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Cassini Mission and Milgromian Dynamics


The findings of the Cassini mission, which orbited Saturn from 2004 to 2017, provided an opportunity to test Milgromian dynamics (MOND) – an alternative theory to dark matter.


GS III: Science and Technology

Dimensions of the Article:

  1. What is Dark Matter?
  2. MOND Theory: An Alternative to Dark Matter
  3. Current Perspective
  4. Cassini Spacecraft Overview

What is Dark Matter?

  • Dark matter, similar to ordinary matter, occupies space and possesses mass. However, it does not reflect, absorb, or emit light to a degree that we can detect yet.
  • Scientists have estimated that dark matter constitutes about 27% of the universe, but its exact nature remains a mystery.
  • Researchers hypothesize that dark matter forms a vast, web-like structure throughout the cosmos.
  • This structure acts as a gravitational scaffold, attracting most of the ordinary matter in the universe.
  • It is established that dark matter is not made up of known particles, and the quest to identify its components continues.

MOND Theory: An Alternative to Dark Matter

Background and Proposition:
  • One of the significant puzzles in astrophysics is why galaxies rotate faster than Newton’s laws of gravity predict based on their visible matter.
  • The concept of dark matter was introduced to explain this discrepancy.
  • Despite being a widely accepted theory, dark matter has not been directly observed and does not align perfectly with the Standard Model of particle physics.
  • To address this issue, Israeli physicist Mordehai Milgrom proposed an alternative theory called Milgromian Dynamics (MOND) in 1982.
Key Concepts of MOND Theory:
  • MOND suggests that Newton’s laws fail under very weak gravitational fields, such as those at the edges of galaxies.
  • The theory has shown some success in predicting galaxy rotation without invoking dark matter, although many of these successes can also be attributed to dark matter while preserving Newton’s laws.
  • MOND affects gravity at low accelerations rather than specific distances.
  • Consequently, MOND’s effects, typically observed several thousand light years from a galaxy, could also be significant at much shorter distances, like in the outer Solar System.
Testing and Evidence Against MOND:
  • The Cassini mission, which orbited Saturn from 2004 to 2017, provided a chance to test MOND.
  • MOND predicts subtle deviations in Saturn’s orbit due to the galaxy’s gravity, given Saturn’s 10 AU orbit around the Sun.
  • Cassini’s measurements of the Earth-Saturn distance using radio pulses showed no anomalies expected by MOND.
  • Newton’s laws remain valid for Saturn, challenging MOND’s credibility.
Additional Challenges to MOND:
  • More evidence against MOND comes from wide binary stars.
  • A study demonstrated that MOND’s prediction of faster orbits was highly improbable, akin to flipping heads 190 times consecutively.
  • MOND also fails to explain the specific energy distribution and orbital inclinations of comets in the outer Solar System.
  • Newtonian gravity is more accurate than MOND for distances below a light year.
  • MOND is less effective at larger scales, like galaxy clusters, where it predicts too little central gravity and too much at the outskirts. Newtonian gravity with dark matter aligns better with observed data.

Current Perspective:

  • Despite some issues with the standard dark matter model, such as the universe’s expansion rate and cosmic structures, MOND is not considered a viable alternative.
  • Dark matter remains the leading explanation, although its true nature may differ from current models, or gravity might exhibit stronger effects on very large scales.

Cassini Spacecraft Overview

Mission Background:

  • Cassini-Huygens was a collaborative space mission involving NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) with the goal of exploring Saturn.

Launch Date:

  • Launched on October 15, 1997, the Cassini-Huygens mission aimed to provide comprehensive insights into Saturn and its moons.


  • The mission comprised NASA’s Cassini orbiter, marking the first space probe to orbit Saturn, and the ESA’s Huygens probe, which successfully landed on Titan, Saturn’s largest moon.
Spacecraft Details:
  • Cassini-Huygens was one of the largest interplanetary spacecraft.
  • The Cassini orbiter, weighing 2,125 kg (4,685 pounds), measured 6.7 meters (22 feet) in length and 4 meters (13 feet) in width.
  • Instruments onboard Cassini included radar for mapping Titan’s cloud-covered surface and a magnetometer for studying Saturn’s magnetic field.
  • The disk-shaped Huygens probe, mounted on Cassini, weighed 349 kg (769 pounds), was 2.7 meters (8.9 feet) across, and carried six instruments for studying Titan’s atmosphere and surface.
Mission Highlights:
  • Orbited Saturn from 2004 to 2017, completing 294 orbits around the planet.
  • Provided extensive knowledge about Saturn, including the structure of its atmosphere and rings, and their interactions with the planet’s moons.
  • Discovered six named moons and identified Enceladus and Titan as promising locations to search for extraterrestrial life.
  • Cassini played a pivotal role in advancing our understanding of the Saturnian system and contributed significantly to planetary exploration.

-Source: Indian Express

June 2024