Warning, this post completely spoils the ending of Avengers: Age of Ultron, because science.
Let’s get right into it. Could you turn a city into a humanity-killing asteroid like Ultron seeks to at the end of Avengers: Age of Ultron? The short answer is probably not, the long answer is below.
Seeking a globally enforceable peace, Tony Stark creates the artificial intelligence known as Ultron out of alien code held within the infinity gem in Loki’s scepter. The plan of course backfires, and the intelligence decides that the best way to save humanity is to end it. Ultron, waxing philosophical about the demise of the dinosaurs, devises a plan – re-create the dinosaur’s extinction event with an asteroid of his own.
But how good a plan was it?
“We’re Mad Scientists, Man”
After stealing a ton of vibranium – the fictional material that comprises Cap’s shield – to reinforce the earth underneath the fictional European town of Sokovia, Ultron reveals that his goal is to lift much of the city up into the atmosphere and bring a calamity back down. If we want to estimate how much destruction Ultron has at his disposal, we first have to estimate how big this city/asteroid is.
For simplicity’s sake, let’s guess that Sokovia is at sea level and that Ultron captures one kilometer (0.6 miles) of the city around the church in the center in the lifting process. When the city is lifted, it looks nearly like a half-sphere of Earth. That half-sphere would be two cubic kilometers of dirt and rock.
And if all that soil and rock beneath the city has a density between that of soil and crust, then the total mass of Ultron’s makeshift asteroid would be around four trillion kilograms. That’s about the same mass as all the food the world produces each year.
Mass is only part of the equation. Ultron also needs speed. The faster the city is going on impact, the closer to extinction humanity gets.
The Strings of Gravity
To get the speed Ultron is looking for, the A.I. could simply let the city drop from a great height. When an object is raised above a massive body like the Earth, it gains potential energy, more specifically gravitational potential energy. The total of this potential energy is the theoretical maximum energy that an object in free fall will hit the Earth with.
In the film, Ultron is able to half-execute his plan by dropping the city (with the help of some thrusters we’ll ignore) at altitude before The Avengers vaporize it. According to a line Captain America delivers at about the same time the key on the lifting device is turned, “the air was getting a little thin.” If we guesstimate they were getting to a height where altitude sickness could set in, Sokovia was around 2,400 meters up when it started to fall.
A free-fall from this height (ignoring air resistance) would impart 100,000 trillion Joules of energy to the surface where the rest of Sokovia sat. Because science communication dictates that I compare this to a nuclear blast, this amount of energy is about half that of the largest nuclear bomb ever tested, Tsar Bomba.
In nerdier terms, that’s about the yield of a small photon torpedo.
“That’s the Endgame Up There”
While a 50-megaton impact is a gargantuan amount of energy, it’s not an extinction-level event. For that Ultron would need either a bigger rock or more height to fall from. So, if The Avengers didn’t stop him, how high would he have to bring the city to ensure an impact like that which killed the dinosaurs?
If the threshold for human-ending impacts is around 100,000 megatons, we can rearrange the gravitational potential energy equation to solve for height. Ultron could drop the city from 6,300 miles (10,000 kilometers) above the Earth and by the time it hit, it would wipe us out.
To create a dinosaur-level extinction event, however, Sokovia would have to come down from 6.3 million miles (34 light-seconds) above the surface.
The problem is that neither situation is really possible. Six million miles away the Earth’s gravitational pull is much less, so by the time it got to Earth the impact energy would still be less than Ultron needs (and if you’re going that far into space, just start pushing the Moon on us Majora’s mask-style).
And to even get six thousand miles up, Ultron would have to accelerate the city to a velocity where it could escape the Earth’s gravity – 25,000 miles per hour (11 kilometers per second). Ask any rocket scientist about the kind of energy it would take to lift four trillion kilograms of city off the Earth and they’d laugh even vibranium out of the room.
The other option would be to increase the velocity of Sokovia with thrusters. But still, the city would have to be going 14 kilometers per second at impact. In short, unless Ultron started outside the orbit of the Moon and gradually rocketed Sokovia towards the Earth (the gravity of which he couldn’t feasibly escape), humanity wouldn’t be going anywhere.
I guess the real hero of The Avengers is Jarvis, who kept Ultron out of our nuclear power plants and water sanitation stations — the best targets for an internet-enabled A.I. — and forced him into playing dinosaur destroyer. You’d think Ultron would have done the math.
Kyle Hill is the Science Editor at Nerdist Industries. Follow on Twitter @Sci_Phile.