Animadversions on the Martian equatorial magnet

Having poasted I took time today to read Marcus DuPont and Jeremiah Murphy, "Fundamental physical and resource requirements for a Martian magnetic shield". Not quite as fundamental as we'd like, is my verdict, and - I suspect - the source of bemusement among the Science Blogosphere over the past month or so.

These two clowns huffed a phat blunt together a year ago and decided, find enough bismuth to make a bismuth-strontium-calcium-copper alloy, and rust it a little. Start coiling the planet with late-'80s "Bisco" (Bi₂Sr₂Ca_nCu_n+1O_2n+6). Repeat until you lay down 100 Eiffel Towers worth, at 3400 km radius. For reference the equatorial radius is 3396.2 km.

Sabine asks-and-answers Where do you get so much bismuth from? Asteroid Mining. - further, I bet you could pluck up a lot of that from the meteorites already dotting the Martian surface. And we'll be dropping comets on here too, say DuPont and Murphy. I expect especially ammonia-heavy comets, they need the nitrogen a sight more than they need the carbon-dioxide. I'm not disputing this much. I'm not disputing even that the colonists want a magnet, as Casey Handmer disputes. Handmer is if anything doubly-wrong in that SML1-Lissajous would flare-protect the higher satellites; this goes beyond the DuPont-Murphy concern.

Mars' tilt is like Earth's so an equator-spanning magnet will leave some less-protected space on the poles. And Mars won't be able to fire off Orions anywhere they like, anymore; so, like here on Earth, the poles will be where those monsters take off. A minor inconvenience because by then I hope Mars will own better ways around Tsiolkovsky.

One major issue I have here is simple longterm logistics. This Bisco has been chosen because it is a super conductor. Our super-fu is still not room-temp, nor even Mars-temp. Poking around, Bisco 2223 hits 107 K. Seems an excellent plan for keeping Jupiter's ions off the Ganymede colonies; Ceres and Callisto, the two likiest pit-stops, don't really need it. Unsure about Mars. DuPont and Murphy did crunch the numbers and figured that keeping the Bisco at 100 K would be the hard part. Underground in the shade, we suppose.

I mean, once we've started keeping the carbon-dioxide in, the air isn't just warming up - it is also conducting heat better. So those underground chambers will need to stay vacuum.

I might further nitpick to add the InSight experiments on the surface which detect about 2 microTesla [doi 10.1038/s41561-020-0537-x]. Crustal field ain't a planetwide field. So: tell us that.

And what's the longterm plan. The next milestone is when Phobos hits the planet. That'll happen sooner if there's an atmo to drag it. I mean, sure; last winter I'd rather whack that moon with another big rock either to push it to areosychronicity or else just to make a big ring out of it, for Deimos to mine the remains. Either of those options is violent and hazardous to the surface. But either way, the Martian Congressional Republic has a problem on its hands.

After the Phobos problem is solved - or Mars is rebuilt after the Big Crash, doesn't matter: our Sun is heating up. Our own good green Earth is done-for in 500 million years. That's going to mean more "mass loss" therefore radiation for Mars. Although if we're starting small, like to keep in a 0.2 bar atmo, in shallah we can keep adding more cable as needed.

More to the point is getting started. Mars is bumpy. To wrap the coil around its equator requires a lot of ups and downs. The Tharsis Bulge is there, and I think so is the Marinaris. Not only is this going to warp the topology of our magnet, but even if it works our descendents must secure it - until the heat death of the Sun. This includes cooling it, because it has to be below 100 K which Mars doesn't get naturally. The authors convince me that the power needs aren't a problem - if all goes well. - but, tell it to a Texan: what if a sector gets whacked by a meteor? or falls down a rockslide? Yeah I know: a Mars-side magnet remains better entropy-management than balancing a (bigger) magnet at SML1. Still, it sucks.

Also I dislike how DuPont and Murphy calculated the ram pressure of the solar wind in Mars' near space. There's the problem: they... calculated it, rather than measuring the actual rads. This was done from the mass loss rate from the Sun and the inverse square law. Cosmic rays are, I recall, correspondingly worse - by the same law. All this is the sort of thing Martians want measured directly... which may be done from any orbiter around Mars. I propose it can be more-directly extrapolated from Geiger meters, as measure sieverts / day. Calculations based on GCSE physics are well and good for extrasolar planets, but frankly we can do a lot better around a planet as well-known as Mars.

Their density for these superconductors is hilariously wrong at 3 g/cm³. The numbers I find [10 Nov.] run from 5.7 (Rebco) to 6.31 (the Bisco they actually want). So, double their mass estimate.

I don't know why they even included the paragraph on Can the device fit in a rocket?. Uh. No? Not even on an Orion, dood. They will moot shifting material from the Asteroid Belt, later on. Which also will require some hardcore thrust, albeit with lower delta-V. That paragraph, I am sorry, needs deletion; and replacement, with some logistics on the delta-V budgets between Mars and relevant asteroids. Luckily these are heavy-metallic so within 3 AU of the Sun.

UPDATE 11/26-8: Suppose there's a 90 kilotonne ring of satellites over the equator. It's already magnetised. Magnetise it some more! Quite literally, from 110 km up, ain't no mountain high enough ain't no valley low enough. Although, it does freeze the place.