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You are in charge of the world, with technology about the same as we have now, except that a while back the leaders of the world decided that nukes are too powerful, so they destroyed them all and noone knows how to make them anymore.

The space program has also died out, so you do not have any probes.

When some astronomers tell you that there is a large asteroid coming in 5 days and the impact will kill all human life on Earth, what do you do to prevent the impact?

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    $\begingroup$ Five days? Nothing... that is much too short a time frame to implement any kind of space operations. Earth is doomed. Keep it under wraps to prevent the last few days to descend into utter chaos... let the thing hit... so long, and thanks for all the fish. If it is so big that it will kill all life on Earth then you have no chance in hell to move it. And nukes would have been pointless anyway. $\endgroup$ – MichaelK Jun 15 '17 at 10:32
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    $\begingroup$ yes but is there some kind of misel that can throw it of course $\endgroup$ – Foxy Jun 15 '17 at 10:33
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    $\begingroup$ Size is only the one problem.... the other is the time frame. Despite what Hollywood wants you to believe, space operations require a minimum of months of planning. The Space Shuttle programme showed with utter clarity that the dream of "shuttling" into space if a dream yet very far away. $\endgroup$ – MichaelK Jun 15 '17 at 10:38
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    $\begingroup$ But I think your problem is you are going about this the wrong way: you have a story you want to tell... a story where the Earth is under threat, and eventually will be saved (I presume) by the actions that take place during the narrative. You want to know a reasonable time-frame and size of the asteroid to make this story work, along with a realistic action taken to affect the save. Turn the question around as such instead, and you are more likely to get a good answer. $\endgroup$ – MichaelK Jun 15 '17 at 10:39
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    $\begingroup$ The prohibition on nuclear weapons and space probes makes no difference. Under the conditions you have specified, nothing could be done even with nukes and rockets available. $\endgroup$ – Mike Scott Jun 15 '17 at 15:33
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If the asteroid is still 5 days away it might need a very small angular deflection to have it miss Earth.

You obviously need "something" able to reach the asteroid to do anything useful (barring magic, of course).

We are developing fairly powerful lasers, but they might not be enough to break the body.

Your best bet is a swarm of the rockets used to penetrate bunkers, these will drill a fair hole on the surface of the asteroid thus providing a highly directional explosion; effectively a rocker burst. If You can hit the side of the asteroid with enough such charges you might be able to deflect its trajectory enough to miss Earth without need to obliterate it.

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  • $\begingroup$ May I know reason of downvote, please? $\endgroup$ – ZioByte Jun 15 '17 at 10:41
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    $\begingroup$ The answer completely ignores the premise stated that we have no rockets $\endgroup$ – MichaelK Jun 15 '17 at 10:44
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    $\begingroup$ @MichaelK: I did not ignore it. I just read that differently. The question states: "The space program has also died out so you do not have any probes". "Probes" and "rockets" are two different things in my limited English command. I was postulating usage of some (powerful) ballistic rocket to deliver conventional warfare, without the need of manned (or remote controlled) general-purpose "probe". Thanks for clarification. $\endgroup$ – ZioByte Jun 15 '17 at 11:04
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    $\begingroup$ TBH the premise is "no space program" not "no rockets". So we might have the ordonance, just no means of delivery... $\endgroup$ – Guran Jun 15 '17 at 11:05
  • $\begingroup$ So you are saying we hold a bunch of rockets on a five day stand-by but nothing we actually want to use them for? That does not make sense at all. Rockets are perishable wares... and they are flippin' expensive. $\endgroup$ – MichaelK Jun 15 '17 at 11:07
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Based on the answers given to this question, considering you rule out space technology, you (and the planet you rule) are pretty much doomed.

Unless you're a Jedi, you won't be able to move this thing with just the power of your mind. You'll have to send something up there to do something about the asteroid. Either retrofit an existing rocket or build one from the ground up (though again, you may not have time to start from scratch) and make sure it is capable of delivering objects beyond Earth orbit. Also, create a payload. This is going to be the thing that will interact with the asteroid

Nevertheless 5 days notice is not enough even to launch a rocket. Consider that when US went to the Moon (and they knew with a huge notice that somebody was going to be sent out there) the only preparation they did for the event of a malfunction of the spaceship was a funeral speech.

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    $\begingroup$ Said speech is amazing, though. $\endgroup$ – Draco18s Jun 15 '17 at 13:31
  • $\begingroup$ I believe United Launch Alliance when explaining the launch price in dispute with SpaceX stated that they provide US government with "any time" launch capability. $\endgroup$ – user61244 Jun 15 '17 at 20:08
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You find a deep hole, stock it with food and supplies, and hope to survive the impact. There is nothing you can do. If the object was years out, a small change in its vector would cause it to miss the Earth. Five days out, we could launch all of our nukes at it, and MAYBE break it up enough that most of the larger pieces would evaporate in the atmosphere. With five years of warning, humanity could deflect it. Five day won't even be enough to get the ammo and food shipped to your bunker..

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If we assume that the asteroid is the equivalent of the Chicxulub impactor (so not quite big enough to kill all life on earth, but probably enough to wipe out humanity), aimed at the centre of the Earth (for working out the worse-case scenario), and that the Earth's gravity had no effect on the asteroid (for ease of working it out), we wouldn't be able to do anything about it.

This asteroid would be 15km wide, and if we assume that it is roughly spherical (for ease of calculation...see jokes about spherical cows) and has a density of 2000 kg/m3, the mass would be 1.77 trillion tonnes. If we could act instantly, we would have 5 days to deflect it by about 7000 km, so 1400 km/day or 16.2 m/s. The energy required to move mass m (in kg) at velocity _v (in m/s) is at least 1/2mv2. For 1.77 trillion tonnes (=1.77 x 1015 kg) by 7000km in 5 days, it would require 4.645 x 1017 Joules, equivalent to about 110 megatonne explosion, assuming we can focus all of that energy onto the asteroid rather than simply having it throw out equally in all directions.

However most of the energy from such an explosion would probably break up the asteroid into smaller fragments, rather than moving it, so resulting in a rain of fiery asteroid fragments impacting the Earth, rather than a single large one. It could be done with rockets pushing it, in theory, but that's out of the question (literally), so I'm not sure we can put up the necessary amount of energy in the right spot.

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  • $\begingroup$ 15 km diameter would probably be enough to cause societal collapse and mass extinction, but I don't think that it would cause a 100% fatality rate with exactly 0 survivors, as the question states. $\endgroup$ – Ambrose Winters Jun 16 '17 at 19:27
  • $\begingroup$ Well, it wiped out all megafauna 65 million years ago, along with many of the plants, so with the collapse of our ecosystem, we'd probably die out. I would agree that you'd want a larger asteroid to be sure, so feel free to scale up the asteroid as necessary. a doubling of the diameter of the asteroid would increase its mass (and thus the energy required to move it) 8-fold $\endgroup$ – Pak Jun 16 '17 at 19:50
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    $\begingroup$ Spherical cow! $\endgroup$ – Ender Look Jun 16 '17 at 19:53
  • $\begingroup$ Thanks for the edit, @EnderLook; I'd forgotten I'd put that bit in! $\endgroup$ – Pak Jun 16 '17 at 20:20
  • $\begingroup$ The way to do it is with a series of bombs, not just one. So long as no bomb imparts movement even close to the rock's escape velocity it won't break up even if it's a gravel pile. You may have to wait a while between bombs for gravity to do it's work, though. With the 5-day timer, though, this isn't an option--all you can do is spread the booms out and hope the rock can take it. $\endgroup$ – Loren Pechtel Jun 16 '17 at 23:58
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Given modern day Earth the only hope of possibly pulling this off would be if there was a large (considerably bigger than anything we've ever flown) deep space rocket ready for launch. Strip it's cargo, replace it with nukes (and good luck disabling the safeties on enough boom for the job) and head out. The astronauts (if it wasn't manned they're in suits and laying on padding--it's a one-way mission anyway so the lack of a re-entry system is irrelevant and they need nothing but air, water and power) push the bombs in the right directions (they have to be spread apart to avoid fratricide) and hope it's a very, very solid rock as the required deflection is going to put an awful lot of stress on it and if it breaks up there's no hope of stopping it.

Pak already showed we need 110 megatons for 5 days out assuming 100% efficiency, I would be amazed if we could get the intercept even two days out so we need 275mt and then 10x this for the efficiency of the crude use of them we are making. Given the most efficient bombs built we still need over 450 tons of bomb--that's a lot of rocket!

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  • $\begingroup$ My answer also assumed that all of the energy from the explosion was focused on the asteroid. In practice, a nuclear explosion in space will throw its energy out in all directions, so only a small amount of it would hit the asteroid; 50% at most, if the nuke is sitting on the surface; if it was inside, it would shatter the asteroid, which might throw out enough energy to do it. The explosive power required would, as you suggest, probably be much higher to take into account the imperfect transfer of energy. $\endgroup$ – Pak Jun 16 '17 at 19:11
  • $\begingroup$ @Pak I wasn't objecting to your number, just using it as a base to work from. I do agree you could get a better efficiency if it were on the ground but that would require soft-landing it, something that there would be no hope of doing in the time available. The bombs will have to fire with a simple radar altimeter so I used NASA's numbers for a crude Orion drive efficiency. There's also the issue that the closer the bomb the more concentrated the energy and the more likely you are to break it rather than shove it. (Think hammer vs mallet.) $\endgroup$ – Loren Pechtel Jun 16 '17 at 22:27
  • $\begingroup$ It's OK, Loren Pechtel; I'd assumed that you weren't objecting to my numbers. I agree with your answer; I just added a clarification that should have been in my original answer. I edited my answer with some of that. $\endgroup$ – Pak Jun 16 '17 at 22:34
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ZioByte, touched on it, but a large laser array could do the trick. But it doesn't have to destroy the asteroid, merely heat it to the point of vaporizing part of the asteroid. This vapor will act as a small jet deflecting the asteroid from Earth.

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    $\begingroup$ But not 5 days away. $\endgroup$ – ozone Jun 15 '17 at 12:00
  • $\begingroup$ That's true for at the scale of power in that article, but they also only go up to 1 MW. Probably because this article is examining space based and not ground lasers. But depending on trajectory and whether or not we are luckily out fitted with several GW lasers (Why would we be? I don't know, that's up to OP) it can be manageable in a world free of 'reality-check' $\endgroup$ – Firelight Jun 15 '17 at 12:17
  • $\begingroup$ It's true no matter how big your laser. You can't generate the necessary acceleration without blowing the asteroid apart (and thus facing a rain of rubble.) $\endgroup$ – Loren Pechtel Jun 15 '17 at 19:14
  • $\begingroup$ @LorenPechtel I am not sure where you are getting that idea $\endgroup$ – Firelight Jun 15 '17 at 20:07
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    $\begingroup$ Why would you fire a laser in one shot? $\endgroup$ – Firelight Jun 16 '17 at 10:55

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