Solar redirection

Among Casey Handmer's 17 November Starship recommendations was for warming Mars. He'd have Elon run a honkin' mirror or several at SML2. Maybe L1 too. Consider this a sequel to "Librations".

Everything here applies equally to redirecting extra solar energy (which I will call "exergy") to Venus' floating cities and low-orbit satellites, including its orbital ring. Or for that matter toward Mars' satellites especially Deimos.

My first animadversion to that is that L1 and L2 are metastable. Any satellite at either point needs stationkeeping. Luckily, the Sun provides propellant in the form of solar-wind, so a sail might do it for low mass. Unluckily the planet sits in the way of the L2 focus, shadowing it. At L1, we have another problem: the point blocks the sun from the planet. So each orbit stands to be a wide one: Lagrange, not Lissajous. Besides that, we have that L2 is further from the planet so gets less Solar irradiance. So L1 is the priority for a heat-up-now solution; if on a wide-enough halo it might even coëexist with Lissajous ad hoc flare-protection.

For Venus in particular, I want L2-Lissajous for a permanent installation, being shady and easiest-access to the rest of the System. SVL2 is, thus, a net consumer of energy, not an exergetic mirror. I have pondered L2-Lagrange to aid this station but such must be stationkept as well, and jostle with visitors.

We cannot get around that L2-Lissajous on this scale cannot sail so needs propellant. The Ebrahimi Alfvén engine is the highest-efficiency propellant-accelerator thus far mooted, short of thermonuclear fusion.

L2-Lissajous aside, at Venus' orbit is easily solar energy to spare, even after stationkeeping, to beam useful power to the inner satellites and L2. For planets further out L1, L3/4/5, and finally L2 present a progressively tougher call: larger mirror means more propellant and more endo-ergy. For Mars "we'll make it up in volume" comes to mind . . .

For the longer term, a planet might swing mirrors at L4 and L5 where they don't need stationkeeping and they're closer the Sun than L2. Should some go astray, we don't care too much and a high-orbit rescue can retrieve them with reasonable delta-V. These are wider halo orbits so will be using some of that solar radiation to turn the mirror. To even out exergy, where the planet's orbit is elliptic (read: Mars) best to orbit so their periods match the planetary highs and lows.

My problem with all these distant points is attenuation. Assuming tightbeams, the Inverse Square Law is the floor, which can assume vacuum. But. L4 and L5 are always 60° from the orbiter, fore and aft respectively. That means each forms an equilateral triangle; so the light-distance between that Point and the orbiter is always the orbiter's astronomical-unit. Space is almost a vacuum here in the solar-bubble, but still an imperfect vacuum. Best-case, Mars gets an extra sun or two, each shining at maybe a tenth irradiance over the whole globe, whose "noon" revolves around 1/6 of a day before and/or after true noon. Venus (and Earth absent orbital-ring), by contrast, won't want the extra warming; our L4/L5 tightbeams will be limited to radio. By the way, radio aside: I class a Flaz Gaz Heat Ray at Martian distance as severe unobtanium.

A high polar orbit is less silly. Again, here each satellite shall be uneven in what gets directed where. This is mitigated (in lieu of stationkeeping) by having two (or more) polar mirrors, so that at the time of year the Sun is shining edge-on to one, the other one is Sun-facing.