Moons of Pluto

Two additional moons were imaged by astronomers of the Pluto Companion Search Team preparing for the New Horizons mission and working with the Hubble Space Telescope on 15 May 2005, which received the provisional designations S/2005 P 1 and S/2005 P 2.

[14] An intense search conducted by New Horizons confirmed that no moons larger than 4.5 km in diameter exist out to distances up to 180,000 km from Pluto (6% of the stable region for prograde moons), assuming Charon-like albedoes of 0.38 (for smaller distances, this threshold is still smaller).

[15] The orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0.4° and eccentricities less than 0.005.

Small-body impacts could eject debris off of the small moons which can form into a ring system.

[18] All of the outer circumbinary moons are also close to mean motion resonance with the Charon–Pluto orbital period.

This suggests that Nix and Hydra were instead captured material, formed around Pluto–Charon, and migrated inward until they were trapped in resonance with Charon.

[18][23] However, New Horizons imaging found that they had not tidally spun down to near a spin synchronous state where chaotic rotation or tumbling would be expected.

It could actually be possible to spend a day on Nix in which the sun rises in the east and sets in the north.

The nearly circular orbits of the smaller moons suggests that they were also formed in this collision, rather than being captured Kuiper Belt objects.

Their grey color is different from that of Pluto, one of the reddest bodies in the Solar System.

This is thought to be due to a loss of volatiles during the impact or subsequent coalescence, leaving the surfaces of the moons dominated by water ice.

[1] An alternative hypothesis is that the collision happened at about 2,000 miles per hour, not powerful enough to destroy Charon and Pluto.

The faster rotation of Pluto back then, with one rotation every third hour, would have created a centrifugal force stronger than the gravitational attraction between the two bodies, which made Charon separate from Pluto, but remained gravitationally bound with each other.

Charon, which is massive enough to have collapsed into a spheroid under its own gravitation, is highlighted in light purple.

Charon has an angular diameter of 4 degrees of arc as seen from the surface of Pluto; the Sun appears much smaller, only 39 to 65 arcseconds.

This proximity further ensures that a large proportion of Pluto's surface can experience an eclipse.

These are much larger than the Sun's angular diameter, so total solar eclipses are caused by these moons.

As such, Styx has no annular eclipses, its widest axis being more than 10 arcseconds larger than the Sun at its largest.

However, Kerberos, although slightly larger, cannot make total eclipses as its largest minor axis is a mere 32 arcseconds.

The next period of mutual events due to Charon will begin in October 2103, peak in 2110, and end in January 2117.

During this period, solar eclipses will occur once each Plutonian day, with a maximum duration of 90 minutes.