Earth has 20 years before the impact of a 20 mile wide asteroid. What would the timeline to save humanity look like? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Earth has 20 years before the impact of a 20 mile wide asteroid. What would the timeline to save humanity look like?

The social and political results are really tough to predict. If you haven’t already, you might want to read Neal Stephenson’s SevenEves to see how a thoughtful SF writer imagines the world would react to a similar kind of global disaster (though Stephenson assumes an even more harrowing 2-year lead time).

It’s a lot easier to consider the scientific & technological issues, so I’ll focus on them instead.

First off, could there be an undiscovered 20-mile-wide asteroid headed our way in the foreseeable future? The answer is a qualified yes. The killer could not come from the asteroid belt. Astronomers have already plotted the orbit of every asteroid that size (and a lot smaller) in great detail. There is no object that size that could plausibly hit Earth any time in the next few thousand years—probably not in the next few million years.

There is one way that an object like that could be on its way without anyone knowing, however. If it were a giant comet or dislodged Kuiper Belt Object coming toward us on an extremely elliptical path (ie. falling almost straight toward the Sun), it would be very hard to detect. We plausibly might not spot it until it was somewhere between the orbits of Uranus and Neptune. An object at that distance would, in fact, take about 20 years to reach us.

Keep in mind that 20-mile-wide objects are extremely rare, relatively speaking. There is no record of Earth being struck by anything that size in the past two billion years. The likelihood of it happening in the next few years is, well, astronomically small. But let’s play out the scenario.

OK, what would happen if the asteroid struck? We’re talking about an object that is 2–3 times the diameter of the asteroid that hit us at the end of the Cretaceous. Given its steep path toward the Sun, it would be moving at a high velocity as well. It might pack 100 times the energy of the impact that ushered the old dinosaurs off the scene. This would be a full-on extinction-level event. There’s no precedent in the history of complex life on Earth, so we can only extrapolate.

All of Earth’s surface would be set on fire. There would be tremendous earthquakes and tsunamis, followed by massive volcanism around the impact zone. The ozone layer would be destroyed. The oceans would turn acidic. The Sun would be blotted out, probably for decades. All surface infrastructure would be destroyed. Most complex species would surely perish in the aftermath.

Now that we have the scope of the problem mapped out, we can think about the response. Idea one: Can we deflect the thing? The most likely approach, based on a recent study (Scientists design conceptual asteroid deflector and evaluate it against massive potential threat) would be to explode multiple nuclear warheads right next to the asteroid, vaporizing part of its surface and changing its orbit.

If NASA, Roscosmos, the ESA, and the Chinese space agency (plus private industry) started cooperating right away, I can imagine that a coordinated set of launches could take off in about 2 years, and would intercept the asteroid about 10 years after that. Unlike many of the cynics here, I think that kind of international cooperation would happen. The threat is just too huge. All other concerns would become secondary; there will be no nations and governments to worry about if this thing hits.

But will it work? That’s really hard to say. We’ve never attempted anything like this. The closest is missions like Hayabusa-2 and OSIRIS-REx, which will rendezvous with asteroids and touch the surface, or New Horizons, which did a precision high-speed flight to Pluto. But we’d be trying to deflect an object while knowing little about its shape and composition. Probably we would launch the nukes as quickly as possible, then adapt the mission parameters as we learned more about the asteroid.

While our nukes are en route to the asteroid, people on Earth would undoubtedly be making survival plans in case the effort fails. One option is to go deep underground. Going very deep in the ocean is another possibility. Either way, you’d need a source of energy that could last decades, along with a closed-loop life support system to refresh the water and air. You will also want to create storehouses of seeds, livestock, and other critical species that need to survive the impact.

Every government would start building these, and would almost surely impose martial law to make sure people don’t storm the facilities. Billionaires would build their own shelters, and plenty of people would try to break into these, too. Whatever selection criteria are used to determine who gets into a shelter, there will be mass rioting. It wouldn’t be a pretty time.

The other survival option is to go into space, and all the major governments would surely work on that, too. Low Earth orbit, where the International Space Station circles, is too vulnerable to debris that would be thrown up by the impact. That leaves three option: geosynchronous orbit, the Moon, and Mars. The first is the easiest to reach but offers no local resources — a big problem if you’re going to need to be self-sufficient for a few decades. The Moon is less accessible but offers better survival options. Mars is the least accessible, but the most hospitable once you get there. Difficult trade-offs.

Governments and entrepreneurs would probably try their hand at all three space options, but this is a really, really hard problem. We have no idea how to build a fully self-sustaining space colony in any of those locations, and 20 years isn’t that much time to figure it out. Then again, people would be willing to try even high-risk expeditions, since the alternative is potential extinction.

It’s interesting to consider the different possible aftermaths. What if deflection is successful—what happens to all the survival colonies? What happens to the governments of Earth?

What if the asteroid strikes? Not all of the survival colonies will make it though. Some independent groups might make it through. What will happen 50 years later when people emerge? Will the first survivalists to come out end up ruling the planet? What will happen when the space colonists return? Will they even want to come back to a devastated planet? Lots to think about here.

Fortunately, the likelihood of it all happening is very, very, very small.

This question originally appeared on Quora. More questions on Quora:

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* Space Exploration: How soon could humanity spread to other worlds?

* Asteroids: Does NASA also track large asteroids that could hit some other planet?

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