I was looking at this plot of speed over time for the Cassini probe.
What's going on at the right, after mid-2004? Why does it keep slowing down and speeding up again?
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It's explained on the wikimedia page for the image:
The various gravity assists form visible peaks on the left, while the periodic variation on the right is caused by the spacecraft's orbit around Saturn.
Since Saturn is moving relative to the Sun, when Cassini is at a point in its orbit that causes it to move in the same direction relative to Saturn that Saturn is moving relative to the Sun its speed should be higher (relative to the Sun) than when its motion relative to Saturn is in the opposite direction as Saturn's motion relative to the Sun (it's possible it's also not moving in a circular orbit, I'm not sure).