Pluto's ascending and descending nodes

Pluto's ascending and descending nodes, the points at which its orbit crosses the ecliptic, are currently separated from Neptune's by over 21°.^[64]This alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) could alter aspects of Pluto's orbit (such as its orbital precession) over millions of years so that a collision could be possible. Sony VAIO VGN-BZ560 Battery Some other mechanism or mechanisms must therefore be at work. The most significant of these is that Pluto lies in the 2:3 mean motion resonance with Neptune: for every two orbits that Pluto makes around the Sun, Neptune makes three. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. Sony VAIO VGN-BZ560N24 Battery This pattern is such that, in each 500-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 a similar distanceahead of Pluto. Pluto and Neptune's minimum separation is over 17 AU. Pluto comes closer to Uranus (11 AU) than it does to Neptune.^[63] Sony VAIO VGN-BZ560N30 Battery The 2:3 resonance between the two bodies is highly stable, and is preserved over millions of years.^[65] This prevents their orbits from changing relative to one another; the cycle always repeats in the same way, and so the two bodies can never pass near to each other. Thus, even if Pluto's orbit were not highly inclined the two bodies could never collide. Sony VAIO VGN-BZ560P Battery Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto and Neptune's orbits does not change.There are several other resonances and interactions that govern the details of their relative motion, and enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 2:3 mean motion resonance). Sony VAIO VGN-BZ560P20 Battery First, Pluto's argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates around 90°.^[66] This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the Solar System, preventing encounters with Neptune. This is a direct consequence of the Kozai mechanism, Sony VAIO VGN-BZ560P22 Battery which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° (= 90°–38°). The closest such angular separation occurs every 10,000 years.^[65] Sony VAIO VGN-BZ560P28 Battery Second, the longitudes of ascending nodes of the two bodies—the points where they cross the ecliptic—are in near-resonance with the above libration. When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, Sony VAIO VGN-BZ560P30 Battery and it comes closest to the Sun at its peak above Neptune's orbit. In other words, when Pluto most closely intersects the plane of Neptune's orbit, it must be at its farthest beyond it. This is known as the 1:1 superresonance, and is controlled by all the Jovian planets.^[61] To understand the nature of the libration, imagine a polar point of view, Sony VAIO VGN-BZ560P34 Battery looking down on the ecliptic from a distant vantage point where the planets orbitcounter-clockwise. After passing the ascending node, Pluto is interior to Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune's expense. Sony VAIO VGN-BZ561 Battery This moves Pluto into a slightly larger orbit, where it travels slightly slower, according to Kepler's third law. As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit Sony VAIO VGN-BZ561N20 Battery (near the opposing node to where we began). The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune again at the original node. The whole process takes about 20,000 years to complete. Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the New Horizonsspacecraft is expected to arrive there. Sony VAIO VGN-BZ561P20 Battery Pluto's visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion.^[2] To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.^[70] It looks star-like and without a visible disk even in large telescopes, because its angular diameter is only 0.11".Sony VAIO VGN-BZ562P Battery The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Sony VAIO VGN-BZ563P Battery Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time. Current maps have been produced from images from the Hubble Space Telescope(HST), which offers the highest resolution currently available, and show considerably more detail, Sony VAIO VGN-BZAAFS Battery resolving variations several hundred kilometres across, including polar regions and large bright spots.^[74] The maps are produced by complex computer processing, which find the best-fit projected maps for the few pixels of the Hubble images.^[75] The two cameras on the HST used for these maps are no longer in service, so these will likely remain the most detailed maps of Pluto until the 2015 flyby of New Horizons.^[75] Sony VAIO VGN-BZAAHS Battery These maps, together with Pluto's lightcurve and the periodic variations in its infrared spectra, reveal that Pluto's surface is remarkably varied, with large changes in both brightness and colour.^[76] Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moon Iapetus. Sony VAIO VGN-BZAANS Battery The colour varies between charcoal black, dark orange and white:^[77] Buie et al. term it "significantly less red than Mars and much more similar to the hues seen on Io with a slightly more orange cast".^[74] Pluto's surface has changed between 1994 and 2002-3: the northern polar region has brightened and the southern hemisphere darkened. Sony VAIO VGN-BZAAPS Battery Pluto's overall redness has also increased substantially between 2000 and 2002.^[77] These rapid changes are probably related to seasonal condensation and sublimation of portions of Pluto's atmosphere, amplified by Pluto's extreme axial tilt and high orbital eccentricity.^[77] Spectroscopic analysis of Pluto's surface reveals it to be composed of more than 98 percent nitrogen ice, Sony VAIO VGN-CS118E/Q Battery with traces of methane and carbon monoxide.^[78] The face of Pluto oriented toward Charon contains more methane ice, while the opposite face contains more nitrogen and carbon monoxide ice. Observations by the Hubble Space Telescope place Pluto's density at between 1.8 and 2.1 g/cm³, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass. Sony VAIO VGN-CS118E/R Battery Because decay of radioactive minerals would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. The diameter of the core should be around 1,700 km, 70% of Pluto's diameter. Sony VAIO VGN-CS118E/W Battery It is possible that such heating continues today, creating a subsurface ocean layer of liquid water some 100 to 180 km thick at the core–mantle boundary.^[80][82] The DLR Institute of Planetary Research calculated that Pluto's density-to-radius ratio lies in a transition zone, along with Neptune's moonTriton, between icy satellites like the mid-sized moons of Uranus and Saturn, and rocky satellites such as Jupiter'sEuropa. Sony VAIO VGN-CS11S/P Battery Pluto's mass is 1.31×10²² kg, less than 0.24 percent that of the Earth,^[84] while its diameter is 2,306 (± 20) km, or roughly 66% that of the Moon.^[8] Pluto's atmosphere complicates determining its true solid size within a certain margin.^[9] Pluto's albedo varies from 0.49–0.66. The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Sony VAIO VGN-CS11S/Q Battery Once Charon's gravitational effect was measured, Pluto's true mass could be determined. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter more accurately, while the invention of adaptive optics allowed them to determine its shape more accurately. Sony VAIO VGN-CS11S/W Battery Among the objects of the Solar System, Pluto is much less massive than the terrestrial planets, and at less than 0.2 lunar masses, it is also less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object in the asteroid belt. Sony VAIO VGN-CS11Z/R Battery It is less massive than the dwarf planet Eris, a trans-Neptunian object discovered in 2005. Given the error bars in the different size estimates, it is currently unknown whether Eris or Pluto has the larger diameter.^[87] Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km. Sony VAIO VGN-CS120J/P Battery Determinations of Pluto's size are complicated by its atmosphere, and possible hydrocarbon haze. Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, which are derived from the ices of these substances on its surface.^[89] Its surface pressure ranges from 6.5 to 24 μbar. Sony VAIO VGN-CS120J/Q Battery Pluto's elongated orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out, and fall to the ground. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate into gas. This creates an anti-greenhouse effect; Sony VAIO VGN-CS120J/R Battery much as sweat cools the body as it evaporates from the surface of the skin, this sublimation cools the surface of Pluto. Scientists using the Submillimeter Array have recently discovered that Pluto's temperature is about 43 K (−230 °C), 10 K colder than would otherwise be expected.^[91] The presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with average temperatures 36 K warmer 10 km above the surface. Sony VAIO VGN-CS190JTB Battery The lower atmosphere contains a higher concentration of methane than its upper atmosphere.^[92] The first evidence of Pluto's atmosphere was first suggested by N. Brosch and H. Mendelson of the Wise Observatory in Israel in 1985,^[93] and then definitively detected by the Kuiper Airborne Observatory in 1988, from observations of occultations of stars by Pluto.^[94] Sony VAIO VGN-CS190JTP Battery When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually.^[93] From the rate of dimming, the atmospheric pressure was determined to be 0.15 pascal, roughly 1/700,000 that of Earth.^[95] In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory, ^{Sony VAIO VGN-CS190JTQ Battery} James L. Elliot of MIT,^[97]and Jay Pasachoff of Williams College.^[98] Surprisingly, the atmospheric pressure was estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, ^{Sony VAIO VGN-CS190JTR Battery} causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the north pole's now continuously dark ice cap.^[99] Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto's atmosphere. ^{Sony VAIO VGN-CS190JTW Battery} Another stellar occultation was observed by the MIT-Williams College team of James Elliot, Jay Pasachoff, and aSouthwest Research Institute team led by Leslie Young on June 12, 2006 from sites in Australia.^{[100] Sony VAIO VGN-CS190NAB Battery} In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery ofethane on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere. ^{Sony VAIO VGN-CS190NAC Battery} Pluto has five known natural satellites: Charon, first identified in 1978 by astronomer James Christy; Nix and Hydra, both discovered in 2005,^[102] S/2011 (134340) 1 (provisional name, also known as P4), identified by the Hubble Space Telescope in 2011,^[103] and S/2012 (134340) 1 found in 2012 and referred to as P5.^{[104] Sony VAIO VGN-CS190NAD Battery} The Plutonian moons are unusually close to Pluto, compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere radius, the stable gravitational zone of Pluto's influence. For example,Psamathe orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. ^{Sony VAIO VGN-CS190NBB Battery} In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty",^[105] although others have pointed out the possibility of additional objects, including a small ring system. The Pluto–Charon system is noteworthy for being one of the Solar System's few binary systems, ^{Sony VAIO VGN-CS190NCA Battery} defined as those whose barycentre lies above the primary's surface (617 Patroclus is a smaller example, the Sun and Jupiter the only larger one).^[107] This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet.^[108] The system is also unusual among planetary systems in that each is tidally locked to the other: ^{Sony VAIO VGN-CS190NCB Battery} Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured.^[109] Because of this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of gravity.^{[67] Sony VAIO VGN-CS190NCC Battery} Just as Pluto revolves on its side relative to the orbital plane, so the Pluto–Charon system does also.^[68] In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. Two additional moons of Pluto were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005, ^{Sony VAIO VGN-CS50B/W Battery} and receivedprovisional designations of S/2005 P 1 and S/2005 P 2. The International Astronomical Union officially named Pluto's newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra (Pluto III, the outer moon, formerly P 1), on June 21, 2006.^[111] These small moons orbit Pluto at approximately two and three times the distance of Charon: ^{Sony VAIO VGN-CS51B/W Battery} Nix at 48,700 kilometres and Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon. Observations of Nix and Hydra to determine individual characteristics are ongoing. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. ^{Sony VAIO VGN-CS52JB/W Battery} Sizes are estimated from albedos. If the moons' albedo is similar to Charon's at 35%, then their diameters can be estimated at 46 kilometres for Nix and 61 kilometres for the brighter Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km, respectively. ^{Sony VAIO VGN-CS60B/P Battery} At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% of Pluto's.^[112] The discovery of the two small moons suggests that Pluto may possess a variable ring system. Small-body impacts can create debris that can form into planetary rings. ^{Sony VAIO VGN-CS60B/Q Battery} Data from a deep optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope suggest that no ring system is present. If such a system exists, it is either tenuous like the rings of Jupiter or is tightly confined to less than 1,000 km in width.^[106] Similar conclusions have been made from occultation studies. ^{Sony VAIO VGN-CS60B/R Battery} Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. This notion has been heavily criticised because Pluto never comes near Neptune in its orbit.^[119] Pluto's true place in the Solar System began to reveal itself only in 1992, ^{Sony VAIO VGN-CS61B/P Battery} when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is believed to be the source of many short-period comets. Astronomers now believe Pluto to be the largest^[h] member of the Kuiper belt, a somewhat stable ring of objects located between 30 and 50 AU from the Sun. ^{Sony VAIO VGN-CS61B/Q Battery} Like other Kuiper-belt objects (KBOs), Pluto shares features with comets; for example, the solar wind is gradually blowing Pluto's surface into space, in the manner of a comet.^[120] If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.^[121] Though Pluto is the largest of the Kuiper belt objects discovered so far, ^{Sony VAIO VGN-CS61B/R Battery} Neptune's moon Triton, which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is believed to be a captured Kuiper belt object.^[122] Eris (see below) is also larger than Pluto but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called thescattered disc. ^{Sony VAIO VGN-CS62JB/P Battery} A large number of Kuiper belt objects, like Pluto, possess a 2:3 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.^[123] Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal; a component of the original protoplanetary disc around the Sun that failed to fully coalesce into a full-fledged planet. ^{Sony VAIO VGN-CS62JB/Q Battery} Most astronomers agree that Pluto owes its current position to a sudden migration undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself, which became its moon Triton, locking others into resonances and knocking others into chaotic orbits. ^{Sony VAIO VGN-CS62JB/R Battery} The objects in the scattered disc, a dynamically unstable region overlapping the Kuiper belt, are believed to have been placed in their current positions by interactions with Neptune's migrating resonances.^[124] A 2004 computer model by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice suggested that the migration of Neptune into the ^{Sony VAIO VGN-CS71B/W Battery} Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. ^{Sony VAIO VGN-CS72JB/W Battery} The resultant expulsion of objects from the proto-Kuiper belt could also explain the Late Heavy Bombardment 600 million years after the Solar System's formation and the origin of Jupiter's Trojan asteroids.^[125] It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration perturbed it into a resonant capture. ^{Sony VAIO VGN-CS90HS Battery} The Nice model requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk; these may have included the bodies which became Triton and Eris. ^{Sony VAIO VGN-CS90NS Battery,Sony VAIO VGN-CS90S Battery,Sony VAIO VGN-CS91HS Battery}