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Are we sure this means it's happening sporadically? I suppose that's one possibility, however another possibility that might account for non-repeatable observations historically has been observational error.
Originally posted by Droogie
Scientists were hoping to gain more insight into the anomaly when the Rosetta spacecraft swung by Earth on Nov. 13 2009 to pick up a gravitational boost for its journey to rendezvous with a comet in 2014.
However, in a major disappointment – which had deepened the mystery — the Rosetta spacecraft did not experience the flyby anomaly during this swingby of Earth in 2009, even though the same spacecraft did experience the anomaly when it flew by Earth 2005, but didn’t in 2007.
I think it's intruiging that it happens sporadically, and unpredictably at this point.
A number of proposed explanations for acceleration towards the sun have been pursued. These are defined as the following: observational errors, an unaccounted for real deceleration, and explanations that would essentially be New Physics.
The possibility of observational errors, which include measurement and computational errors, has been advanced as a reason for interpreting the data as an anomaly. Hence, this would result in approximation and statistical errors. However, further analysis has determined that significant errors are not likely because seven independent analyses have shown the existence of the anomaly (Pioneer anomaly) as of March 2010
Originally posted by EnlightenUp
reply to post by bigfatfurrytexan
Centrifugal force doesn't really exist.
Originally posted by 4nsicphd
It is really rather meaningless to try and discuss this issue without describing it in terms of second or higher approximations of perterbational calculus. Become conversant with Leonhard Euler's work...
Again I don't make too many assumptions like that, which is why I find this paper interesting:
Originally posted by Astyanax
I think it's safe to assume that multibody perturbations, as well as more exotic stuff like relativistic effects, are already being accounted for here, and that this strange acceleration is supplementary to all that.
The empirical formula proposed by J. D. Anderson et al.  to reproduce the data on the Earth flyby anomalies is derived from special relativity (SR). The transverse Doppler shift together with the addition of velocities account for the Doppler data. Time dilation together with the addition of velocities account for the ranging data.
I don't make too many assumptions that I don't have to. These are relatively tiny anomalies. Here is some material on observational error, I'm pretty familiar with this stuff:
Originally posted by Droogie
Regarding the Rosetta craft, I assumed the anomaly didn't occur since the excerpt of the article stated that it didn't.
(p1)Experimental Error in Physics, A Few Brief Remarks… [What Every Physicist SHOULD Know]
Repeatability is the cornerstone of Science!
(p3)No observation or measurement is truly repeatable!
The challenge is to understand the differences between successive measurements
Some observations differ because they are genuinely unique!
Some are different because of RANDOM CHANCE
Most real measurements are a combination of BOTH
Summary of Earth-flyby spacecraft is provided in table below.  The Rosetta data is for its first flyby in 2005; the second flyby produced no significant anomalous increase, and the third a negligible decrease.
As I said, it may happen sporadically as they suggest. But you have your years wrong. There was a slight increase in the Rosetta flyby in 2005 over what was expected, then in 2007 there was no increase over what was expected, and in 2009 there was a slight decrease observed but the amount wasn't considered significant so that's why they called it no anomaly.
When they say they got disappointed when the craft didn't experience the anomaly during it's fly-by in 2009 and 2005, I assume they where looking for it and that it wasn't due to an observational error. And that's why this "further deepened the mystery", and that's also why I assume it happens sporadically.
Originally posted by Arbitrageur
The mixture of plus or minus signs on the flyby anomalies tends to suggest more of a random cause than a new physics, doesn't it?
As the vehicle approaches a planet, one which does in fact have a slingshot effect on the craft, gravity pulls the vehicle toward the planet. In doing so it changes the direction of the vehicle and increases its velocity. Theoretically, the same gravitational force must be overcome to leave the planet’s gravitational field, assuming there is no extra resistance to overcome. The exiting velocity should logically decline by the amount gained while approaching the planet.
Yes, the spacecraft leaves the planet around which it was slung with the same speed, relative to the planet, at which it arrived.