# Solar System Engineering

In this column, Cecil pontificates on the feasibility of moving Mars or Venus into Earth’s orbit, and whether or not those planets would become habitable.

Regardless of whether those planets would become habitable at their new distance from the Sun, it doesn’t matter. Those orbits will not be stable, because what’s to be done with the Earth in the meantime? Neither Mars nor Venus could be put into stable orbits, even at an Earth-Sun Lagrange point; they’re way too massive. In turn the moved planet will destabilize Earth’s orbit, for a lose-lose situation.

*** Ponder

While I agree with Cecil that the idea of moving planets around for our
convenience is ballsy, it almost pales in comparison to a suggestion made
back in 1959 by a chap named Freeman Dyson - to take FULL advantage of
solar power by building a sphere around the Sun with a radius at an
appropriate distance from the Sun to be in a life-friendly zone (PEDANT WARNING-
in our solar system, about 93 million miles, 149 kilometres, 1 astronomical unit).
If you want a little more geeky followup on this, I found this link:
Zoom Astronomy Glossary: D

TTFN,
OzoneBoy

A Dyson sphere, like a ringworld, is unstable. Has anyone ever done the math to determine whether the energy needs to run stabilizing jets is economical?

For that matter, I suppose three planets at a separation of 60˚ would at least be metastable. Assuming that we already have planet-moving tech, couldn’t that suffice?

What if Mars and Venus were in orbits outside the plane of the other planets? Each inclined at 90, but perpendicular to each other as well as earth’s orbit? And not exactly at 1 AU distant from the sun?

Although you could not put Mars or Venus into the L1, L2, or L3 libration points, you could put two bodies of equal mass in each other’s L4/L5 (equilateral triangle) points for a system that is at least marginally stable, albeit easily perturbed. (It’s essentially a degenerate case of a Klemperer rosette.)

If you can generate and control enough energy to move planets, however, you can certainly build and maintain your own permanent habitats. At that point, there is little reason to move worlds or, indeed, live on planetary surfaces at all, as you get much better utilization of a mass to living or arable surface area using a large spun habitat.

Stranger

Mars would be about 3 or 4 hundred thousand times more massive than a 100km asteroid. I don’t know how much momentum would be transferred on each flyby but maybe a “few thousand” times with a “few score” asteroids would do it. It would be years between each flyby for each asteroid so this would be a slow process. We would have to keep up the effort as civilizations rose and fell. And if we ever stopped managing the process we would have all these huge asteroids flying around the solar system in highly elliptical orbits - not safe. And also: How do you shove 100km-sized asteroids around? I guess you’d use smaller asteroids to fly by the big ones. So you’d begin the process by sending out trillions of little autonomous mass driver robots that would each latch on to a rock in the asteroid belt and start pushing it towards Jupiter.

If we wanted to move Earth we could have the asteroids fly by the moon, which would transfer the extra momentum gravitationally to the Earth, tugging it to a higher orbit. Then we’d avoid the tidal effects and there would be less of a chance of getting hit. We could just slam the asteroids into the moon, but then they could only be used once; we would not be able to get the repeated transfer of energy and momentum from Jupiter.

A Dyson sphere is neutrally stable - even if one side of the sphere is close to the Sun, there will be no tendency for that side to get closer to the sun (Shell theorem - Wikipedia).

The Ringworld is very unstable - the amount of offcenterness of the Ring will double every 2 months, which means even if it starts off only an inch off center, it will collide with the Sun in about a century (the doubling time starts out pretty constant, but gets much shorter after a while).

With a caveat or two: planets at least have the advantage of being statically stable, they aren’t going to fall to pieces if they go a mere ten thousand years without any maintainence. Now maybe some really advanced tech on the order of self-repairing megastructures will change the rules in the future, but I’d like to see it first.

Second, there are reasons to suppose that the larger and less biologically dense an ecosystem is, the more ecologically stable against boom/crash cycles it is. In other words, if you could perfectly seal the habitat and just let it go on its own forever, what would happen inside? For a habitat size we could currently build (with steel cable, about four miles in diameter, a few hundred square miles of surface area), it would only remain humanly habitable by constant artificial adjustment. Left to its own devices, nothing larger than weeds and insects could sustain itself. To get anything like the size ecosystem typical of planets (remember you have to include the cubic miles of atmosphere and ocean in the equation), you need nanotube cable at a minimum, and maybe unobtainium. Or a really big turtle and four elephants.

Actually, it would be quite possible to have two large planets in essentially the same orbit. They could be co-orbital just like Janus and Epimetheus. The problem would be getting the planets into a co-orbital state in the first place. Just trying is likely to bugger up the solar system and send one of them up Uranus’s way.

There is of course a solution to that problem. (Link also provides some additional information on moving planets about).