Habitat Bennu won't be all that

In 2019 Maindl considered a hollow solid asteroid around “radius 300 m that is spun up to provide 0.3 g_⊕”. Maybe 4 Mega-Pascals would do, for hard rock; compare 1 bar = 100 kPa. A year ago Peter Miklavcic et al. pointed out that near 1 AU we just got rubble-piles as fly apart at 10 Pascals. Take Itokawa and Bennu: 500m-diameter of trash rock each. We now learn Itokawa is a 4.2By pile, since its parent asteroid shattered. Rubble piles are hard to destroy past that; any further collisions just burrow their way inward and stop – adding to the rubble. I assume Bennu similar: 125 cubic megameters.

Each pile of debris should therefore own some iron-nickel meteors buried down in the silicon and KREEP. That’s good news for the local miners and, later, blacksmiths. But.

The bad news for us with near-Earth asteroids is their location… near Earth. The jumble cannot be hoped to disassemble itself on its way in. And the chunks of iron within them stand to be hefty. This new one in Antarctica is 17 kg – after it fell. What if it had been shielded on its way in?

Miklavcic et al. propose better “Habitat Bennu” with 800 “grains” of broken rock and regolith in a high-tensile bag. That bag needs 200 MPa hoop-stress so: “carbon-fiber”. Picking on Bennu I guess because it's already spinning and also has water (UPDATE 3/22 unlike Diomorph).

By the way I entirely endorse Don’t Look Up to do something with all this debris as long as we’re shifting it to a friendlier location. Power is supposedly delivered via solar: they say panels, George Smith will suggest mercury. The usual cosmic-ray and flare worries apply. So: inner location; we use a big magnet against solar ions and, of course, there’s lots of rock here. Earth-crossers can do STL4/L5. It’s not enough mass to raise tides over here; our tides hopefully won’t raise tides over there over the life of the habitat.

Much as I’d like settling a big centrifuge of ancient dark, 3 ms^-2 isn’t even Mercury/Mars tier. Also the pressure against the bag – they calculate – will be 7 kPa even without the 100 kPa of nitrox we like. Inside the finer dust stands to scrape lungs like lunar dust. Maybe the architects can spray most of the inside dust with goop so it’s mostly clay, before landing any colonists on it. So I don’t think we’ll be walking the surface. I’m seeing a canister buried in the regolith with its own means to seal in the air: a campsite. We do however get a radiation-shield and 3 is better than 0.

Humans want something up to 9.8 m/s² and space to walk around on. The next phase is a larger 2.8 km diameter, 3e+8 kg. Bring Itokawa along to Habitat Bennu, and more. Piece by piece, we must assume; which pieces we’ve agreed to fuse into grains once there. Venereans can bargain down to 8.7 m/s².

I concede with SFF Chronicles that this is a “Tough SF” blogpost or even an incomplete syfy story. The bag of 200 MPa “carbon-fibre” for the 0.3 g partial-habitat is, yes, skirting unobtanium. Although: we might just be able to get it done with present tech. The lower levels should be fused first into something that doesn’t rip the bag. As we spin up: fuse all those rocks even harder. Also I wonder about making the cement something more rubbery, itself. Then when more meteors ping it from below / outside, all our dirt doesn’t spin away. On this assumption, a few more small craters won’t hurt anyone and Bennu’s in-house astronomers should be able to spot anything comparable to Bennu itself.

I am more unsure how well the classic O’Neill or torus-wheel actually works in space; on Earth we assume a fixed hub. Although a 60-degree part of that torus might spin nicely against a counterweight, starting modestly at 200 doodz.

HOW? 3/1: David W. Jensen says replicating robots, twelve years for Atira. Huh. Maybe.