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Thread: City / crater / asteroid question

  1. #1
    ....... Harlequin's Avatar
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    City / crater / asteroid question

    Coastal city of eight million (chicago sized, perhaps).

    Direct hit by very large asteroid object... Big enough to leave an impact crater of several miles.

    Would crater flood almost straight away from the ocean?
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  2. #2
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    I suppose that if the city was coastal and the crater was below the sea level, and the crater extended to the ocean I would hazard a 'yes' guess. As far as how quickly the crater fills up, I would probably model that after the levee breach during hurricane Katrina in New Orleans.

    But, I would have to think that an impact of that magnitude would generate quite a large tsunami, which would probably have an even more dramatic effect.

    Just a guess. No real world experience.

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    Swan in Process Siri Kirpal's Avatar
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    Sat Nam! (literally "Truth Name"--a Sikh greeting)

    Would depend on the height of the crater wall. And as said above, proximity to ocean.

    Agreeing with the possibility of a tsunami.

    Am thinking it might trigger earthquakes too, beyond the quake of impact.

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  4. #4
    Tending bar by the litterbox. Thomas Vail's Avatar
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    Quote Originally Posted by Harlequin View Post
    Coastal city of eight million (chicago sized, perhaps).

    Direct hit by very large asteroid object... Big enough to leave an impact crater of several miles.

    Would crater flood almost straight away from the ocean?
    You say it's a coastal city and the impact crater is several miles across - ipso facto as the crater should extend a good ways into the ocean as well, it's going to very quickly be filled with sea water, once cool enough to not immediately boil it all away.

  5. #5
    Heckuva good sport frimble3's Avatar
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    Quote Originally Posted by Thomas Vail View Post
    You say it's a coastal city and the impact crater is several miles across - ipso facto as the crater should extend a good ways into the ocean as well, it's going to very quickly be filled with sea water, once cool enough to not immediately boil it all away.
    Yes, to extending into the ocean.
    I think the chief other thing to consider is how high the rim of the crater is. That might be sufficient to act as a dam, or, partial dam.
    Last edited by frimble3; 08-14-2017 at 05:22 AM.

  6. #6
    Get it off! It burns! Dennis E. Taylor's Avatar
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    The meteorite doesn't have to center on the city. You can pick a size of meteor that will give you, for instance, a crater that ends a quarter mile from the water. Ground shock, etc, will still be enough to get rid of anything not immediately pulverized. Take a look at pictures of the Barringer Crater for an idea of what you could get.

    Once the crater has been created, there would probably be enough cracks, partial sewer pipes, and maybe simple collapses of the crater wall (unstable soil on the shore side?) to allow the water to flow in. For time required to fill it in, calculate the volume of the crater (for a rough estimate, you could consider it a cone rather than a spherical section), and find out the flow rate of the small river of your choice. Remember that your numbers only have to be reasonable, not correct.
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  7. #7
    Feeling lucky, Query? jclarkdawe's Avatar
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    Don't forget the ground water in the area. Even if the crater wall is relatively intact, ground water will leak into the area.

    You might want to read https://en.wikipedia.org/wiki/Impact_event

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  8. #8
    practical experience, FTW MaeZe's Avatar
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    Quote Originally Posted by Thomas Vail View Post
    You say it's a coastal city and the impact crater is several miles across - ipso facto as the crater should extend a good ways into the ocean as well, it's going to very quickly be filled with sea water, once cool enough to not immediately boil it all away.
    Some craters have evidence of molten rock flowing into the crater, but most of the time the heat is blasted out out the crater.

    Tycho and Giordano Bruno Crater (Lunar impacts)
    [Giordano Bruno]At the bottom of the back wall is an intriguing whorl in the smooth, dark rock. Debris slid into the molten rock and caused the viscous puddle to flow in a circular motion before it froze solid.

    [Tycho] An isolated mountain rises 2 kilometers (1.2 miles) at the center of the crater. It was thrust upward through a pool of impact melt when the crustal rock rebounded very quickly after the impact. A covering of that now-solid impact melt coats the summit. It appears darker than material farther downslope on the peak and has a sharp, brittle edge.
    Scientists Solve Mystery of Meteor Crater's Missing Melted Rocks
    Scientists have tried to explain why there's not more melted rock at the crater by theorizing that water in the target rocks vaporized on impact, dispersing the melted rock into tiny droplets in the process. Or they've theorized that carbonates in the target rock exploded, vaporizing into carbon dioxide....

    "If the consequences of atmospheric entry are properly taken into account, there is no melt discrepancy at all," the authors wrote in Nature.

    "Earth's atmosphere is an effective but selective screen that prevents smaller meteoroids from hitting Earth's surface," Melosh said.

    When a meteorite hits the atmosphere, the pressure is like hitting a wall. Even strong iron meteorites, not just weaker stony meteorites, are affected.

    "Even though iron is very strong, the meteorite had probably been cracked from collisions in space," Melosh said. "The weakened pieces began to come apart and shower down from about eight-and-a-half miles (14 km) high. And as they came apart, atmospheric drag slowed them down, increasing the forces that crushed them so that they crumbled and slowed more."
    So it depends on the speed of the impact. On the Moon there is no atmosphere to slow meteorites down. On Earth, impact craters vary as to how much heat and molten rocks are generated.


    As for the crater filling with sea water, an author can choose the characteristics of the impact:

    Size and make-up of the object, speed of impact, distance to the ocean, ground water, take your pick.

  9. #9
    ....... Harlequin's Avatar
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    Oh sweet, brains to pick! *rubs hands*

    Okay, not a rock, if that helps. Giant metal spaceship. I haven't worked out the speed because I'm not clever enough ;-) It starts in or near the planet's atmosphere (appears in the wrong place going full speed, can't slow down sufficiently and slams into city).

    Yes, I imagine the crater would extend into the ocean, but I wondered if debris and what have you would create a barrier (at least temporarily), plus rim of crater itself. Equally, it could as another poster suggests, not land dead on the coast--still effectively wiping the city out but not being half in the water on the start. I'm sure anything on the actual coast which survived initially would get washed away in short order.

    If the heat is dispersed, would it not still be extremely hot near the impact site?
    Last edited by Harlequin; 08-14-2017 at 01:53 PM.
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  10. #10
    Let's see what's on special today.. Bufty's Avatar
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    I guess 'Giant metal spaceship'- not being a solid or unyielding/dense object- creates a new ball of wax completely in physics terms. But if it's fantasy, the spaceship construction, trajectory and impact result can surely be whatever you wish it to be story-wise.
    Last edited by Bufty; 08-14-2017 at 03:11 PM.
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  11. #11
    ....... Harlequin's Avatar
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    True, but there's still limits to believability, and being relatively uneducated in science I don't know where that boundary lies. If it was totally unrealistic that it takes a couple of hours for the ocean to breach and fill a massive crater, then I'd work around that (for example).
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  12. #12
    Let's see what's on special today.. Bufty's Avatar
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    Quote Originally Posted by Harlequin View Post
    True, but there's still limits to believability, and being relatively uneducated in science I don't know where that boundary lies. If it was totally unrealistic that it takes a couple of hours for the ocean to breach and fill a massive crater, then I'd work around that (for example).
    You can create your own degree of believability. All the elements are fluid. Impact speed, crater depth, crater shape, proximity to ocean, ocean current or tide, is it high tide at impact or low tide, is it day/night, raining/sunshine, is the crater rim at the ocean blocked by the remains of the spaceship, is the rim simply built up earth, what's the material on the ground, is it sand, earth, gravel, rock, rubble...

    I suspect the believability will lie in the writing and the readers' imagination, not the detailed science/physics.

    Why not write the scene as you imagine it and then build on that after checking for possible obvious errors of logic/science.

    No idea what the story is about, but are you looking to describe the event as it happens, or is it only the resulting chaos and aftermath that is experienced/witnessed by a POV character? Can't imagine who (other than an aircraft pilot) would be in a position to witness the actual event and describe it with anything other than incredulous non-scientific comments. Depending upon your time factor, the approaching craft would no doubt be detected by defence instrumentation.

    Good luck.
    Last edited by Bufty; 08-14-2017 at 04:06 PM.
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  13. #13
    practical experience, FTW MaeZe's Avatar
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    If the impact is a spaceship and not a meteorite, it would have to be huge.

    Nomenclature side step:
    Meteoroid - the object in space
    Meteor - the visible object in the atmosphere
    Meteorite - the object once it hits the ground

    The speed of meteoroids in space can be tremendous. In addition, the direction the meteorite hits from can add the speed of the Earth coming at it, subtract the speed of the Earth it is catching up to, or neither if it comes at us perpendicular to the Earth's orbit. The angle the meteoroid enters the atmosphere affects the impact speed. And the material the meteoroid is made of affects whether it breaks up in the atmosphere or hits the Earth intact.

    You may just need to go with pure fiction here if this is a spaceship.

  14. #14
    practical experience, FTW Bolero's Avatar
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    Or is the city at sea level with a dyke to keep the sea back?
    And you demolish the dyke. Would remove some calculation.

    Do you need it permanently flooded by the sea? Or spring high tide sweeps in and out again?
    "People don't live on the Disc any more than <....> they live on balls. Oh, planets may be the place where their body eats it tea, but they live elsewhere, in worlds of their own which orbit very handily around the centre of their heads." Terry Pratchett, The Last Continent.

  15. #15
    ....... Harlequin's Avatar
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    It's meant to be a long ago thing--so it does end up permanently flooded, but need not be from that specific event alone.
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  16. #16
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    For me the realism is very difficult but not impossible. If you read back to the weight of ore thread, you'll see some of my comments on mass and spaceships - mass is not good for spaceships, not good at all. So if your spaceship isn't particularly massive, in order to achieve your desired effect, you'll need a lot of speed.

    You say it appears in the wrong spot, doing full speed. What I picture is the ship is in FTL, warp speed, hyperspace, whatever, and the navigator didn't have his coffee and dropped out at the wrong spot. In some space movies ships emerge from FTL and kinda plop in place, or glide along at gentle speeds. In this case, you need to be going fast. The ship will likely be torn up in the atmosphere and would create a huge plasma fireball. The molten wreck can cause the crater. The death toll would spread pretty darn far.

    I have no idea how fast your ship will need to go, but your scenario brings this to mind. https://what-if.xkcd.com/147/

    So to book end it, you'll probably want to keep your ship somewhere under 1/4 the speed of light.

    For consistencies sake, unless you had a massive improvement in technology (perhaps developed precisely because of this catastrophe) I'd recommend making sure all your out-of-FTL events involve sufficient deceleration time.

  17. #17
    practical experience, FTW MaeZe's Avatar
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    A ship traveling FTL or even 25% of light speed could indeed provide the element needed for a large impact to make up for lack of mass. That gives all sorts of leeway there on the resulting crater being very believable.

    Another option to make a larger crater credible would be to have the ship use an antimatter drive.

    Antimatter to ion drives: NASA's plans for deep space propulsion
    Last edited by MaeZe; 08-15-2017 at 08:19 AM.

  18. #18
    ....... Harlequin's Avatar
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    and the navigator didn't have his coffee and dropped out at the wrong spot.
    Gave me a grin while having my own coffee this morning.

    Ship is fleeing, and can be totalled, but some people in it need to survive. Sorry to keep adding confounding details. This is what I get for trying to do fantasy world building *after* MS is done. Yes, it does bork the planet very badly.


    A ship traveling FTL or even 25% of light speed could indeed provide the element needed for a large impact to make up for lack of mass. That gives all sorts of leeway there on the resulting crater being very believable.


    Hurrah, leeway! I'll take it ;-) I don't know enough about antimatter to touch on that >.> But the tech is never explained or used again, so hopefully that won't matter.

    Pleased to see ships with solar sails on that link. I've been a little bit in love with those since Urth of the New Sun.
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    I'm not sure how to do survivor + crater miles wide. Maybe escape pods right before atmospheric entry? The escape pos would also be going super fast though, but that detail would be easier to overlook.

  20. #20
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    Probably easiest is have more than one ship. One crashes, rest don't.
    Happiness, is just a word to me
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    Sounds settled then. I'd probably be somewhat vague about the speed. 1/4 speed of light will probably make the planet uninhabitable. Terminal velocity won't destroy the city without some secondary explosion like someone suggested.

  22. #22
    practical experience, FTW MaeZe's Avatar
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    Quote Originally Posted by M.C.Statz View Post
    ... 1/4 speed of light will probably make the planet uninhabitable.
    No so sure about that, you also have a relatively small mass. I think vague about the speed is fine though.

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    Quote Originally Posted by MaeZe View Post
    No so sure about that, you also have a relatively small mass. I think vague about the speed is fine though.
    Well it really depends. In one way, you are right. I did the math. The space shuttle Enterprise (real one) is 75,000 kg. If it's traveling at the 1/4 the speed of light, it has the kinetic energy of 3.5 million Hiroshimas.

    By contrast, the meteor that killed the dinosaurs was about 10 billion Hiroshimas.

    So if you had about 3,000 space shuttle Enterprises going 1/4 the speed of light you could come close to wiping out all life on Earth.

    However, if you hurled a fully loaded Madrid Maersk (which clocks in at 200,000 tons) at that speed, you'd get roughly one dinosaur killing event.

    So if we're talking a luxury interstellar spaceliner carrying a handful of dignitaries, you're probably only talking a half decade of a nuclear winter/major volcanic eruption type event - cold weather and crops failing. If you're shipping a massive amount of alien artifacts for the kiddies back home, you could get to a permanent sanitization of the planet without too much trouble.

    Billion =10^9
    Last edited by M.C.Statz; 08-16-2017 at 01:54 AM.

  24. #24
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    The planet can die, dinosaur style, so I'm fine with that either way. Aliens can fix later, and they do mostly. Ish. Just several billion natives dead. It's all good.

    On that note, how much of the planet would die after a year of no sunlight, do you reckon? >.>
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  25. #25
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    Quote Originally Posted by Harlequin View Post
    The planet can die, dinosaur style, so I'm fine with that either way. Aliens can fix later, and they do mostly. Ish. Just several billion natives dead. It's all good.

    On that note, how much of the planet would die after a year of no sunlight, do you reckon? >.>
    No idea, sorry

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