Once again Dante Lauretta statement about Bennu's regolith, this time I post an exact transcription from
the recording of the teleconference on YouTube (47:17 - 51:22):
Ivan Carone (AFP): „What have you learned from Bennu’s soil thanks to this operation and, to make it simple, if someone was to walk on Bennu what do you think it would be like?”
Dante Lauretta: „(...) i’ll just go back a little bit in time here and then and I did this after the TAG event I went back to a science team meeting from 2014 where we were discussing the possible interactions of TAGSAM with the Bennu soil or the regolith during the collection event. We had multiple teams trying to model this process. It seems like it should be straightforward but it’s actually pretty chellenging because you’re in a microgravity environment and all of the literature and all the previous studies that we were looking at the gravity field is a major component in describing the soil behavior in response to various forces. So, the question we were grappling with is how does the soil beheve when you remove gravity from the equation and all the other forces start to play a role. We’re particulary interested in understanding the cohesive forces between grains and any frictional forces that could arise due to the compression. The predctions ranged from we would penetrate three meters to we would penetrate three millimeters! So, it was really not helpful for me, except that we had a kind of a range of predictions based on inferred regolith properties.
Since we penetrated, we estimate, about that 48 cm, that’s much much closer to a very low cohesion regolith model. Basically there’s almost no forces between the grains that keep them bound together. They are not sticking to each other in any way. So when you slide two grains past each other there’s no force that really seems to be measasurable that’s prohibiting that motion. So, when the TAGSAM head made contact with the regolith, it just flowed away like a fluid.
And I think that’s what would happen to an astronaut if she were to attempt to walk on the surface of the asteroid. She would sink to her knees or deeper, depending on how loose the soil was until you hit a larger boulder or some kind of bedrock.
So, that’s already I can guarantee you the team is off now that we have this ground truth calibration data set to re-evaluate models of asteroid geology, and especially soil response and soil creep, and mass wasting, and other movements on the surface of the body. So, it’s fascinating that there was so little resistance to the spacecraft from the asteroid surface. Basically it’s kind of like a ball pit at a kid’s playground right you kind of jump into it and you just sink in. And that’s what the spacecraft did. And so luckily we had those back away thrusters to reverse the direction of motion, or we might have just flown all the way through the asteroid
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Ivan Carone (AFP): "But do you think it could go very deep, like this kind of low cohesion?"
Dante Lauretta: "Yeah, I believe the model suggested, like I said, to three meters of penetration in a completly cohesionless environment, and that seems to be the way the soil responded. We were moving at almost full velocity after we made contact, and the deceleration that we experienced before the back away thrust was primarily from the TAGSAM, recall that the TAGSAM collects material by ejecting nitrogen gas, it’s basically like a cold gas thruster. And if we hadn’t done that and we hadn’t fired the back way thrusters, there was no resistance that’s measurable coming from the actual asteroid regolith."