ Jul 31 '19 at 2:11
  • $\begingroup$ @JonathanHarley I know this is counter-intuitive, but things actually move faster in lower orbits. They will scrape each other with orbital velocity. Instead of an impact, you basically have a cosmic belt sander. Again, doesn't seem survivable. This is basically what subrunner said in his answer. $\endgroup$
    – Ryan_L
    Jul 31 '19 at 3:38

The only sensible way for two planets to collide at low enough relative velocity not to vaporize a substantial fraction of their combined masses is by orbital decay of a binary planet system. Unfortunately, planets aren't rigid balls; at some point before crustal contact, one or both planets would begin to fragment as the combination of tidal forces and centripetal acceleration exceeded the combination of gravity and the tensile strength of the rock forming the crusts and mantles.

There are a number of good answer saying why this isn't going to work. There is at least one way some life could survive (monocellular extremophile life, anyway): a collision so violent that pieces of planet are completely ejected from the newly reformed, larger planet without being melted.

This happens all the time (geologically speaking) with meteoric impacts -- there are many samples of Earth's Moon and Mars known to have fallen to Earth, and the only reasonable way for those fragments to have escaped their home bodies is by being ejected during an impact event. The fragments are rather small, but there is still a persistent hypothesis that life may have started on Earth from an impact of an object carrying sporulated bacteria from another world (possibly not even one in the Solar System).

  • 1
    $\begingroup$ I like what you wrote. I waffled overly verbosely about gradually reducing the orbital velocity of tidally-locked bodies to soften the impact, in an answer far far below. But, reading your answer, I think you're right about tidal / centrepetal forces wrecking the arrangement before contact. $\endgroup$ Jul 31 '19 at 17:22

As the other commenters have mentioned, surviving a collision would be impossible: one aspect of the definition of "planet" is that the celestial body must have enough mass to gravitationally reshape itself into a sphere. If the peanut were to reshape itself (as it must, because it's made of two planets, which even on their own can turn in to a sphere) nobody would survive, the whole thing would be a giant ball of lava like the proto-earth. However, worldbuilding isn't about saying "no," so I can come up with a couple of options for you:

  1. Instead of planets, the colliding bodies could be much smaller. Maybe your beings live on the surface of Ultima Thule, or rather I should say, half live on Ultima and half live on Thule. The collision of those bodies was not so violent that they got reshaped into spheres, so it may have been survivable for the native tholin-eating lifeforms
  2. Instead of colliding, your planets could have a near miss. You could then dial in whatever degree of gravitational disruption you wanted, and as a bonus once it was over everyone could climb out of the bunkers and return to a survivable world like the one they were born in.


I absolutely agree with the other answers declaring this to be unsurvivable. But I'm going to try and make the case it could be survivable, just because I don't think anyone has tried sufficiently hard at thinking of ways it could be done.

Firstly, everyone has assumed the planets would inevitably approach at interplanetary collision velocities. This does not have to be true! If we're going to give ourselves the best chance of surviving, then we have a bit of preparatory engineering work to do.

So, we start with the planets orbiting each other, in a binary Klemperer rosette. Then, we induce the collision by gradually slowing them down, so that they gradually approach each other, until they just touch - still orbiting around their barycenter. Their relative velocity at impact is zero.

To minimize disruption at the area of impact, they'd have to be tidally locked, too, so their surfaces don't scrape across each other.

Exactly how to reduce their orbital velocities with such finesse is unspecified. Perhaps you can paint one half of the planet white and the other black, then wait a long time? There are fictional precedents of such planetary engineering feats, but they involved a reaction-less, inertia-less drive.

Without any orbital velocity, the planets would messily coalesce, churning crusts and mantles and cores, ejecting huge chunks into space to rain down afterwards, shedding every last whisp of oceans and atmosphere.

But with the orbital velocity, that can't happen. The planets remain suspended, each looming across half a sky. They won't remain the same shape of oblate spheroid as they originally were, of course. They'll flex and buckle, thrusting parts of the planet clear out of the atmosphere, while other parts drown in oceans of magma, hundreds of miles deep. God's own storms as half the atmosphere slews off into space. The sloshing oceans scraping a mile-deep layer off the crust as they go. Tidal forces yawning open all the old tectonic seams, and popping a hundred new ones too, as the rocks liquify under local strain variation.

And yet... somehow, the result is kind of stable. Not geologically, maybe. But for hours, or maybe years, they remain, a curious whirling hourglass of two kissing spheres. Long enough for someone who somehow predicted which small patch of crust would remain intact, to stand on it, in a space-suit, in a bunker, and say "I survived the event", before getting picked up and flown the heck out of dodge. Just before the 500 mile wave of ferociously radioactive liquid iron spurts out from the core, taking everything in its path. Who can say?

I'd originally planned to speculate about thousands of survival capsules spread around the planet, each equipped with miraculous ways of deriving power, and oxygen and water and... crap, what are they going to do for food? No, that's not going to work. Long term survival is, I concede, impossible.

Update: I concede the idea from other answers that, unless the planets were made of something unbelievably strong, tidal forces as they approached would flex and destroy their spherical shapes, generating tremendous heat, and tearing apart the planets into giant smears of molten rock. This would happen long before the planet surfaces could touch. So this answer is a nice poetic idea, but I don't believe it could happen after all.

  • $\begingroup$ You know, writing this answer reminds me that, as a teenager, those sections in the old Niven/Pournelle novels, that parts in italics, that describe the impersonal cataclysmic events, in between the parts where people do people things. Those were my favorite parts. $\endgroup$ Jul 31 '19 at 2:58

It's not possible to make this happen, because of power output.

An object falling must release its potential energy. This usually occurs by transferring it into kinetic energy, but there are other options. A skydiver at terminal velocity bleeds his or her potential energy into thermal energy of themselves and the air around them.

A falling planet has a lot of potential energy.

For simplicity, lets assume one planet falls into the other. We'll make one planet smaller than a planet can be. The dwarf planet Ceres is not quite big enough to be a planet, and weighs in at 10^21kg. Combine that with a gravitational pull of 9.8m/s^2 for Earth and a height of 100km. Why 100km? Well, that's the Kaman line. A fall from higher than that might be gentle, but once the other planet starts entering the atmosphere, it is going to decelerate very quickly.

Multiply them together, and you get 10^26J. Now we get to look at one of my favorite pages on Wikipedia, Orders of Magnitude (Energy). On it, we find that the earth receives roughly 5*10^24J of energy from the sun every year. So we're looking to dissipate as much energy as 20 years worth of solar input.

Now I don't have the numbers for how much extra solar energy a planet can have without killing off it's life, but I think it's reasonable to assume that doubling the heat energy entering the planet would probably wipe out life as we know it. So you certainly can't have the planets mate in anything less than 20 years.

And there's no way you could manage to orbit in the atmosphere for 20 years. The amount of drag would be intense, generating life-altering wind storms at best. At worst, fireballs that turn you all into crispy critters. And that's assuming somehow you manage to get the planets into a magically perfect orbit around each other and bleed off the energy.

So I'd say there's only one solution. You need more celestial bodies. Let the two planets just barely touch, then fling a pair of blackholes past each side, using the tidal forces to arrest the movement towards one another. This lets you bleed the energy into the black holes, resolving the heating issue.

... of course there will be the issue of the massive cataclysmic seismic forces that are going to come into play at that point. But I think this path offers the possibility that at least one cockroach survives, so I'm going to call it a technical victory.

  • $\begingroup$ You could control the rate of approach by having the planets orbit each other. They could remain indefinitely suspended, just touching, if they were tidally locked, so that the area of contact was stationary. (Still an absolute death sentence, but this softens some of the absolutes in your answer, eg "no way you could manage...") $\endgroup$ Jul 31 '19 at 17:26
  • $\begingroup$ @JonathanHartley That would work for when the planets are in geosynchronous orbit, but not any other orbital altitude. So I suppose, in theory, there is a planetary density where that might work, just large enough to put the centers of mass in geosync when touching. Myself, I did not read the word "collision" in the question to consider that case, but it is an intriguing counterargument. It looks like your answer argues "it may be stable on the order of years," and my answer suggests how many years it would need to be stable to be survivable. $\endgroup$
    – Cort Ammon
    Aug 1 '19 at 0:39
  • $\begingroup$ Hey Cort. I don't think what you say is quite right. When we say "tidally locked", we mean the planets' rates of spin have been matched with their orbital velocity - so by definition, they are ALWAYS in geosynchonous orbit. In this scenario, as we subsequently modify the orbital velocity, we also modify the rate of spin to keep them tidally locked, so that they remain in a geosynchronous orbit at all times. $\endgroup$ Aug 1 '19 at 2:37
  • $\begingroup$ @JonathanHartley Ahh, I see. I was only considering the case where one was tidally locked (such as how the moon is tidally locked to us, but not the other way around). In that case, geosynchronous orbit and tidally locked would be two different things. $\endgroup$
    – Cort Ammon
    Aug 1 '19 at 2:53
  • 1
    $\begingroup$ @JonathanHartley I think some of the challenge is that I just wrote an example which was basically a moon crashing into the Earth. So all of my own interpretations were tainted by how our moon/Earth system operates. For example, I naturally assumed that tidal forces would cause tides, in the sense we experience on Earth, which are changing several times a day. $\endgroup$
    – Cort Ammon
    Aug 2 '19 at 17:04

Not the answer you're looking for? Browse other questions tagged or ask your own question.