What happens when Earth is hit by a 10km wide banana…

This week I watched Don’t Look Up. I was impressed. Others may have different opinions, but to me it came across as a razor-sharp satire on the current zeitgiest, a social commentary of the highest order and a dark comedy with some solid science behind it. The scary part wasn’t the concept of a planet-killer comet arriving but the fact that much of the human conduct it portrayed was only barely hyperbolic. Ouch.

Of course it’s given me an opportunity to unleash my inner geek. What impressed me about the science was that, apart from a handful of exceptions (including the spaceflight sequences, but clearly for humorous purposes), the science was generally spot on, right down to the way that astronomical bodies are actually discovered. The orbital calculations in the opening scenes show how that part is really done, though of course these days astronomers use computers. One thing they didn’t show is the standard back-check, in which older astronomical photos are checked for signs of the new object, once its orbit is known. That has the double purpose of enabling further refinement of the orbit and allowing a certain amount of ‘damn, we didn’t spot it then’ to go on.

“Earth. How peaceful it looks.” An image I made with my Universe software, showing the path of a 10 km-wide banana about to strike the Pacific. More on that below.

The idea of a large comet hitting Earth isn’t far-fetched. It’s happened before – which is why we’re here and the world isn’t populated by Mr T. Rex Esquire. Not something to lose sleep over: the risk of anything that big hitting in anyone’s lifetime is miniscule. But the chance of smaller objects striking is far higher and a ‘city buster’/’regional impact’ such as the Tunguska impactor of 1908 is quite plausible. Had it struck five hours later, it would have hit St Petersburg. Yah. NASA has recognised the point, though personally I think the name ‘Planetary Defense Coordination Office’ is far too bureaucratic. However, the super-cool name – SPACEGUARD – which Arthur C. Clarke coined in 1973 for his novel Rendezvous with Rama, has been taken (the SPACEGUARD website is here).

Movies have been made before about a killer asteroid or comet. Most of them ‘Hollywoodise’ the concept – all you have to do is nuke the thing and it turns into a zillion fragments that then fall harmlessly to Earth. Right? Wrong. The problem is that the net kinetic energy isn’t significantly changed, and it’s this energy that causes most of the problem. A single 10km wide comet will certainly destroy Earth’s ecosphere. But (say) 30-odd impacts of fragments, all striking at the same time across a slightly wider area, will essentially be as devastating. This was implicitly recognised in Don’t Look Up as far as I could tell.

The other problem with the Hollywood version is that, every time (including in Don’t Look Up), spacecraft are portrayed as soft-landing on the object at the last moment. This is impossible with current technologies. Don’t forget, it’s hurtling towards us at 70 kilometres a second. To rendezvous with it in the times portrayed, you’d need to fly a brachistochrone curve orbit to get there fast enough (which requires ridiculous amounts of fuel), then decelerate to a stop, then accelerate up to 70 kilometres a second so as to match velocities with the object. No current propulsion system can do it, and systems with the specific impulse to achieve it – such as exotic antimatter beam-core engines (roughly, Heinlein’s ‘torch’) or Robert Zubrin’s scary uranium-salt nuclear motor not only don’t exist – they will likely never exist.

A cometary object, in this case a 10 km wide banana, approaches Earth at 70 km/second. Another image I made with Universe.

Current thinking is that if you DO have to use a nuke – very much a last resort – then it needs to be targeted in such a way that the blast will nudge the intruder into an orbit that doesn’t impact Earth. Carefully, so the object doesn’t break up. Other methods include hitting it with lasers, a simple kinetic impact by a heavy enough spacecraft, gravity tractors (yes, they’re a thing) and zapping it with a suitably powerful laser, not so much to boil it away as to create a thrust that changes it orbit. There’s also the idea of hitting one side with black (or white) paint, harnessing the Yarkovsky Effect to create asymmetric radiation which, over time, can also thrust the object sufficiently to alter its orbit.

None of these have been tested – though there’s a mission under way right now to test the kinetic impact idea – and gear to achieve them doesn’t exist. The main problem is the time factor. Most of the realistic ways of nudging a Mount Everest sized object away from a collision course would take years to work. Problems include the fact that, while some meteors might well be fairly solid, others are not – they’re puff-balls held together by very few cohesive forces and minimal gravity. Each demands a different method to deal with it. The key word here is ‘years’.

I’ll go into this in more detail in another post. For now – well, it’s geek time. I mean more geeky than what I’ve just written. I can simulate impacts with some astronomy software I’ve got, so I modelled what would happen if Earth was struck by a 10-kilometre wide banana moving at 70 km/s, which is the highest typical speed of a comet around Earth’s distance from the Sun. Don’t laugh, the mass worked out to 1.5 x 10<exp>14 kilos and it’s the kinetic energy that counts. Also, the software assumed the banana was a sphere (it’s an old joke, but true in this case…) Watch as the Pacific ocean is turned into a giant sundae of lava and banana bits.

A screen GIF I made showing a few seconds after impact… each sparkle is a separate object with trajectory calculated by proper n-body mathematics, not via quick-and-dirty patched conics. For various reasons, Universe switches off the high-end graphics when it’s simulating the real physics (and still cranks my high-end CPU).

Copyright © Matthew Wright 2022