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This is a small but critical part of my story; I need to kill off most of humanity (leaving a 5- or 6- digit number of survivors) and render the planet uninhabitable long-term, but survivable for a short time (as short as a few hours is OK). So I've been thinking about this scenario:

A large comet/asteroid impact turns one whole side of the planet to lava; everyone on that side is vaporized instantly and the atmosphere is pushed into space.

On the downstream (non-impact) side, there are earthquakes that would register 12-15 on the Richter scale, if the Richter scale went that high (I get that it's logarithmic). In addition there are tornado-force winds as the atmosphere rushes to fill the void on the upstream side of the planet, and torrential rainfall as water condenses out of the suddenly rarefied air.

Most people on the downstream side die from injuries and there is only a short time before the air pressure drops to unsurvivable levels and temperatures become lethal due to equalization with the lava on the upstream side.

Does this make reasonable sense, physics-wise? It doesn't have to be perfect since most of the story takes place afterwards and not on Earth (and we're not coming back), so we don't have to worry too much about details.

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    $\begingroup$ The asteroid would have to be pretty large to shove a huge fraction of the atmosphere to escape velocity. So large I'd imagine the impact would liquify the crust anyway. I think it's fair to say the comet couldn't eject much more than its own mass from Earth. Some of that mass may be atmosphere, but a good portion will be molten rock and dust. $\endgroup$
    – BMF
    Oct 21, 2020 at 16:20
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    $\begingroup$ How about a large comet makes a very close pass around the sun and breaks up. The debris swarm is headed for Earth and hits progressively around the globe over a day or so? $\endgroup$
    – Slarty
    Oct 21, 2020 at 16:56
  • $\begingroup$ "uninhabitable long-term" - uninhabitable for sheltered, or unsheltered humans? $\endgroup$
    – Alexander
    Oct 21, 2020 at 16:59
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    $\begingroup$ Why "1 comet/asteroid"? Levy-Shoemaker 9 broke apart and hit Jupiter multiple times. $\endgroup$
    – Michael Kutz
    Oct 21, 2020 at 16:59
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    $\begingroup$ The most common, "hit by a meteor, now we're all gonna die!" scenario is the meteor kicks up enough dust to blanket the earth in the equivalent of a nuclear winter. Solar-dependent power is gone. Everything gets ice-age cold. Almost everybody's dead in 3-6 months. It takes centuries for the dust to settle. $\endgroup$
    – JBH
    Oct 21, 2020 at 17:19

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A large comet/asteroid impact turns one whole side of the planet to lava

This will melt the crust. The shock that led to the formation of the Moon was probably similar to that. It involved an impactor about the size of Mars, which is orders of magnitude more massive than the dino killing asteroid.

When the dino killer asteroid hit the Earth, it heated all of the atmosphere by a few degrees in very little time. Many animals on the other side of the planet were literally cooked alive. This seems to be a hot topic among paleontologists, pun intended:

This global "impact firestorms" hypothesis, initially supported by Wolbach, H. Jay Melosh and Owen Toon, suggests that as a result of massive impact events, the small sand-grain-sized ejecta fragments created can meteorically re-enter the atmosphere forming a hot blanket of global debris high in the air, potentially turning the entire sky red-hot for minutes to hours, and with that, burning the complete global inventory of above-ground carbonaceous material, including rain forests.

With a Mars sized impactor, you would not just turn one half of the crust into magma. You would send the planet back into the Hadean phase. This kind of impact would heat up the atmosphere to Venus levels immediately. It also causes the planet to reshape itself in very short time - the Earth would literally soften up for a while, and the global Earthquake will destroy anything and everything on the surface. Your number of survivors will be a hard zero.

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  • $\begingroup$ This pretty much what I'm looking for; chances for survival would be zero. But the carnage is not instantaneous, so within a few hours some people could be rescued. $\endgroup$
    – Crash Gordon
    Oct 22, 2020 at 16:51
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    $\begingroup$ @CrashGordon If you mean hours before the impact, yes. Once the impactor is below lunar orbit people may already be unsaveable. Theia hit Earth at about 4 km/s, and simulations look like this (check the second impact, the "grazing" one). Notice that even before the impact, both planets bulge. $\endgroup$
    – The Square-Cube Law
    Oct 22, 2020 at 16:57
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    $\begingroup$ Yeah—came here to say this. The crust would melt and the atmosphere would overheat and the oceans would boil and the debris ejected into space would blot out the sun, the source of all life on earth. So yeah, the humans would die. Just make your space rock significantly smaller so that the humans opposite the impact site only have to deal with a mini ice age and agricultural die-offs as a result of the ash in the sky. $\endgroup$
    – HolocronCollector
    Oct 23, 2020 at 2:12
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This makes some sense. It would definitely kill people. However, the hurricane-force winds part is probably not realistic. A meteor impact that melts half the Earth won't just make air get flung into space: the air will become no longer exist as it dissociates into its constituent atoms, which probably means no vacuum. You can't just "fling" air. The rain part doesn't make sense to me, as it's likely that the ratio of water/air is the same (remember, the water vapor on the impact side was dissociated into O2 and H2). Additionally, an impact large enough to turn half the planet to lava would almost surely destroy the whole Earth. The meteor would likely have mostly kinetic energy, and for enough kinetic energy to turn into thermal energy to actually melt half the Earth, the meteor would have to disobey the laws of physics (as a meteor with enough KE to do that would just shatter the Earth and fling the pieces a couple billion miles through the galaxy, instantly destroying the whole thing, and in that case likely won't melt half the Earth, since the Earth won't be able to stop it and turn enough of its KE into heat [Kinetic energy - heat occurs when something bumps into something else and slows down/stops via friction of the normal forces; such a massive/fast meteor wouldn't stop]). If half the Earth actually DID turn into lava, and half the atmosphere id disappear, the planet would become uninhabitable as not enough atmosphere and oxygen would remain to support life (the atmosphere shields us from the hazards of space). In addition, the Earth would probably collapse if half of it turned into a liquid (and likely settled into the core), as a hemisphere is not a viable planet shape. Finally, your "impending doom" is going to happen another way: even if the meteor wasn't big/fast enough to melt half the Earth, the shockwave from the impact would race around the Earth, killing everyone it hits as dust from the blast is flung into the air, obscuring the sun. This even happens FAAAAR before half the Earth melts from an impact (which would shatter the Earth anyway). I'd say that people would have time to leave Earth before the shockwave flattens them. A meteor that would do this would probably be about small continent size (Europe).

(also pretty sure Richter 15 means the Earth explodes)

P.S. if only a little bit of the Earth around the impact melts, lava falling from the sky seems possible and would be cool.

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  • $\begingroup$ I'm mostly trying to think how to describe the situation as the planet is dying. I was trying to think of what would happen in the aftermath of a large impact and my mental image had one hemisphere splashing out like water from a pebble, carrying air with it. Maybe that was not accurate. $\endgroup$
    – Crash Gordon
    Oct 22, 2020 at 16:54
  • $\begingroup$ Your energy scales are way off: shattering the earth takes thousands of times more energy than melting the crust $\endgroup$
    – QuadmasterXLII
    Oct 22, 2020 at 16:57
  • $\begingroup$ I do like the "lava rain" bit. $\endgroup$
    – Crash Gordon
    Oct 22, 2020 at 17:05
  • $\begingroup$ My point is that an asteroid carrying enough energy to melt the crust likely would actually transfer all that energy into heating the Earth and melting it. $\endgroup$
    – The_CIA
    Oct 22, 2020 at 18:32
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There's this YT video.

https://www.youtube.com/watch?v=PENT_hnyO-o

It's supposedly from Discovery Channel, so it should have at least resemblance of realism, but I think it doesn't account for the part that on planetary scales, everything's a liquid, so impact like this should have created shockwave through the planet that would lensed on the other side of the planet and blew the crust there (https://en.wikipedia.org/wiki/Mantle_plume#The_impact_hypothesis); It is theorized that one such impact created the Siberian traps, the megavolcano that almost ended all life on Earth during the Great Dying extinction event (83% of all species went extinct at that time).

What definitely wouldn't have happened is the air rushing "in" the impact crater; quite the reverse would happen actually, the impact will create a shockwave that will grind everything on the surface into very fine dust and pebble as it travels along the surface. So basically first the people will feel the impact with the earthquakes spanning the entire planet, then the opposite site from the impact should explode into an instant supervolcano that will make Yellowstone blush (Probably making it's own less powerful shockwave in the process), and then those that survived will be blasted away by the supersonic atmospheric shockwave that arrives from the direction of the impact. If they survive the earthquakes and do not live close enough to the opposite site to be blown up by the fresh supervolcano, I think they would have rougly one or two hours left to live.

If you scale the impactor to being "just" about the size of one that set off the Siberian eruption, then it wouldn't be as dramatic and the humans would been able to survive for longer, but perhaps for too long for the purposes of your story (years to centuries until the rising temperatures and massive die-offs would finally pushed them to go join the dinosaurs)

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  • $\begingroup$ I'm going to have to watch that video a few times. Very helpful. $\endgroup$
    – Crash Gordon
    Oct 22, 2020 at 17:01
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I am not convinced that an asteroid strike big and destructive enough to make the earth uninhabitable for humans would allow them to survive long enough to work out a way to escape from the planet.

There are some other scenarios for human life ending on Earth that you could consider. These would give your survivors more chance to hatch an escape plan:

  • Global warming
  • Pandemic
  • Volcanic activity
  • World wars - using nuclear or biological weapons
  • Psychological breakdown (depression and suicide)
  • Declining fertility
  • Over population
  • Artificial Intelligence
  • Nano-technology (the infamous grey-goo)
  • Increased solar activity

I am sure there are others.

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    $\begingroup$ I never said humans were going to work out a way to escape :- ) $\endgroup$
    – Crash Gordon
    Oct 22, 2020 at 17:02

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