With bad news from Cassini, is dark matter’s main rival theory dead?

Sub: Science and tech

Sec: Space sector

What is the Cassini mission?

Cassini–Huygens, commonly called Cassini, was a space-research mission by NASA, the European Space Agency, and the Italian Space Agency to send a space probe to study the planet Saturn and its system, including its rings and natural satellites.
The Flagship-class robotic spacecraft comprised both NASA’s Cassini space probe and ESA’s Huygens lander, which landed on Saturn’s largest moon, Titan.

What is Milgromian dynamics or MOND?

In 1983, the physicist Mordehai Milgrom initiated a new research program in cosmology, called MOND (for MOdified Newtonian Dynamics), or Milgromian dynamics.
In three papers, Milgrom proposed a set of postulates describing how Newton’s laws of gravity and motion should be changed in regimes of very low acceleration.
Modified Newtonian dynamics (MOND) is a theory that proposes a modification of Newton’s second law to account for observed properties of galaxies.
The main postulate of MOND is that gravity starts behaving differently to what Newton expected when it becomes very weak, as at the edges of galaxies.
MOND is quite successful at predicting galaxy rotation without any dark matter, and it has a few other successes.
Its primary motivation is to explain galaxy rotation curves without invoking dark matter, and is one of the most well-known theories of this class.
It has not gained widespread acceptance, with the majority of astrophysicists supporting the Lambda-CDM model as providing a better fit to observations.
MOND only changes the behavior of gravity at low accelerations, not at a specific distance from an object.
This means that, although MOND effects would typically kick in several thousand light years away from a galaxy, if we look at an individual star, the effects would become highly significant at a tenth of a light year.

Bad news for CASSINI:

Cassini mission, which orbited Saturn between 2004 and its final fiery crash into the planet in 2017. Saturn orbits the Sun at 10 AU.
Due to a quirk of MOND, the gravity from the rest of our galaxy should cause Saturn’s orbit to deviate from the Newtonian expectation in a subtle way.
This can be tested by timing radio pulses between Earth and Cassini.
Since Cassini was orbiting Saturn, this helped to measure the Earth-Saturn distance and allowed us to precisely track Saturn’s orbit.
But Cassini did not find any anomaly of the kind expected in MOND. Newton still works well for Saturn.

Bad news for MOND:

MOND predicted that such stars should orbit around each other 20% faster than expected with Newton’s laws.
MOND also fails to explain small bodies in the distant outer Solar System.
Comets coming in from out there have a much narrower distribution in energy than Mond predicts.
Newtonian gravity is strongly preferred over MOND on length scales below about a light year.