Moons of Pluto
James Christy spotted Charon on the 22nd of June 1978 while examining photographic plates at the United States Naval Observatory. This discovery came nearly half a century after Pluto itself was found in 1930. The presence of this large companion forced astronomers to revise their estimates of Pluto's size and mass significantly. Before 1978, scientists assumed all observed light and gravity belonged solely to the dwarf planet. Two additional moons appeared in images taken by the Pluto Companion Search Team using the Hubble Space Telescope on the 15th of May 2005. These objects received provisional designations S/2005 P 1 and S/2005 P 2 before receiving official names later that year. The International Astronomical Union officially named Nix and Hydra on the 21st of June 2006. Kerberos emerged from search efforts for Plutonian rings and was announced on the 20th of July 2011. Styx was discovered during a hunt for potential hazards facing the New Horizons spacecraft and announced on the 7th of July 2012.
Charon measures about half the diameter of Pluto yet holds nearly one eighth of its total mass. This massive ratio places the system barycenter outside either body at approximately 878 kilometers above Pluto's surface. Pluto and Charon orbit this common center point rather than one revolving around the other. They remain mutually tidally locked so that each always presents the same face toward its partner. The IAU General Assembly considered reclassifying them as a double planet in August 2006 but abandoned the proposal. Both bodies appear as perfect spheres within measurement uncertainty despite their small size. The gravitational interaction creates a unique binary dwarf planet configuration unlike any other known pair in the solar system. Their shared rotation period matches their orbital period exactly, creating a synchronized dance across space.
Four smaller moons circle the central pair at distances ranging from 42,700 kilometers to 64,800 kilometers from the system barycenter. Styx sits closest among these outer satellites while Hydra orbits furthest away. All four possess irregular shapes with Nix measuring roughly 42 kilometers and Hydra spanning 55 kilometers on their longest axes. Kerberos and Styx measure only 12 and 7 kilometers respectively. These objects maintain nearly circular prograde orbits lying in the same plane as Charon. Styx, Nix, and Hydra exist in a three-body Laplace resonance with orbital periods in a ratio of 18:22:33. Nix and Hydra share a simple two-body resonance with a ratio of 2:3 between them. Styx and Nix maintain an 9:11 resonance while Styx and Hydra hold a 6:11 relationship. The Pluto-Charon pair generates strong tidal forces that vary by 15 percent peak to peak at the location of the outer moons. This gravitational field creates a highly compact region where stable orbits for additional moons are extremely limited.
Scientists suspect the entire satellite system formed from debris after a massive collision similar to the impact thought to have created Earth's Moon. High angular momenta within the system can only be explained by such a violent scenario rather than captured Kuiper Belt objects. Nearly circular orbits suggest these bodies formed during the collision event itself. Their grey color differs sharply from Pluto which ranks among the reddest bodies in the solar system. This difference likely stems from volatile loss during the impact or subsequent coalescence leaving surfaces dominated by water ice. An alternative hypothesis suggests the collision occurred at about 2,000 miles per hour without destroying either body completely. They remained attached for up to ten hours before separating again due to centrifugal force exceeding gravity. Faster rotation periods back then allowed material to escape both bodies and form the four known small moons. No additional moons or rings were found by New Horizons despite expectations of extra debris from such an event.
Transits occur when one moon passes between Pluto and the Sun while occultations happen when Pluto blocks a satellite from view. These events align with specific points in Pluto's orbit near its perihelion and aphelion. Charon appears eight times larger in diameter than our Moon does from Earth due to its proximity. A single eclipse by Charon covers 64 times more area than a lunar eclipse viewed from Earth. Only the hemisphere facing Charon experiences solar eclipses because tidal locking keeps that side fixed toward the companion. Smaller moons cast shadows elsewhere on the surface with Nix spanning 3 to 9 minutes of arc and Hydra covering 2 to 7 minutes. Total solar eclipses are caused by these smaller moons rather than just the large Charon. Styx has no annular eclipses since its widest axis exceeds the sun's angular size by over 10 arcseconds. Kerberos cannot produce total eclipses as its minor axis measures only 32 arcseconds at maximum. The next period of mutual events involving Charon begins in October 2103 and peaks during 2110 before ending in January 2117.
The New Horizons spacecraft visited the Pluto system in July 2015 returning detailed images of all five known moons. Images achieved resolutions up to 330 meters per pixel for Nix and 1.1 kilometers per pixel for Hydra. Lower resolution data covered Styx and Kerberos confirming their irregular shapes and high obliquity states. An intense search confirmed no moons larger than 4.5 kilometers exist out to distances of 180,000 kilometers from Pluto. This finding rules out any additional significant satellites orbiting within six percent of the stable region for prograde orbits. Data also suggests no ring system is present despite earlier hypotheses about debris rings forming around small moons. Deep-optical surveys using the Advanced Camera for Surveys on Hubble supported these conclusions alongside occultation studies. All four outer circumbinary moons were found to be at high obliquity levels suggesting they either formed that way or were tipped by spin precession resonance. Mark R. Showalter noted that Nix can flip its entire pole making rotation appear almost random compared to other known moons like Saturn's Hyperion.
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Common questions
When did James Christy discover Charon?
James Christy discovered Charon on the 22nd of June 1978 while examining photographic plates at the United States Naval Observatory. This discovery occurred nearly half a century after Pluto itself was found in 1930.
What are the official names and discovery dates for the four smaller moons of Pluto?
The International Astronomical Union officially named Nix and Hydra on the 21st of June 2006, Kerberos emerged from search efforts and was announced on the 20th of July 2011, and Styx was discovered during a hunt for potential hazards facing the New Horizons spacecraft and announced on the 7th of July 2012. These objects received provisional designations S/2005 P 1 and S/2005 P 2 before receiving official names later that year.
How does the size and mass ratio of Charon compare to Pluto?
Charon measures about half the diameter of Pluto yet holds nearly one eighth of its total mass. This massive ratio places the system barycenter outside either body at approximately 878 kilometers above Pluto's surface.
Why do scientists believe the Pluto satellite system formed from a collision?
Scientists suspect the entire satellite system formed from debris after a massive collision similar to the impact thought to have created Earth's Moon because high angular momenta within the system can only be explained by such a violent scenario rather than captured Kuiper Belt objects. Nearly circular orbits suggest these bodies formed during the collision event itself.
When will the next period of mutual events involving Charon occur?
The next period of mutual events involving Charon begins in October 2103 and peaks during 2110 before ending in January 2117. Only the hemisphere facing Charon experiences solar eclipses because tidal locking keeps that side fixed toward the companion.