Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits are aligned so that they can never collide or even approach closely. Several factors contribute to this.
At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed from above, it is also the farthest above the ecliptic. This means Pluto's orbit actually passes about 8 AU above that of Neptune, preventing a collision.[74][75][76] Pluto's ascending and descending nodes, the points at which its orbit crosses the ecliptic, are currently separated from Neptune's by over 21°.[77]
However, this alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) such as orbital precession would adjust Pluto's orbit so that a collision could be possible over millions of years. Some other mechanism or mechanisms must therefore be at work. The most significant of these is that Pluto lies in the 3:2 mean motion resonance with Neptune: for every three of Neptune's orbits around the Sun, Pluto makes two. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is configured so 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 distance ahead of Pluto. In fact, Pluto and Neptune's minimum separation is over 17 AU. Pluto actually comes closer to Uranus (11 AU) than it does to Neptune.[76]
The 3:2 resonance between the two bodies is highly stable, and is preserved over millions of years.[78] 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.[76]
Other factors
Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto's and Neptune's orbits does not change.[74][79] However, 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 (in addition to the 3:2 mean motion resonance).
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°.[79] 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,[74] which relates the eccentricity of an orbit to its inclination, relative 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.[78]
Second, the longitudes of ascending node 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°, 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.[74]
To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-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. This moves Pluto into a slightly larger orbit, where it travels slightly slower, in accordance with 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 (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 once again at the original node. The whole process takes about 20,000 years to complete.[76][78]
Satellites
Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005.[80]
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. In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty."[81], although others have pointed out the possibility of additional objects, including a small ring system.[82]
Charon
The Pluto-Charon system is noteworthy for being the largest of the solar system's few binary systems, defined as those whose barycentre lies above the primary's surface (617 Patroclus is a smaller example).[83] This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet.[84] The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon. If one were standing on Pluto's near side, Charon would hover in the sky without moving; if one were to travel to the far side, one would never see Charon at all.[85] 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.[86]
Nix and Hydra
Two additional moons of Pluto were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005, and received provisional 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.[87]
These small moons orbit Pluto at approximately two and three times the distance of Charon: 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, and are very close to (but not in) 4:1 and 6:1 mean motion orbital resonances with Charon.[88]
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. Sizes are estimated from albedos. The moons' spectral similarity to Charon suggests a 35% albedo similar to Charon's; this value results in diameter estimates of 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. At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% of Pluto's.[89]
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. 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.[82]
Similar conclusions have been made from occultation studies.[90] In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km (or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometres.[91]
| Name | Discovery Year | Diameter (km) | Mass (kg) | Orbital radius (km) (barycentric) | Orbital period (d) | |
|---|---|---|---|---|---|---|
| Pluto | /ˈpluːtoʊ/ | 1930 | 2,390 (70% Moon) | 13,050 × 1018 (18% Moon) | 2 040 (0.6% Moon) | 6.3872 (25% Moon) |
| Charon | /ˈʃærən/, /ˈkeɪrən/ | 1978 | 1,205 (35% Moon) | 1,520 × 1018 (2% Moon) | 17,530 (5% Moon) | |
| Nix | /ˈnɪks/ | 2005 | 88 | 1 × 1018 | 48,708 | 24.9 |
| Hydra | /ˈhaɪdrə/ | 2005 | 72 | .391 × 1018 | 64,749 | 38 |
Kuiper belt
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 was heavily criticized because, as explained by its orbit, Pluto never comes near the planet.[92]
Pluto's true place in the Solar System began to reveal itself only in 1992, when astronomers found a population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This belt, known as the Kuiper belt after one of the astronomers who first speculated on the nature of a trans-Neptunian population, is believed to be the source of many short-period comets. Astronomers now believe Pluto to be the largest[10] of the known Kuiper belt objects (KBOs). Like other 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.[93] If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.[94]
Though Pluto is the largest of the Kuiper belt objects discovered so far, 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.[95] 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 the scattered disc.
A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.[96]
Formation
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. 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. The objects in the scattered disc, a dynamically unstable region beyond the Kuiper belt, are believed to have been placed in their current positions by interactions with Neptune's migrating resonances.[97] 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 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. 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.[98] 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.[99] 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.[98]
Exploration
Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto.[100] No serious attempt to explore Pluto via spacecraft occurred until the last decade of the 20th century. In August 1992, JPL scientist Robert Staehle telephoned Pluto's discoverer, Clyde Tombaugh, requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later remembered, "though he's got to go one long, cold trip."[101] Despite this early momentum, in 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.[102]
After an intense political battle, a revised mission to Pluto, dubbed New Horizons, was granted funding from the US government in 2003.[103] New Horizons was launched successfully on January 19, 2006. The mission leader, S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.[104]
In early 2007 the craft made use of a gravity assist from Jupiter. Its closest approach to Pluto will be on July 14, 2015; scientific observations of Pluto will begin 5 months prior to closest approach and will continue for at least a month after the encounter. New Horizons captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI).[105] The images, taken from a distance of approximately 4.2 billion kilometres, confirm the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper Belt objects.
New Horizons will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterise the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyse Pluto's neutral atmosphere and its escape rate. New Horizons will also photograph the surfaces of Pluto and Charon.
Discovery of moons Nix and Hydra may present unforeseen challenges for the probe. Debris from collisions between Kuiper belt objects and the smaller moons, with their relatively low escape velocities, may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for micrometeoroid damage that could disable the probe.[82]
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