Research on the Cassini spacecraft did not find a galactic anomaly of the kind predicted in the MOND theory.

What is MOND theory? 

• Propounded by: Israeli Physicist Mordehai Milgrom initiated a new research program in cosmology, called MOND (for MODified Newtonian Dynamics) or Milgromian dynamics, in 1983.
• Basic Idea: Milgrom proposed a set of postulates in three papers describing how Newton’s laws of gravity and motion should be changed in regimes of very low acceleration.
  • Milgrom’s postulates were designed to explain the asymptotic flatness of galaxy rotation curves without the necessity of postulating the existence of “dark matter”
• Aims: MOND seeks to replace Newtonian dynamics and general relativity to account for the ubiquitous mass discrepancies in the Universe without invoking the dark matter, an inherent part of the Newtonian Theory of Standard Dynamics.

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Limitations of the MOND theory

• The Cassini Mission: Due to a quirk of MOND, the gravity from the rest of our galaxy should have caused Saturn’s orbit to deviate from the Newtonian expectation subtly,  But Cassini did not find any anomaly of the kind expected in MOND.
  • The timing of radio pulses between Earth and Cassini was tested, which allowed for the precise tracking of Saturn’s orbit.
  • Finding: The study found out that no matter how the calculations are tweaked,  given how MOND would have to work to fit with models for galaxy rotation, it cannot fit the Cassini radio tracking results.
• A test provided by wide binary stars:  MOND predicted that Two stars that orbit a shared centre several thousand AU apart should orbit around each other 20% faster than expected with Newton’s laws.
  • But a detailed study rules out this prediction, with the chance of the MOND theory prevailing being the same as a fair coin landing heads up 190 times in a row.
• Failure to explain small bodies in the distant outer Solar System: Comets coming in from there have a much narrower energy distribution than Mond predicts. 
  • These bodies also have orbits that are usually only slightly inclined to the plane that all the planets orbit close to. Mond would cause the inclinations to be much larger.
• MOND theory also fails on scales larger than galaxies and is unable to explain the motions within galaxy clusters.
• Gravity Anomaly: MOND cannot provide enough gravity either, at least in the central regions of galaxy clusters. However, on their outskirts, MOND provides too much gravity. 

Significance

• Dynamics of individual galaxies: MOND predicts quite accurately the observed dynamics of individual galaxies of all types (from dwarf to giant spirals, ellipticals, dwarf spheroidals, etc.), and of galaxy groups, based only on the distribution of visible matter (and no dark matter).
• General Laws of galactic dynamics: MOND’s basic tenets predicted the general laws of galactic dynamics (with some additional, plausible, non-MOND-specific requirements) are well obeyed by the data, with a0 appearing in these laws in different, independent roles.
• MOND has unearthed a number of unsuspected laws of galactic dynamics, predicting them a priori, and leading to their subsequent tests and verification with data of ever increasing quality. 
  • One of these phenomenological laws is the baryonic Tully-Fisher relation, which is underlain by the MOND mass-asymptotic-speed relation (MASR)
• New Tools: MOND, as a set of new laws, affords new tools for astronomical measurements, such as of masses and distances of far away objects in ways not afforded by standard dynamics.

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