David Jensen's mechanical math, in "Autonomous Restructuring of Asteroids into Rotating Space Stations", wasn't great. On looking into Jensen a bit more, I noticed Figure 2-1 using the same formula to deduce the "material" from the volume: divide by 4. So how's his material math?
Scrolling down, I saw Table 2-4 had Ryugu with type Cg; Atira of course being S. Table 3-6, though, has Ryugu as S; and Bennu also as S. Table 3-6 also has wildly different "packed" material than Figure 2-1; more like dividing by 1.95. What's up with that?
Absent a sample from Bennu direct, we know the densities of various carbonaceous chondrite. They have a range: 2.42 to 5.66 g/cm3; Mathilde at 1.3 ±0.2(!). Basically we know that the overall asteroid is rubble therefore porous; this "1.19" density is covering for at least half of the asteroid being empty space between rocks. And, for C's, the rocks are porous too.
To sum up, to get the usable material from Bennu begs the question. What needs doing is to get a sample. We're getting that in a few weeks. Jensen's "what-the-hell, divide by four" attitude might actually... work, as rule-of-thumb, if Bennu's rocks prove as porous as Bennu overall.
Now, if Bennu were instead S-type; that's different. Stony chondrites rarely get as porous as carbonaceous chondrites. Divide-by-four can work for Ryugu and Bennu; but not for Atira.
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