Discussion below Are all or some geostationary satellites tidally locked to the Earth? got me thinking about Pluto and Charon. According to Wikipedia:
Charon and Pluto orbit each other every 6.387 days. The two objects are gravitationally locked to one another, so each keeps the same face towards the other. This is a case of mutual tidal locking, as compared to that of the Earth and the Moon, where the Moon always shows the same face to Earth, but not vice versa. The average distance between Charon and Pluto is 19,570 kilometres (12,160 mi). The discovery of Charon allowed astronomers to calculate accurately the mass of the Plutonian system, and mutual occultations revealed their sizes. However, neither indicated the two bodies' individual masses, which could only be estimated, until the discovery of Pluto's outer moons in late 2005. Details in the orbits of the outer moons revealed that Charon has approximately 12% of the mass of Pluto.
Question: How "locked" are Pluto and Charon? How much does each librate as seen from the other? Charon's orbit has an eccentricity of only about 0.0002 but I think the total apparent libration motion depends upon orbital inclination and the alignment of each body's rotational axis.
Related to lunar libration motion quantification:
- How to get lunar L, B, C parameters from the Moon's 3x3 rotation matrix from the Python package Skyfield?
- What are the "Moon L, B, C" angles shown in this solar eclipse simulation?
To give some idea of what libration looks like here are two GIFs of our Moon's libration from Who does these mesmerizing simulations of the phases of the Moon? And how?