Pluto-Charon System

Pluto-Charon System

An oblique view of the Pluto–Charon system showing that Pluto orbits a point outside itself. The two bodies are mutually tidally locked.

Pluto's orbit around the Sun

Orbit of Pluto—ecliptic view. This "side view" of Pluto's orbit (in red) shows its large inclination to the ecliptic.

Pluto is a planetoid consisting of a minor planetary system of five known moons. It is located beyond Neptune in the Kuiper belt, classified as a trans-Neptunian object.


800px-Pluto-01 Stern 03 Pluto Color TXT

Pluto's orbit appears to cross that of Neptune when viewed from directly above, however, the two objects' orbits are aligned so that they can never collide or even approach closely. Pluto's orbit passes about 8 AU above that of Neptune, preventing any collision.[1][2][3] Pluto is protected by its 2:3 orbital resonance with Neptune: for every two orbits that Pluto makes around the Sun, Neptune makes three. Each cycle lasts about 495 years. This pattern is such that, in each 495-year cycle, the first time Pluto is near perihelion, Neptune is over 50° behind Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be nearly 130° ahead of Pluto.[4]

The 2:3 resonance between the two bodies is highly stable, and has been preserved over millions of years.[5] This prevents their orbits from changing relative to one another, and so the two bodies can never pass near each other. Even if Pluto's orbit were not inclined, the two bodies could never collide.[3] The long term stability of the mean-motion resonance is due to phase protection. If Pluto's period is slightly shorter than 3/2 of Neptune its orbit relative to Neptune will drift, causing it to make closer approaches behind Neptune's orbit. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly more slowly, according to Kepler's third law. After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently sped up, that Pluto orbit relative to Neptune drifts in the opposite direction until the process is reversed. The whole process takes about 20,000 years to complete.[3][5]

New HorizonsEdit


Artist's impression of New Horizons' close encounter with the Pluto–Charon system.

On July 14, 2015, the New Horizons spacecraft flew 7,800 mi above the surface of Pluto and sent back images of the planetoid and its large (and intriguing) moon Charon. Many of the images show unexpected beauty and complexity on Pluto’s surface. Jeff Moore will give a public speech, disclosing the latest photos and current thinking among the New Horizons team members about Pluto, its moons, and the unexplored frontier that lies beyond.[6]

Jeff Moore is a Research Scientist in the Space Science Division at NASA’s Ames Research Center. His research has focused on the geologic lifetime of landscapes and crustal materials of other planets. His published work explores the roles of impact cratering, volcanoes, and tectonism on terrestrial planets and outer planet satellites. Dr. Moore is the Imaging Node Leader for the New Horizons mission and has worked on the Mars Exploration Rover, the Mars Reconnaissance Orbiter, and the Galileo Mission to Jupiter, among others. He is the recipient of the 2013 Distinguished Scientist Honor Award at NASA Ames.[6]


  1. Wan, Xiao-Sheng; Huang, Tian-Yi; Innanen, Kim A. (2001). "The 1:1 Superresonance in Pluto's Motion". The Astronomical Journal 121 (2): 1155–1162. doi:10.1086/318733. Bibcode2001AJ....121.1155W. 
  2. Hunter, Maxwell W. (2004). "Unmanned scientific exploration throughout the Solar System". Space Science Reviews 6 (5): 501. doi:10.1007/BF00168793. Bibcode1967SSRv....6..601H. 
  3. 3.0 3.1 3.2 Malhotra, Renu (1997). "Pluto's Orbit". Retrieved March 26, 2007. 
  4. Williams, James G.; Benson, G. S. (1971). "Resonances in the Neptune-Pluto System". Astronomical Journal 76: 167. doi:10.1086/111100. Bibcode1971AJ.....76..167W. 
  5. 5.0 5.1 Alfvén, Hannes; Arrhenius, Gustaf (1976). "SP-345 Evolution of the Solar System". Retrieved March 28, 2007. 
  6. 6.0 6.1