[X] My Assistant

[nprev]

Surprised we didn't already have a thread. DART launched successfully at 0621 UTC today (23 Nov 21). Mission page here, encounter (as in collision) with small satellite of 65803 Didymos in late Sep/early Oct 2022.

[Explorer1]

Wow, those streamers, it looks quite catastrophic! Much more violent than the SCI on Ryugu, as expected!

[vikingmars]

Wow, those streamers, it looks quite catastrophic! Much more violent than the SCI on Ryugu, as expected!

Wow ! Yes: Dimorphos must be made of very loose material and may have been partially destroyed, if not entirely. We will see...

[mcaplinger]

Dimorphos must be made of very loose material and may have been partially destroyed, if not entirely.

Regardless of what the images look like, that seems very unlikely, at least if you believe "Spacecraft Geometry Effects on Kinetic Impactor Missions", Owen et al, https://iopscience.iop.org/article/10.3847/PSJ/ac8932/pdf

However, they did a ton of work and then at the end

It seems likely, based on observations
during the surface sampling in the OSIRIS-REx mission... that the weak material limit is
the most likely case. In fact, if Dimorphos’ surface is as weak
as that observed during the SCI experiment, it could be significantly
weaker than even the weak limit presented here.

I haven't read the paper in enough detail to know if their modeling is truly appropriate or just detailed but in an unrealistic way. But the spacecraft only had a mass of about 500 kg and Dimorphos is of order 10**7 more massive and DART wasn't going that fast.

On the other hand, I was thinking of this:

Han Solo: That's what I'm trying to tell you, kid; it ain't there... It's been totally blown away.

[fredk]

Regardless of what the images look like, that seems very unlikely

My guess was also that it was extremely unlikely that Dimorphos could be destroyed. Then I did a spherical-cow/back-of-the-envelope calculation:

For mass 535 kg at 6.65 km/s, DART had kinetic energy of around 10^10 J.

For a radius of 80 m and density of around 1.86 g/cm^3 (as assumed in Owen etal), the gravitational binding energy of Dimorphos is -3/5 GM^2/R ~ -10^7 J.

So DART had ~1000 times as much kinetic energy as would be needed to completely disperse Dimorphos (ie to "rest at infinity"), ignoring any mechanical cohesion in the moon.

That was surprizingly high to me - I guess the point is gravity is weak and Dimorphos is small. Still, in reality that huge kinetic energy has to overcome mechanical cohesion and much of it will also go into heat, rather than overcoming gravitational potential energy. And of course much of the ejecta will greatly exceed escape velocity, so you're "overdispersing" a small amount of ejecta at the expense of "underdispersing" (or not dispersing at all) the rest.

So in the end the destruction (or not) of the moon is determined by the mechanical and thermal details, which presumably Owen etal have modeled sufficiently.

[mcaplinger]

I guess the point is gravity is weak and Dimorphos is small.

If you believe "REACTION OF DIMORPHOS' STRUCTURE TO THE DART IMPACT" https://www.hou.usra.edu/meetings/lpsc2021/pdf/2041.pdf then only 2.5e-3 of the kinetic energy isn't dissipated inelastically. That's still more than the gravitational binding energy, but I suspect the real system is a lot more complicated than any of these models. The whole concept of gravitational binding energy is somewhat abstract and ignores how the energy would be transported through the body, other sources of cohesion, etc.

TBH, I'm still not quite sure how this mission came to be or if it really tells us anything practical about how to deflect asteroids, but it was cool