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An asteroid is hurtling towards Earth with the same trajectory as the one that killed the dinosaurs. Current human civilisation gets one year warning and will do everything it can to secure our survival. What is the smallest size of this asteroid such that, despite all our efforts, we would still be wiped out?

Assumptions

  • Size could be mass, or something else like diameter. If necessary assume asteroid is spherical and uniform, (unless there's some better way you know of for modelling asteroids)
  • In every particular except size (mass or diameter)--e.g., velocity, density, angle of impact--the asteroid is identical to the Chicxulub impactor
  • Humans have 1 year of warning. Assume everyone will cooperate and treat the asteroid as singular matter of utmost priority, and that all reasonable resources can and will be diverted to methods for deflection or breaking it up etc
  • Asteroid is a solid cosmic object on a collision course with Earth. If it needs to be much larger than Earth to fulfil these criteria, and/or gravitational effects need to be considered, so be it
  • For humanity to 'survive' it's sufficient that a substantial proportion, say 10%, of the human race survives. However a) they must survive permanently, not temporarily escape only to be faced with an uninhabitable world where they'll die months later, b) no escape into space or anything like that.

I am mainly interested in what the most powerful technologies humanity could have ready within one year would be, how much of them and what explosive power, if humanity devoted all resources practically possible to the problem as a matter of species survival--and what size of asteroid would be too big for them to make a difference.

The more technically detailed the answer, the better!

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    $\begingroup$ Welcome to worldbuilding. Mind that in the title you ask about surviving, in the body you ask about being wiped out. And asking about what is the largest size of an asteroid that could wipe us out makes no sense. $\endgroup$
    – L.Dutch
    Aug 25, 2020 at 5:20
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    $\begingroup$ I think that the question title and intro are a bit misleading. You ask about the largest possible asteroid humanity could survive; that makes me think of whether we would survive the impact. I'm pretty sure that any impactor short of Theia would be survivable by a small group of people in a massive bunker, yet diverting an asteroid of that size with a one year warning is beyond our wildest dreams. Why don't you ask for the most effective methods to divert or destroy asteroids that are large enough to wipe out humanity, since you're interested in that technology rather than the survivability? $\endgroup$
    – KeizerHarm
    Aug 25, 2020 at 8:27
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    $\begingroup$ Your question is still two questions at once: asteroid survivability and methods of asteroid deflection/destruction, both of them are vague. The first one because speed and angle matters as much as size: there's many factors. And the question asking for all the possible technologies available to prevent the destruction is almost too broad to stand on its own, let alone coupled with the survivability aspect. You should really hone in and ask either for the biggest asteroid we could survive without efforts, or the most effective ways to neutralise a Chicxulub-size impactor with one year warning. $\endgroup$
    – KeizerHarm
    Aug 25, 2020 at 15:39
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    $\begingroup$ To put it another way: there are possible asteroids that would wipe us out, but be easy to deflect. There are also asteroids impossible to deflect that are not going to kill nearly as many people. Your question has two unknowns: asteroid specifics, and deflection method specifics. You can't be vague about both those things, I would advise to put one of them in stone and let us solve the other one $\endgroup$
    – KeizerHarm
    Aug 25, 2020 at 15:44
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    $\begingroup$ Do not underestimate the site, narrow questions with ostensibly obvious conclusions often elicit the most creative responses :) Multiple smaller questions is the way to go when you're doubting about whether the scope is too broad $\endgroup$
    – KeizerHarm
    Aug 25, 2020 at 20:43

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Checkout Purdue University's online impact effect calculator, at:

https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/

Diameter is not the only factor at play. You need to consider angle, density, and depth. A massive asteriod of ice gently brushing past the ocean is of lower consequence than an iron asteroid of the same size hitting granite at a 90 degree angle.

Assuming worst case (Iron, 72km/s, into rock)

Your question is a little unclear, "What can we survive?" vs "What do we have no chance of surviving?". 1km is clearly survivable if you're prepared and on the other side of the planet. 50km is clearly not survivable. In between is a sliding scale of other factors, including luck, impact site, weather patterns, impact angle, and human ingenuity.

Can we stop it?

I haven't mentioned any "Go into space and stop it" style missions, 1 years notice is officially not enough time get our act into gear. NASA needs a minimum of 5yrs. Almost all of the deflection strategies we know of (eg https://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Collision_avoidance_strategies) require getting a spacecraft to the asteroid years before impact, and that can only happen with more warning, and good launch windows.

Theoretically we could get our act into gear anyway, but watching us bumble with COVID19 I think this is a bit of a stretch, but lets assume we do for sake of example.

For example, using "Apophis" asteroid as an example, which is an asteroid very close to impacting Earth, assuming it is impacting in exactly 1 year from today, we have a few launch windows for a kinetic impact: enter image description here There are 7 other trajectories, earliest launch date is Sep 4th 2020, Last launch date is in Jan 26 2021. Impact for those launches is between Feb 27 2021 and Apr 16 2021, which would give between 6 and 4 months for the trajectory to diverge after the impact.

The larger the asteroid, the harder, or earlier, or more often, you need to use to your kinetic impactor. Apophis is a relative simple one, it's only 325m in diameter. We'd launch a few missions (as a backup), but a single kinetic impactor with 6 months for the change to diverge could save the day.

Landing something on the asteriod, or flying near it (Gravity tractor, Ion beam, Mass driver, attaching a rocket engine, attaching a solar sail, releasing steam in front of it, tether an ballast, etc.), which are our best hope of deflecting a planet killer, are a lot trickier to do with only a years notice. Using the same asteroid orbit, but assume the mass is too large for a kinetic impactor, we only have 1 launch window (Feb 11th 2021), with an arrival date of May 2nd. enter image description here That only gives 3 months for it to deflect the asteroid before impact. Most of our known methods need years to deflect, so the larger asteroids are essentially unstoppable with only 1 years warning.

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  • $\begingroup$ You are not answering " What is the largest size of this asteroid such that, despite all our efforts, we would still be wiped out?" $\endgroup$
    – L.Dutch
    Aug 25, 2020 at 11:25
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    $\begingroup$ @Ash Good answer; it would be good as L.Dutch says to get an upper bound. We are interested only in "What do we have no chance of surviving?", with the caveat that a small number of people (<10% of human population) hunkering down in bunkers or escaping into space because they have the means doesn't count as survival $\endgroup$ Aug 25, 2020 at 15:25

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