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I am trying to think of a natural event that causes very big impact to human lives - massive movements of people, shift of power, etc. This event happens in a medieval period on an earth-like planet.

I was thinking that a large asteroid passing very close to earth can cause significant perturbations. I would like to know what disasters can such an event lead to.

Fires is the obvious one. What about earthquakes due to the gravity of the asteroid that change the relief of the surface of the planet? What about giant waves that lead to floods?

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  • $\begingroup$ This is a case where I have the Math to tell at a glance that humanity would be doomed, but I still have to run it to know how much doomed we are. I love this type of question, +1. $\endgroup$ – Renan Jun 20 '18 at 22:29
  • $\begingroup$ I think I've seen this already, here or on another stack... $\endgroup$ – Mołot Jun 20 '18 at 23:22
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    $\begingroup$ A large asteroid would likely have some cling-ons. Having some of those smaller pieces impact could cause some of the devastation you desire. $\endgroup$ – Tracy Cramer Jun 21 '18 at 0:08
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The largest asteroid in the solar system is, arguably, Ceres. The second largest is Vesta. Then there is Pallas. All of these bodies are two orders of magnitude less massive than our Moon, therefore their gravity is around 1~9% that of our satellite. Any other asteroid is three or more orders of magnitude less massive than the Moon.

If these asteroids passed close to the Earth, they would have tidal effects. How strong the effects would be would depend on distance. The gravity force between two bodies is given by the following formula:

$$ F = G \frac{m_1m_2}{r^2} $$

Where $G$ is a constant ($6.674 \times 10^{−11} Nkg^{–2}m^2$), $m_1$ and $m_2$ are the masses involved, and $r$ is the distance between the bodies. This means that the Moon ($7.34 \times 10^{22}kg$, $384,400 km$ away from us) pulls the Pacific ocean ($6.8 \times 10^{20}kg$) with the following Force:

$$ F = 6.674 \times 10^{−11} Nkg^{–2}m^2 \times 10^{-11} \frac{(7.34 \times 10^{22}kg) \times (6.8 \times 10^{20}kg)}{(3.844 \times 10^{8}m)^2} \approx 2.254 \times 10^{16}N$$

If Ceres ($8.95 \times 10^{20}kg$) passed by us at a distance of 4,000km above the surface of the Earth, the force on the Pacific would be...

$$ F = 6.674 \times 10^{−11} Nkg^{–2}m^2 \times 10^{-11} \frac{(8.95 \times 10^{20}kg) \times (6.8 \times 10^{20}kg)}{(10^7m)^2} \approx 4.05 \times 10^{17}N$$

You may be wondering why a value of $10^7m$ in the division, when I mentioned 4,000km above us. That's because distances in the formula are between centers of mass. The radius of the Earth makes up for the bulk of those $10^7$ meters.

Ceres is much less massive than the Moon, but as you can see, because it would pass so close, forces would go up by one order of magnitude. Ceres would have the pull of twenty Moons. Replacing Ceres with Pallas, for example, does not make the situation much better... At a quarter of Ceres's mass, that would still produce a pull five times stronger than the Moon's.

Back to the simulation with Ceres, we would have some devastating tides... We would have a global tsunami event, in which every single seashore in the planet would experience oceanic retreat and a large wave going kilometers into the country - not necessarily in this order - before the sea level returned to normal. In fact, in some places it would not return to normal, for water would become either permanently trapped in, or permanently sucked out. The displacement of oceanic mass might also trigger quakes (provoking secondary tsunamis) and a few volcanoes might be pressed into spilling out lava, but that would be it. Millions of people on the shores would die, but humanity would live.

The worst impact would be a change in Earth's orbit. It would become a little more excentric from the encounter. I don't have the math in me to say by how much, but I believe the climate patters would change forever worldwide. Wild life does not hande that very well when it takes centuries to happen, let alone when it happens in the span of a year (which we would need in order to experience the whole magnitude of the climate change). A mass extinction event would be more probable than not.

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    $\begingroup$ With the Earth moving at 30 km/s and the asteroid passing by fast enough not to get caught in orbit or get pulled into the atmosphere, how long would the asteroid exert a pull greater than the moon on the planet? $\endgroup$ – Tracy Cramer Jun 21 '18 at 0:32
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    $\begingroup$ @TracyCramer It would probably never get caught - the path would have to be just right for the Moon to reduce its speed, but that's more unlikely than likely. Otherwise, if it is not in a collision trajectory, it is in a flyby one, regardless of speed. $\endgroup$ – Renan Jun 21 '18 at 1:17
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    $\begingroup$ Love the answer. Out of curiosity why did you choose a 10,000km passing distance? Astronomically speaking that is incredible close. Also, if this was a flyby, would it have such a huge effect as you say? Could it actually cause such tidal issues? $\endgroup$ – Jonnyboy Jun 21 '18 at 7:12
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    $\begingroup$ @Renan Was thinking about this more and wondering if that distance would be inside the Roche limit and actually cause the asteroid to split up? $\endgroup$ – Jonnyboy Jun 21 '18 at 7:35
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    $\begingroup$ @Renan, Great answer! Please note that the 10,000 km distance is not surface-to-surface. The classical $Gm_1m_2/r^2$ formula assumes point-masses. A more accurate model depicts a continuum of differential masses, some of them closer, some further apart. In practice, the point-mass distance is equivalent to somewhere in between surf-to-surf and center-to-ctr. In turn, crossing the Roche limit implies massive amounts of derbis falling into the Earth and Moon, a transient ring, lots of new near-Earth asteroids (threatening to fall a few orbits later), and possibly a new satellite in the long term $\endgroup$ – Rafael Jun 21 '18 at 13:13
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The question is how rare you are willing to tolerate your asteroid to be in order for your story to make sense. The bigger the object and closer the pass-by, the rarer. An interstellar object of enough mass (a Rogue planet maybe), passing close enough, can have whatever effects you want, including removing the Earth from Heliocentric orbit, dooming it to eternal cold.

A smaller or further passer-by—yet extremely rare—could alter the Earth's orbit or axial tilt. Even a slight change in these and you can get massive climate implications. Take for example the theory that the Sahara became a desert because of a change of less than a degree in axial tilt. In this case, the change in tilt is cyclical and is expected to repeat itself every 41,000 years. Make it happen faster and there you go.

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A small asteroid impact could have a devastating (though localized) effect - something akin to the Tunguska impactor, but over a populated area, for instance.

An asteroid passing through enough of Earth's atmosphere to cause any significant amount of heating would likely lose so much of its momentum in the process (that's what generates the heat, after all, through friction) that it would impact. If it was big enough to cause notable heating, its impact will probably be too big for your story; we're talking a planetary-level extinction event.

If the asteroid doesn't hit Earth, the only real mechanism for affecting it is gravity. The gravity of the Moon generates tides, obviously, but isn't thought to have a significant effect on things like fault lines and earthquakes. (In some of the Jovian moons, gravity causes tidal flexing of the crust of the moons, but obviously Jupiter is much, much larger than any close-passing asteroid.) Lunar tides also build up a resonance - the fact that the Moon always follows the same path makes the tides stronger. An asteroid wouldn't do that, because it only makes one pass, so its "tides" would be weaker.

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  • $\begingroup$ I disagree. Even fairly small objects have been observed to pass through the atmosphere and head back out. The problem here is that anything big enough to dump a lot of heat into the atmosphere is big enough to have been disrupted (it's within the Roche limit) and that means you'll have debris impacts. $\endgroup$ – Loren Pechtel Jun 21 '18 at 1:00
  • $\begingroup$ @LorenPechtel Small objects can skim the atmosphere, but they don't do anything to Earth in the process. A larger body that dumps enough heat to cause fires (as in the question) seems unlikely to successfully bounce, to me anyway. Maybe I'm wrong, in which case that seems like a good answer. $\endgroup$ – Cadence Jun 21 '18 at 1:11
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    $\begingroup$ Drag goes up at the cross section, mass at the volume. Thus the bigger an object the easier it is for it to punch through and head back out. $\endgroup$ – Loren Pechtel Jun 21 '18 at 1:18
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Not fires, because if it were close enough to cause heating then it would definitely impact. However, a lot of floods would happen due to the tidal effects that would take place. (I am assuming that the asteroid in question is on the same scale as the moon due to how you worded the question.) Also, it might block out the sun temporarily, causing cold temperatures, and crops to die, causing in turn food shortages.

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  • $\begingroup$ heating from the asteroid touching the atmosphere isn't the only way to cause fires. The tidal effects (like the flooding you mention) could simply cause buildings to collapse, and wiring would set the building materials on fire. Also, not sure how fast the asteroid would be going, but I would assume fast enough to not have to worry about crops dying from no sun. If not, that would be the least of our worries. $\endgroup$ – Aethenosity Jun 20 '18 at 23:39
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No significant effects.

If an asteroid passed us by so close as to graze the atmosphere without bouncing, it means it'd be on a collision course.

Otherwise, to provoke temporary, but significant effects on the planet, it'd have to be at least the mass of the Moon and pass us by at, say, 150,000 Kms. Tidal effects would be strong enough to create floods and earthquakes

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