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In several NASA animations of missions, I've noticed that the spacecraft is rotating while traveling in deep space. This was true of Curiosity when that video came out. Now the Juno mission demonstrates the same behavior.

I saw one related question, but there, stability of rocket boosting is given as a justification. In the videos I'm referencing, they're mid-travel, just floating in space, so that doesn't really answer it for me.

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It's done in the interest of stability, both inertial and thermal.

Inertial stability: basically if something is spinning, it resists attempts to change its spin axis. So, if you align your spin axis with, say, the axis along which your thrusters point, it's easier to keep yourself pointed in the right direction while you burn. After your burn, you still want to keep some attitude profile (say, to keep your antenna pointing at Earth), and spin-stabilization is an easy way of doing that.

Thermal stability: on your spacecraft, whichever surface can see the Sun is hot, and whichever surface cannot is cold. This can introduce thermal gradients, which are generally undesirable. However, if you introduce a spin, now you have a symmetry around your spin axis, and thus less of a thermal gradient -- this is often referred to as a "barbecue roll".

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    $\begingroup$ It also allows smoother starting of liquid engines -- the fuel is forced to one end of the tank. Although there are tanks, like those used in the shuttle's OMS pods, that have internal structures that make this unnecessary. $\endgroup$ – Erik Oct 11 '13 at 21:34
  • $\begingroup$ @Erik, when you spin a spacecraft along its thrust axis, the fuel is forced to the outside of the tank. But when the engine is running, the fuel is forced towards the bottom of the spacecraft. For common tank designs like the sphere you'd need two fuel pipes running from the tank to accommodate both scenarios. Aren't ullage motors (small solid rockets that give an initial acceleration to force the fuel in the desired direction) more common to help start a liquid engine in 0 G? $\endgroup$ – Hobbes Oct 15 '13 at 10:48
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    $\begingroup$ @Hobbes the shuttle's OMS tanks (and other tanks for engines that have to start in zero g) have screens inside them to collect fuel via surface tension and capillary action for zero g startup. Without them, you wouldn't be able to start the engines in zero g reliably. Spinning the vehicle (if possible) would make these screens unnecessary. Finally, you can spin and thrust at the same time, so only one feed line is required for "gravity" fed operations. $\endgroup$ – Erik Oct 16 '13 at 14:09

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