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Originally Posted by Tensor
Here ya go CM. As usual, Jerry is making objections to GR without knowing whereof he speaks.
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Put your thinking caps on: GR provides a geometric solution to the Mercury Procession problem. Four dimensional; but it is not that different from Keplar trying to use perfect shapes to plot the orbits of all of the planets. GR is so widely accepted because it resolved the one outstanding issue with solar Newtonian mechanics. Hell, I'm arguing Newton is off on outer planet masses by factors of more than two. Where GR fits in all of this is absurdly academic.
Meanwhile, from the Planetary Society's Emily:
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Originally Posted by Lakadawala
To make a long story short, the actual gravity data differed from the model by quite a lot, which had Smith and his coworkers scratching their heads for a while: "Initially, we thought there was something wrong with the data, something wrong with the position of Mercury, or something wrong with the model of the spacecraft, but none of those seemed to explain this. It was after about two weeks of trying to find some physical explanation that we thought, 'this must be gravity,' so we tried solving for a gravity anomaly under the spacecraft at the point of closest approach." That is, they inferred there was some excess mass buried invisibly under Mercury's crust near MESSENGER's path. Smith showed that they were able to account for about 95% of the problem deviation using a single mass anomaly positioned about 10 degrees south of the equator at about 60 degrees longitude. There's no real conclusions about the interior structure of Mercury that you can draw from this one flyby -- much more data is needed.
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Just as it was necessary to use a major gravity anomaly to explain the close flyby of Galileo of Ganymede, the very first pass of Mercury is about three sigma out of wack.
What the hell? If you remember my predictions about Mercury, I expected something quite opposite: Mercury should be lighter than Newtonian estimates, so I'm 180 degrees wrong, right? Maybe. Once again, not enough information: I figured Mercury would provide less gravitational braking than expected. This article does not say whether the assist was more or less than expected. To achieve gravitational braking, you cross in front of the planet's orbit and let gravity reign in some momentum.
mmm...Ok, I can see a couple of solutions, but more information is needed. If this was our closest pass to Mercury, and closest by a fairly long shot, previous passes may have under estimate the
Newtonian mass/density of Mercury that we will measure once in orbit. The other solution would require an inside to outside pass - I just don't know enough about Messengers path. What is clear, is that the earlier-than-planned orbital correction last year was indeed due to an unexpected gravity-related event...because, like Ganymede and Europa, it can't be written of as atmospheric.