The Puzzle Over Saturn's Orbit

The Puzzle Over Saturn's Orbit (contd)

Posted: 31 May 2011 09:10 PM PDT

If modified theories of gravity are correct, we ought to see the effects in 
the orbit of Saturn. But nobody is quite sure whether we do or not

Many astronomers think our universe is filled with  mysterious dark stuff 
that exerts a gravitational pull on big things like galaxies.  In fact, 
most galaxies spin so fast that they would fly apart unless there were a 
substantial amount of this dark gloop holding them together.


But if dark matter does fill our galaxy, we ought to see it in our Solar 
System. There's no shortage of dark matter detectors looking for the stuff. 
Most have drawn a blank and those that do claim to have seen it have been 


There is an alternative hypothesis, however.   This is the idea that 
Newton's equations of motion work in a different way at the very low 
accelerations that stars experience as they orbit a galaxy.


The equations that describe this so-called Modified Newtonian Dynamics or 
MOND are non-linear and so lead to other predictions. "An important 
consequence of the non-linearity is that the gravitational dynamics of a 
system is inï¬,uenced by the external gravitational environment in which the 
system is embedded," say Luc Blanchet  at the Universite Pierre et Marie 
Curie and Jerome Novak at the Universite Denis Diderot, both in Paris.


This is called the external field effect and it ought to have a measurable 
influence on the Solar System, particularly on the precession of the 
perihelion of the planets.


Today, Blanchet and Novak calculate the size of this effect and compare it 
to the best data we have of planetary motion.

It turns out that the planets most effected are the distant gas giants: 
Saturn, Uranus and Neptune. And the best monitored of these is Saturn, 
since astronomers have been able to follow the motion of the Cassini 
spacecraft as it orbits the ringed giant.


Blanchet and Novak say that the accuracy of these measurements can be used 
to rule out some formulations of MOND. "We find that the precession effect 
is rather large for outer gaseous planets, and in the case of Saturn is 
comparable to, and in some cases marginally excluded by published residuals 
of precession permitted by the best planetary ephemerides."


But the story doesn't end there. One of the best sets of data about 
Saturn's motion has been compiled by the Russian astronomer Elena Pitjeva, 
who heads the Laboratory of Ephemeris Astronomy at the Institute of Applied 
Astronomy in St Petersburg.


In 2005, she published a comprehensive set of data on  Saturn's motion. 
It's this that Blanchet and Novak used to compare their calculations 


But back in 2008, rumours began to circulate that Pitjeva had found 
something strange in more recent data. These were outlined in a paper by 
Lorenzo Iorio at the National Institute of Nuclear Physics in Italy and 
covered by the Physics arXiv Blog at the time.


The bottom line was that Pitjeva had reportedly discovered that the 
precession of Saturn's perihelion, as predicted by general relativity, 
needed to be corrected to fit the most recent data from Cassini.

Pitjeva doesn't appear to have published these data, even now almost three 
years later. And Iorio hasn't updated his paper either.


But it raises an intriguing question. Could the data from Cassini be 
telling us something interesting about MOND?

Perhaps Blanchet and Novak could politely enquire about the status of 
Pitjeva's result and compare it with their calculations. Just to settle the 
matter for curious souls.


Ref: Testing MOND in the Solar System


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