Are the planets where we belive these to be

1. Jun 2, 2010

Bjarne

A year ago a friend of mine told me that have seen a film that claims that it seems that the planets in not exactly where we believe these to be.
When space probes arrive there are small deviations.

Is that really true?

2. Jun 2, 2010

Staff Emeritus
That is logically impossible. How could we believe an object is not where we believe it to be.

3. Jun 2, 2010

cahill8

You can walk outside and point to a planet in the sky. Are you saying the planet isn't exactly there or are you talking about mathematical predictions being wrong?

4. Jun 2, 2010

Bjarne

I am talking about that the course of a space probes visiting a planet must be adjusted a tiny little bit, to arrive the planet exactly where we expect it to arrive.

5. Jun 2, 2010

D H

Staff Emeritus
You are essentially demanding perfect knowledge, Bjarne, and quite a bit of perfect knowledge to boot. Perfect knowledge does not exist in physics. It does not exist anywhere.

6. Jun 2, 2010

Bjarne

No
I am not asking the reason for this. (we don't know).
It should only me inner planets that are affected: Mercury, Venus, Earth and Mars.
I am 99, 9 % sure, -
My friend (a math teacher) (that have seen that in television) haven't misunderstood anything, - he is not that stupid.

Last edited: Jun 2, 2010
7. Jun 2, 2010

Integral

Staff Emeritus
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.

8. Jun 2, 2010

Bjarne

In this case it had nothing to do with gravity assist or Pioneer anomlies.
It should not be known know why that happens / the cause of it

9. Jun 2, 2010

D H

Staff Emeritus
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.

10. Jun 2, 2010

Bjarne

Does somebody know where to read more of this ?

11. Jun 2, 2010

D H

Staff Emeritus
Bjarne, you are asking about multiple and rather distinct areas of knowledge, and areas of rather advanced knowledge at that. People are to this day getting PhDs in solar system dynamics and in spacecraft guidance, navigation, and control.

That, by the way, gives you two sets of keywords ("solar system dynamics" and "guidance, navigation, and control") that you can use as a basis for a search. Please do some research on your own and then come back with more specific questions.

12. Jun 2, 2010

Staff: Mentor

So to sum up and try to answer a broad/vague question more succinctly:
1. No, scientists don't know the location of the planets to absolute mathematical precision, but they do know the location to a very high precision.
2. This fact is not the least bit profound and doesn't imply anything negative about the state of scientific knowledge (there is a vague implication in the OP of a flaw in scientific understanding).
3. If you had a reference to the claim in the OP, it would be very helpful.

13. Jun 2, 2010

DaveC426913

I hyothesize that the OP's math friend is suggesting a possible solution to the Pioneer Anomaly - to wit - that the discrepancy is less about where the probe is, and more about where Earth is when observing the probe.

If so, it's naive.

14. Jun 3, 2010

Bjarne

No this is not the case.
But yes we don't know whether this anomaly I am talking about here, - is caused but the space probes or the planets.
The strange thing is that this anomaly only occurs when visiting the inner planets, - which off course suggest; that it must be the planets (not the space probes) that not are exactly where we expect these to be.

15. Jun 3, 2010

Staff: Mentor

In any case, the "anomaly" you describe does not exist.