Integral said:
Yes, they need to make adjustments. But is it because the planet is not where we expect it to be or because the probe is not where we expect it to be? Same results but way different conclusions.
There you go.
There certainly are errors between the computed state of a planet (state = position, velocity, orientation, and rotation rate) and the true state at some point in time. These will generally be small compared to the errors between the probe's computed state and true state.
Even more importantly, there will be differences between the probe's desired state (here, state is position, velocity, attitude, and rotation rate) and the computed state. There differences can be rather large at times -- i.e., right before the probe starts a maneuver. The purpose of the maneuver is to make the computed state match the desired state.
A simple way to do this is to align the vehicle in the right direction and fire the thrusters for the right amount of time. This is exactly how older probes worked. One problem with this approach: The vehicle wasn't truly pointed in the right direction, and it didn't fire the thrusters for the right amount of time. Force variations of ±10% from design are quite common. Thrust deviations of a few percent is quite common even after calibrating the thruster (and the only way to calibrate the thruster is to fire them in the operational environment).
An arguably better approach is to use the vehicle's navigation sensors (e.g., accelerometers and gyroscopes) to guide and control the maneuver. This is what most, but not all, modern vehicles do. Note that I said "arguably better." This approach is arguably worse than dumb old timed burns. The hardware and software needed for timed burns is rather simple: A clock and software that watches the clock. The modern approach requires a lot of hardware (all those sensors) and a lot of software (software to collect and interpret sensor readings, software to check for and respond to sensor failures, software to compute incorporate the sensor readings into the state, ...) Expensive stuff.
Even with navigation sensors, the maneuvers will not be performed perfectly. Nothing is performed perfectly. The sensors do not sense the state perfectly. A small error in velocity can compound over time to make for a not so small error in position. The correction burns are needed to address the errors in the vehicle's guidance, navigation, and control software and hardware.
