[QUOTE=Giles]
I think that, given enough energy, you could move Venus so that it was a Trojan, at 60 degrees before or after the Earth. That would be a stable position.
[/QUOTE]
The assumption of stationary stable libration points in a restricted circular orbit three-body problem is based on the assumption of M1>>M2>>M3->0, where M1 is the primary (the Sun), M2 the secondary (Earth), and M3 the minor satellite. The case of M1~M2>>M3 (a doublet) is interesting but not applicable here, and M1>>M2~M3 is going to be quasi-chatoic and possibly dynamically unstable (M2 and M3 will oscillate) over the long run, particularly given that the Earth’s orbit has about a 2% eccentricity. (This is assuming that the M2-M3 system has no rotational momentum of its own other than that imparted from rotation about M1; a system that does have such rotation may be more stable if the resonance matches the forced oscillation.) Placing it directly opposite the Earth makes the situation even worse. In general, you don’t want any large masses near Earth’s orbit, period. [thread=337968]Here[/thread] is an old thread on the topic.
Regarding the movement of planets via direct force, there are two problems; one is obtaining sufficient exhaust velocity to eject your propellent from the gravity well of the world you’re moving and the second is applying the force from thrust to the world in some way which causes it to move.
With regard to the first problem, you’re going to have to have sufficient exhaust velocity that your propellent isn’t blunted by the atmosphere or captured by the gravity well; should this happen, the only thing you’ll accomplish is heating up the atmosphere or causing the planet to bounce like one of those wood paddles with a ball connected by a rubber band. The rubber band, in this case, is gravity, which will simply absorb the energy of the thrust and rerelease it as it pulls the exhausted propellent back toward the planet. The surface escape velocity for Venus is about 10.5km/s. Modern chemical reactant liquid propellent rockets have an exhaust velocity of <4,500 m/s, so even ignoring the atmosphere you’re not going to be going anywhere. Ion thrusters and the like have much higher exhaust velocities, but the per-unit momentum is so low that any atmosphere at all will blunt it quickly, turning that energy into randomized heat. So, you’ll get a little nudge, but then you’ll bounce back into position as the mass of the propellent is pulled (and pulls on) the planet.
The second problem is even more significant. Solid ground isn’t when it comes to planetary sizes. The planets are, in fact, so soft and mutable that they’re pulled into round shapes, and even in the case of the Earth pulled somewhat out of round by the forces of their own motion. Driving the kind of impulse directly into the structure of the planet necessary to affect any short-term movement would cause an inert world to crumble into a pile of rock, and a volcanically active one to melt into molten lava. Even if you distribute the forces evenly across a face of the planet, thrusts will have to be very, very low in order to minimize violent seismic activity.
Notions about painting one side silver and letting light pressure move the planet are a complete wash; the pressure of solar radiation, even in the in-system, is negligable for something with the mass/aspect ratio of a spheroid, and the drag the planet receives from the interplanetary medium probably cancels out light pressure (at least around an order of magnitude), so they’ll stay in balance.
The only practical way to move a planet–and I use that term very, very loosely–is to use some other mass to swing by and take away or add some momentum, in a manner similar to a swing-by maneuver done by interplanetary probes, only on a much more massive scale. In other words, you use gravitational coupling between the Primary (your maneuvering body) and the Secondary (the planet you wish to move). You’ll have to do this hundreds of thousands of times to get even a measurable effect, and you’ll have to do it in a very coordinated fashion to first ellipticize the orbit, and then circularize it when you get the planet out to your new habitable orbit. How long this would take depends on how massy your Primary is, but we’re talking on the order of millions or tens of millions of years to move a planet a few million kilometers outward.
If you’ve got the resources, technology, and patience to do this sort of thing then you probably don’t need to move worlds to live on; you can build your own artificial worldlets that are a better use of material than planetary bodies. Just don’t go spinning a giant habitable ring around a central star, there being a well-known stability problem with that particular solution. Klemperer Rosettes are also not so stable, so don’t get cute. Just clear out the Asertoid Belt and build yourself a few million spun habitats between 0.80 and 1.3 AU using water (harvested from Kuiper objects) as shielding and inertial buffer.
Stranger
