Thread by @ImprobablAuthor

Spitballing about DART & Dimorphos. Yes, actual scientists have done TONS of rocket science, impact calcs, modeling, scenarios, etc, & now are analyzing data for making this efficient & viable at redirecting killer asteroids. But this is just for funsies! A 🧵

This was basically a fast refrigerator hitting a stadium sized gravel pile. In my head, that's not what an asteroid was. They're MOUNTAINS. IN. SPACE. But as a lot of recent missions have shown, and asteroid researchers suspected for years, a lot are just loose piles of rubble.

They're loosely held together by static forces and gravity, but not that much gravity. If they're more potato than sphere, there's not enough compression to form them into solid bodies. So, less mountain, more ball pit. At least for these types of asteroids. Really surprised me.

So now we slam a satellite into one. Goal is to change its path a little very early, so something that would have hit the earth (even if they are loose piles of rubble and not unitary bodies, they still have a lot of mass moving very fast, so BAD) is now veered away.

In terms of changing Dimorphos's velocity, DART can't do a whole lot. Assuming a perfectly inelastic collision where DART buries itself into Di, and all KE goes to motion, all momentum transfers to the asteroid, delta v is mass(DART) x velocity(DART) / combined mass

DART m=560kg v=6.58km/sec, p (momentum) = 3684800kgm/sec, KE = 12 GJ
Dimorphos m=5x10^9kg v=0 (in its frame of reference, yes, I know, what about Didymos), p=0, KE=0
After hit, max xfr of momentum, best delta v would be 0.000737m/sec, or 2.7 meters per hour change.

For a perfectly elastic collision, where DART bounces off Di like a billiard ball, both p and KE are conserved. After colliding, DART heads in the opposite direction at pretty much the same 6.58km/sec, but Di has picked up twice as much speed, now at 0.001474m/sec or 5.3m/hour.

So not much speed change in either case, but over time, even that small a change in velocity would cause it to have an altered orbit around Didymos, either a change in inclination, eccentricity, or apogee/perigee. If it worked, further, bigger missions might have greater effect.

Orbital mechanics are weird. You have to consider alterations in the reference frame of the central body (and any other perturbing influences). Push you in direction of your orbit, you add rotational energy. Instead of going faster, you orbit further out and slower!

For DART & Dimorphos, not sure they've put out yet what the goal for change around Didymos was. Of course, a slight change in orbit was hardly what we saw! BOOM, big splash, lots of unexpected filaments to the ejecta, perhaps more matter than expected, even a "comet" tail now!

Another interesting tangent. DART was tiny in comparison to Dimorphos, but moving very, very fast. Not much change in velocity possible because of the difference in masses, but xfr of energy? That's interesting, especially for a loose pile of rubble.

As a loose pile held together by static & weak gravity rather than solid chemical/structural bonds, it doesn't take THAT much energy to blast it apart. Its escape velocity is only 6.4cm/sec or 0.23km/hr. You could easily disassemble it by picking up pieces & throwing them off.

There's even a term for that: Gravitational Binding Energy, how much KE to overcome a body's potential energy and send every piece out to infinity, never to recombine. For Earth, GBE is HUGE, not to mention all the energy it would take to shatter us into easily throwable pieces.

For Dimorphos, GBE is much less, only 6.1MJ, and it's already broken up into throwable chunks. DART had 12GJ of KE, the explosive equivalent of almost 3 tons of TNT, about 2000 times the energy needed to blow up weakly bound Dimorphos!

Now, it's been carefully explained by the orderlies in my ward that's not how impact physics works, but it is a fun thought. This collision is neither perfectly elastic or inelastic. It's messy, but ALSO NOT how traditional impact sims worked either. A lot of folks surprised.

Impact sims/demos always showed a bullet splashing a sandtable to make a crater with ejecta. That I think assumes you're hitting a unitary body with some structural strength like a moon. This is a free-floating gravel/boulder pile.

While I'm assured you can't just blow it up by having KE >> GBE, that most KE is lost inelastically through formation of ejecta and jostling/cronch of the packing peanut-like components of the rubble pile, I do think A LOT of folks were surprised by the splash we saw.

I'm not sure we didn't break Dimorphos up. Blowing it to hell is too much to hope for (and not the point of the experiment) but I think it definitely did more than anticipated. Be interesting to see how much it got pushed and how much it got shattered.

And if we did punch through the rubble pile, how do we make this more efficient? Grant that asteroids aren't how we picture them, not solid. How do I get the max KE and momentum transfer for a particular size and delta v satellite?

Break up the impactor from one body to multiple submunitions, spread the impact out from one site to several simultaneously? Use more mass, less speed? Give up some KE to get a survivable penetrator with a warhead and put that low GBE to good use?

Dunno. Fun thoughts. My congrats to the #NASA, #JHUAPL, #SpaceX, #JPL, #Aerojet, #LLNL, university, and other teams that brought this mission to success. Can't wait to hear results and see the follow-up. Exciting times! #pouroneoutforthedinosaurs

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