Supposing we wanted to move Venus to Earth orbit. (It could be positioned on the opposite side of the Sun)
How hard would it be to make Venus inhabitable after the move?
And what’s a practical way to achieve this feat? Suppose you had a fairly long timeframe (thousands of years, but not millions of years).
I was thinking a solar sail could be fairly cheap, but could it move a planet? Clearly the face of a planet acts as a giant solar sail, yet planets haven’t been blown out of orbit after so many eons of solar exposure.
How massive could a solar sail be made and what sort of forces would be generated?
Does Earth orbit shift appreciably due to solar wind after, say, a million years?
I suppose in theory you could move a planet into the same orbit as another planet. As a practical matter however I’m pretty sure that sooner or later they would collide, or come so close as to throw one or both of them out of orbit.
I think it would be impossible to make the orbits match so precisely that when one was in the precise mipoint of apogee the other was in the precise midpoint of perigee and that the apogees and perigees of both would be at exactly the same distance from the sun. Discrepencies would result in an enventual collision or near miss which would screw things up royally.
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.
Where do you anchor your pushing or pulling things? I remember as a kid reading all those Superman stories where he’s moving an asteroid or a planet. Imagine walking up to him while he’s doing this – it’d look as if he’s doing a headstand. In fact, his pushing effort would quickly cause him to sink into a deep hole.
It’s pretty clear that you have to anchor your device in pretty secure and unfractured bedrock, and then not exceed the binding forces of whatever you have hooked in, or the fracture strength of the rock. You’d certainly have to take it easy. Maybe that solar sail will do the job – but you’d better make it big, so gravity doesn’t pull it down, and give it really strong tether lines.
There is a Larry Niven short story wherein the protagonists, having been marooned in time, use a gigantic fusion drive to try to get an ancient earth spinning again. It mentions that within moments, the drive settles in the now molten bedrock…
Let’s pretend you had a planet which whose rotation is slaved to its revolution (so that the same side always faced the sun). To move the planet, I build a huge rocket engine on the sunward side which expelled its gases into space. In order to move it into a larger orbit, do I need to have constant thrust? Or can I just maintain thrust over a single revolution and then cut the engine?
Not for something of the mass of Venus. It’s speculated that the object that collided with the Earth to produce the moon was originally at a trojan point.
One Face, which borrows some Known Space ideas but isn’t actually a Known Space story. The character in charge of this plan says that he has confidence in the power of the (total conversion) engine but is just hoping the Earth is strong enough to stand the stress.
As far as making Venus inhabitable; one suggestion I’ve seen is to block out the Sun, by, say, a solar sail kept from moving an uncovering Venus. Eventually, the carbon dioxide freezes solid, and you use mass drivers to fling the frozen CO2 elsewhere. Preferably using von Neumann ( replicating ) machines to cut down on the time. Use similar machines to toss ice from one of the outer moons to Venus. Let Venus warm up again and seed with life.
If you want to spin Venus up at the same time, you angle the expelled CO2 to provide a spin force for the planet. You’ll probably have to add lots of rock as well. Another possibility is building a magnetic equatorial ring, and using a similar orbital ring to spin the planet. I’m not sure if that would work; the orbital ring has been suggested for spinning Earth, but Earth has a mangetic field of it’s own and wouldn’t require a ground ring.
My own personal idea; as long as you’re moving planets, move Mercury into orbit as Venus’s moon, to stabilize it’s spin.
I don’t think we have a big enough kazoo to afford them.
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.
I second Stranger’s comment about how weak planets like Venus and Earth are. They’re practically liquid droplets.
As Stranger suggests, you could use an external massive object to pull a planet. However, a slingshot effect would be brief and would require an object to be pretty close - therefore there’d be tidal hell to contend with, a little rough on the trophy. To move the planet without wrecking it, you’d have to use a much larger mass much further away, so the field strength doesn’t change much from point to point on the earth. The Sun, for instance, is close enough that the difference in field strength between the sides of the Earth facing toward it versus facing away from it generate significant ocean tides (I don’t mean the much bigger Moon effect, but rather the small solar effect that IIRC is the difference between spring and neap tides).
