Let's say that for the purpose of terraforming, we need to speed up the rotation of a planet (like Venus), a moon or an asteroid to near Earthian day. How would we go about doing this? Can this be accomplished from within the body itself?
The key thing to doing this is to accelerate some of the mass of Venus outside of the planet, or alternatively bringing in some mass. Thus, there are 2 main things that could be done to alter the rotation speed. These ideas are explained in Wikipedia.
- Carefully plan a series of asteroid impacts in a direction where they can increase the rotational energy of the planet.
- Shoot out some part of Venus, most likely it's thick atmosphere, in to space beyond the escape velocity.
In addition, the wind speed of Venus is higher than the land speed, so putting up some large wind blockers would help to convert some of that energy into rotational energy of the planet. However, this would be a fairly minimal effect, as this is already being done in some scale, although not as quickly as is desired.
So, how else might this be accomplished? Xkcd has a nice comic showing one way, spinning around in circles can cause the motion of the planet to change.
Furthermore, in a what if scenario, they gave one way to change the rotation speed of the Earth, driving around in circles. That would work as well, although very slowly. But in theory a train could be built that circled the planet, solar powered, that would slowly increase the rotational speed of the planet, although it would be quite slow.