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It is hard enough to exactly aim a rocket when you know all the forces on it, but on a long journey there must be any number of minor gravitational attractions to rocks and other mass in space, and the effects must add up considerably over millions of miles.

My assumption is that adjustments must be made based on comparing actual position to estimated position, and correcting accordingly.

From spaceflightnow.com:

The first "deep space maneuver" occurred last Thursday, Aug. 30, more than 300 million miles from home, to begin fine-tuning the craft's course to intercept Earth. The Leros-1b main engine was fired for 29 minutes and 39 seconds, changing Juno's velocity by about 770 mph while consuming around 829 pounds of fuel.

So how often does this measurement need to be done, and how is it done? Triangulation from known stars?

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This is pretty mission-specific. Using Mars Science Laboratory as an example, they initially scheduled for six possible midcourse correction maneuvers. According to the link, the second maneuver imparted about 5.5 m/s total delta-V, whereas the first imparted about six times that amount.

So how often does this measurement need to be done?

The trajectory is measured quite often, with an estimate being provided multiple times per day or even more frequently (depending on the phase of the mission).

and how is it done?

Range measurements are usually taken between a ground station antenna and an antenna on the spacecraft. Think of it as a glorified "time of flight" calculation, in which the time it takes for the signal to travel between the antennas is measured, which in turn produces an estimate of the distance traveled. Sometimes the first time derivative of range (range rate) can be measured as well.

These measurements can be used to update the trajectory, and the difference between the nominal trajectory and the actual one is used to calculate the required correction maneuver.

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Using Voyager as your example, the vast majority of the fuel (hydrazine) was used for attitude control -- pointing the spacecraft at various targets. This of course includes pointing at the Earth for communication with ground control.

Spacecraft, including Voyager, often use a star tracker (like this one from Ball Aerospace) to determine their attitude.

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