Yes, that’s about right. 15³ = 3,375, and if the asteroid is potato or cow shaped, somewhat less. My bad.
On breaking up an asteroid, there was this discussion back in 2010:
In which I wrote the following:
…from Grey’s BOE calcs, one can see that when dealing with a small asteroid (on the order of 100 meters diameter), breaking it up prior to impact may be beneficial. The total atmospheric temperature rise due to burnup is negligible, and we would save ourselves from an impact that could kick up climate-changing amounts of debris into the atmosphere. When dealing with a much larger one (on the order of 10 km diameter), we’re just about screwed either way: this embodies energies on the order of 15 million megatons (the largest nuclear device ever detonated was around 50 megatons). A surface impact would cause ridiculous seismic events and kick up life-extinguishing amounts of debris; if instead we could somehow shatter a meteor that large prior to impact, the atmospheric temperature rise would be on par with the worst clime-change forecasts, except it would happen literally overnight.
Move from a 10km-diameter bolide to a 20km one, and the energy (and therefore the temperature rise from atmospheric burnup) increases by a factor of 8, to almost 100 Kelvins. Anything not living in the ocean would be dead in fairly short order, and I imagine the eventual increase in temps of the oceans’ surface layers would upset the marine ecology so much that life throughout the oceans might eventually die off completely.
Speaking of Seveneves, how would a lifeboat in, say, geostationary orbit do in a Chicxulub scenario? Is that high enough to avoid ejecta?
And what’s with this 100-years-underground business, anyway? What’s keeping you from emerging after two, or five, or ten years and building greenhouses to supplement your canned food stores?
Good point. OK, I concede that smithereening an asteroid would be a net improvement over leaving it intact. Though, of course, if one has the capability of smithereening it, one probably also has the capability to deflect it completely, which is far better yet.
Yeah 100% agreement I suspect that deflection… or even just slowing it down enough … or speeding it up … that it’s a miss … is best. But I can at least imagine that getting something to finesse it way out there might be harder than getting something to explode into it.
The advantage of exploding it way out there, say into three or four pieces, is that each of those parts might have a new slightly different trajectory. One sped up a bit, one slowed down, one angled slightly differently. Maybe one still on exact course but it is smaller than the whole was and maybe deflecting it as it gets closer more achievable?
If all is to hit I’m guessing there would be a sweet spot of least damage? Small enough pieces that there is not ejecta and no smaller, to minimize the cluster bomb effect and to minimize the surface area heating up atmosphere. Ideally into ocean deep?
I don’t think we know enough about the composition of asteroids or comets to be able to reliably predict how many pieces it would shatter into.
If it broke up into pieces, would gravity end up pulling it all back together? It seems like 15km of rock would have a relatively significant amount of gravity and all the pieces might find their way back together.
I know enough to know I don’t know if we know …
But I suspect that we’d know enough to predict enough to have an expectation of two to five parts or a thousand or somewhere in between.
Conceptually yes. But practically no.
Assuming you gave it a pretty good whack, the pieces will be be initially accelerated to a decent speed apart. Meanwhile, the collective gravity of a 15km rock is real close to zero. That mutual gravity of all the chunks will be pulling on each and all of them. But it has to slow their outwards motion to zero, then accelerate them back together. And overcome the secondary collisions since they won’t all arrive back at the center of mass at the same time.
Given enough time, like millennia, the rock might re-coalesce. Assuming we attacked it less than a year before it hits/misses Earth, there won’t have been enough time to re-coalesce much of anything.
Said another way, the “escape velocity” of a 15km rock is real small. Some quickie online calcs say about 5 meters/sec for a solid rocky body; less of course if it’s just at a rubble / ice pile. Any fragment going faster than that won’t ever come back. For scale, 5 meters/sec is a gentle underhand beanbag toss.
And in fact, will re-coalesce, at least any parts that aren’t given too much velocity. The rocks out there have been playing a big game of bumper-asteroid for some 4.5 billion years. They’ve pretty much all collided multiple times during that time. When they do collide, some bits gain enough velodity to escape and become a new asteroid, some get close to escaping and become satellites, and some eventually re-coalesce back to the rock. But as you say, it’ll take millennia.
When they do re-coalesce, they end up making a rather fragile connection. There’s not enough gravity to connect them firmly together, so most asteroids are basically rubble piles. The exceptions are nickel-iron asteroids and those that rotate too fast to hold onto loose rocks on their surface. Some of the latter have actually been observed shedding dust.
My personal feeling is that a repeat of Chicxulub leaves humanity extinct.
And just wanted to add, people advocating for subterranean refugia better make sure they site them properly. And by that I mean that the ideal location is probably not on the other side of the Earth from the impact. The mantle plume will get you long before the mobs of desperate excludees.
Murphy posits that any place people choose to slowly starve to death underground will actually turn out to be close enough to the impact site wiping out not only those people but all of the stockpiled resources that could have been used by survivors somewhere else. It’s like playing Battleship, except they only have one bomb to drop on you, but one bomb wipes out 90% of the board.
Other threads touching on it include:
It’s not like we’d be trying to all survive in one single bunker. There are suitable “bunkers” all over the place. And I put “bunker” in scare quotes because plenty of perfectly normal houses would be adequate for the purpose.
Sure, if you’re right at Ground Zero, or antipodal from that point, nothing’s going to save you. But most people won’t be at those points, no matter where those points are.
It’s a little more than just the antipodal point…mantle plume flood basalt eruptions won’t be any fun for anyone on the planet, and definitely not surviveable for anyone within a thousand km of the antipode or more. We’re talking about millions of km3 of lava erupting.
Are we?
Linked to before. With the other study a model that demonstrated that it hit at a worst possible angle.
It neither supports your claim nor contradicts the cites I’ve given.
Specifically it states that in the context of having the then extant extensive volcanism it is reasonable to hypothesize the impact could have been a trigger of more lava flows.
Do we currently have the same Deccan level existing volcanism?
Didn’t read both cites, I see… the first one shows that an impact is enough on its own. Yes, it will energize some existing mantle plume. Even without having any current Deccan-level eruptions, we do have plenty of current hotspots that could be activated by impact seismicity. They don’t have to be already Deccan-level to be made much, much worse than they are…
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25% of species may have survived, but probably 90+% of any given species individual members were killed.
In the end, I don’t think that a dinasour killer asteroid would make humanity go extinct. We’d have some bunkers, we’d have some areas that were not rendered uninhabitable.
But it would kill billions, it would probably kill 99ish% of us off.