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 ï¬nd 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: arxiv.org/abs/1105.5815: Testing MOND in the Solar System
A découvrir aussi
- Découverte de ruines de lAtlantide aux Caraïbes ?
- (AUTEC - Atlantic Undersea Test and Evaluation Center)
- Astéroïde 2005 YU, un mini-monde arrondi d'environ 400 mètres de diamètre.