[sometimes]

I must admit I don't have an appreciation of how big comets are. And I guess the only force likely to affect is when particles leave the object, so if the whole mass is still travelling as one then I presume there will be no trajectory change, but if some mass is leaving then a slight trajectory change might occur.

The exhaust of a car would have impact I'm sure if the car were in space, without air resistance, and overwhelming gravity. I know that the tiniest forces can have a cumulative effect. We are talking about years of travel in the case of a comet. In this case, it sounds like it takes a bit of a nudge every time it comes close the sun - perhaps something similar as it passes (if does) the bigger planets.

I presume some boffin somewhere will have calculated the size of the activity and worked out relative forces.