DART & HERA, NASA/ESA Asteroid Redirection Missions Options

[marsbug]

I notice that the final partial frame appears somewhat out of focus. This makes sense because the frame width is apparently 16 meters and the telescope aperture is 0.21 meters, so the frame is only about 80 apertures wide. For a camera focused at infinity, the size of the focus spot should be equivalent to the aperture.

John

Ah, I did wonder about that, thank you.

[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.

[marsbug]

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

Looking at this picture: https://www.asi.it/wp-content/uploads/2022/09/6.png
The asteroid regolith is fairly dark material, but Didymos is quite over exposed. At the same time the streamers and debris, while widespread, are still barely visible, with no really bright spots except the one where the mini-moon itself is located. So the streamers are not dense, and don't have any larger chunks in (unless Dimorphos has a pure carbon interior), but the point where the mini moon sits is still bright - which suggests the moon itself is still dense and reflective compared to the streamers, and likely most of it is still in one place.

[Explorer1]

I think it's fair to say, however, there has been a not-insignificant loss of mass; a portion of the debris is on an escape trajectory, another portion will remain in orbit around Didymos, eventually impacting either body or escaping later on, and a portion will have settled back down or not been moved at all. I know there will be more LICIACube images to come (such as seeing the other side system!), but otherwise we will have to wait for HERA to figure out the ground truth of what 'We' just did, and the ratio of these portions, more exactly.

[Marcin600]

By the way, I added the scale indicators to this image: https://www.nasa.gov/sites/default/files/th...049_43695_0.png
(based on the NASA description - width of the image is 31 m - to better visualize the size of what we see on the last full frame of Dimorphos):

these stones are really big ...
(difficult terrain for any eventual lander, rover or human explorer)

Attached thumbnail(s)

 

[vikingmars]

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



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.

Thank you very much for the paper : I went through it and, now, I'm convinced that Dimorphos has survived the impact and remains quite in good shape. The crater would be very visible and, maybe we will see some secondary impacts not only on Dimorphos, but also on Didymos as well. When the ESA spacecraft will reach the system in December 2026, there will be a lot of impressive images to be seen. Maybe some rocks were also thrown in orbit around Didymos by the impact, building some kind of 'secondary moons', making the system even more interesting to visit

[Explorer1]

When the ESA spacecraft will reach the system in December 2026, there will be a lot of impressive images to be seen. Maybe some rocks were also thrown in orbit around Didymos by the impact, building some kind of 'secondary moons', making the system even more interesting to visit

Hopefully they won't be too big! OSIRIS-Rex sized pebbles are just fine, but big chunks are not a good idea! Though I am sure HERA will be well-prepared.
However, I think the gap in time is large enough for the area to clear naturally; Stardust certainly had no issues revisiting Tempel 1 after 6 years.

[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

[fredk]

Indeed, as I mentioned this is a job for gory-details calculations. I did find it interesting and surprizing, though, that the one energy is ~1000 times the other.

My vague understanding of the justification of this was that the models vary by factors of order unity in predictions of the delta v imparted to the moon, due to the uncertainties in the mechanical properties etc, and they wanted to pin down those factors somewhat. I'd still worry that those factors might be significantly different for different asteroids, impact geometry, etc, so indeed it isn't clear what you've learnt...

Yeah, it was cool.

[StargazeInWonder]

Given the above discussion, it may have been a missed opportunity not to have observed the other side of Dimorphos before the impact to check for effects at the antipode (or the location of any putative exit wound, if you will). If the asteroid is that loosely bound, it seems like the spacecraft drilling through the asteroid (or, still more likely, the spacecraft being embedded/destroyed and causing ejecta to leave the antipode) would be a more likely outcome than breaking the asteroid apart. But everyday intuition fails in a case like this, and the theory may not be much better.

[climber]

…..Maybe some rocks were also thrown in orbit around Didymos by the impact, building some kind of 'secondary moons', making the system even more interesting to visit

Just imagine ending up with a ring

[climber]

Very interesting hypothesis here : https://twitter.com/dr_thomasz/status/15749...uEVuWHohPUlr6YQ

[vikingmars]

Very interesting hypothesis here : https://twitter.com/dr_thomasz/status/15749...uEVuWHohPUlr6YQ

Thanks so much Climber: very interesting indeed