At the end of Book 6 of my book series, a magic ritual has caused a large meteorite (around 1/4 the size of the one that caused the chicxulub crater) to ram into the small planet (around the size of Earth's moon). However, the main characters are some time away from their ship when the meteorite hits. If the meteorite hits the opposite side of the planet, how long do these characters have to escape before they are swept away by the shockwave?

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    I suspect this would be greatly affected by the relative velocity of the meteor and the planet... – Qami Sep 14 at 20:06
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    So your main characters have a spaceship but never looked for incoming asteroids before landing on the planet. In the 1820s Biela's Com0nt was calculated to impact Earth in 4339 but it broke up sometime in 1842-1871.… This shows that impacts can be predicted well in advance once the object is discovered. – M. A. Golding Sep 14 at 20:35
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    @M.A.Golding : presumably the "magic ritual" that summoned the meteor wasn't as predictable ;) – Qami Sep 14 at 20:38
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    A meteorite has, by definition, already rammed into the planet. What you're looking for is probably an asteroid. – reirab Sep 14 at 22:24
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    @elemtilas The details given are clearly tied to the story, but the general question of "what's the damage from an impact on the far side of the planet" is, in my opinion, on topic. Any decent answer should be able to say something to that general question. – Cort Ammon Sep 15 at 5:10
up vote 15 down vote accepted

Rough estimation: the shock wave is basically a seismic wave.

The fastest seismic wave (P wave) travel at about 5.5 km/s in granite. Considering that the diameter of the Moon is about 3500 km, and assuming that the propagation of the seismic wave goes along the shortest path and with uniform velocity, we have about 636 seconds, or 10 minutes, before the shock wave reaches the other end of the body.

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    That's a good estimate for when the things will get messy. But seismic wave may not be sufficient to kill the main character. – Alexander Sep 14 at 21:15
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    @Alexander the pressure wave from such an impact would blow the land off the back of the planet.. like a Newton's cradle. The safest place to be is adjacent to the impact zone. – Richard Sep 14 at 23:25
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    @Richard I don't think "Newton's cradle" model is applicable here at all. There is no reason to believe that planet's mantle and crust would behave like elastic pieces. If that was so, for every impact the Earth would have had not one, but two craters. – Alexander Sep 14 at 23:43
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    @Richard, that only applies to completely solid bodies. If there's a fluid core or mantle, the dynamics get complicated, and the antipodal point isn't especially dangerous. – Mark Sep 15 at 1:15
  • Assuming the asteroid get's stopped, the absolute soonest they could be affected is still the speed of sound through the ground though... – Persistence Sep 15 at 6:34

Feeding "1/4 the size of the Chicxulub impactor" and "other side of the Moon" into an Earth impact calculator gives an approximate time of "forever", even with worst-case estimates for the size of the meteorite. Key numbers from the calculation:

The fireball is below the horizon. There is no direct thermal radiation.

The major seismic shaking will arrive approximately 18.3 minutes after impact.

Mercalli Scale Intensity at a distance of 5500 km:

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

The ejecta will arrive approximately 28.8 minutes after the impact. At your position there is a fine dusting of ejecta with occasional larger fragments

The air blast will arrive approximately 4.63 hours after impact.

Peak Overpressure: 5400 Pa = 0.054 bars = 0.767 psi

Max wind velocity: 12.4 m/s = 27.8 mph

Sound Intensity: 75 dB (Loud as heavy traffic)

If your explorers are out wandering, they probably won't notice the seismic effects. Instead, the first evidence they'll get of the impact is a dusting of powdered rock ejected by the impact (I suspect the actual arrival time will be closer to 45 minutes than the given 28.8 minutes, due to the smaller planet size changing the duration of ballistic trajectories). About four and a half hours after the impact, the shockwave you're worried about will arrive, in the form of a loud, protracted rumbling.

In short, it'll be obvious something happened, but your explorers won't be in any danger until secondary effects (weather changes, firestorms) start arriving, probably days after the impact.

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    If the meteor is big enough to 'punch' through the atmosphere all the way to ground, then much ejecta will exit via the evacuated tube and go into near orbit, to rain down planet-wide. – amI Sep 14 at 23:08
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    @reirab, the quick-arrival ejecta gets knocked clear out of the atmosphere by the impact, and follows a ballistic suborbital trajectory to wherever it lands. For a point opposite the point of impact, the fastest possible arrival time is about 45 minutes (for complicated reasons, this depends only on the density of the body, not its size). – Mark Sep 15 at 1:13
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    @ubadub That's the magnitude of the earthquake at its epicenter, not the magnitude that you will be feeling if you are very far away from it. – forest Sep 15 at 8:54
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    While it's a good first attempt, this is a serious misapplication of the program. The program assumes relatively local effects - that is, the terrain is assumed to be flat. As a result, the planetary-scale effects are off. Just as an example, the 18.3 minutes represents a shock wave which propagates along the surface. Obviously, the shock wave propagating through the planet will arrive much sooner (shorter path), not to mention the changes of propagation velocity with depth (2-8 km/s at surface, 13 km/sec in deep mantle). Other issues are similarly distorted (or worse). – WhatRoughBeast Sep 15 at 11:51
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    @ubadub, the Richter scale, and its modern replacements, are a measure of the energy released: a 9.8 earthquake is 9.8 no matter where you are. The Mercalli scale, on the other hand, is a measure of what the earthquake feels like, and how much damage it does. The Mercalli rating of an earthquake varies with location, and 5500 km away from a magnitude 9.8 earthquake, it isn't very high. That's why I included the Mercalli rating and omitted the Richter rating. – Mark Sep 16 at 5:55

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