STL4 is cursed

So is the Sol / Terra Libration 5, for that matter.

This blog has noted a theory that Theia came into an Earth-crossing trajectory from outside. I find an earlier theory: that it was our very own Dopple World. Theia had come in from a libration point - a stable one. In theory (well, per Sean Raymond theory) libration-haloes may own a mass up to the mass of what they're librating against, and stay stable. If greater than Earth then I suppose Earth would be the librant, not the librator, if those be words (need 24.96 times difference for stability). Certainly Jupiter got plenty of "Greek" and "Trojan" asteroids fore and aft. Down here at 1 AU, eventually something moved Theia and Earth out of mutual stability.

The overall theory came from Edward Belbruno and Richard Gott, who came up with Theia in the first place; the Lagrange side of their theory - their Just So Story - was their best idea at the time for "okay buddies, how did Theia get there in the first place". They didn't make a big deal of it, since it was hard to explain and it existed alongside obvious other options like "it got whacked from outside". They figured Venus and Jupiter, also forming at the time, as the perturbers. UPDATE 6/19/22 - Not calculating inter-Dopple tides.

Amarante, Winter, and Tsuchida in 2013 couldn't find how Theia could even form 1 AU from our Sun, such as not simply to enter Earth at the time. That killed the sub-hypothesis such that "outside" was the last one standing. The libration-formation notion accordingly has been forgotten about; but it's important for the history of the thesis, which central thesis is as solid as early-system dynamics get.

One major issue with the libration theory, I think, is that it was falsifiable. We should be seeing more planetesimals in STL4 and L5 [ed. that don't raise tides] than we do - which, as far as ProjectRho will tell me, consist of "Kordylewski clouds", which I cannot even enunciate let alone describe. The closest we get are these weird pseudomoons like Cruithne and this fragment "Kamo'oalewa". Which fragment is Lunar anyway, a "tectite" as we call them when they land upon us. As I suppose we should expect, because the Sun's pull down here makes difficult for asteroids to get captured directly into such haloes.

So: why aren't STL4 and L5 even homes for capture by tectites? Well: Belbruno and Gott have that answer. Venus has settled in 13:8 resonance with us. It follows that, even now, there doesn't seem room in Earth's orbit for a stable satellite in its haloes. tl;dr - Earth has no haloes and, like Ganymede, isn't Lagrangian.

Which is not to say we can't have a short-term floater around Lagrange's points, but long-term they will need station-keeping just like John Norman's antichthon.

As for Venus' Lagrange-points, I dunno if they be libration-foci. On the one hand, Earth might have even more of an effect; on the other hand - the sun might enforce more stability. Venus might not have rubble in its haloes simply because there's no moon there for rubble to be placed into a convenient trajectory. Asteroids as fly by Venus tend to fly faster even than they fly by Earth.