Pluto and Charon Help (page 2)
Introduction to Pluto
Pluto , named after the god of the underworld, follows an eccentric orbit that ranges from slightly inside that of Neptune (29.7 AU) to about 50 AU at aphelion. Pluto and its moon, Charon , named after the ferry boatman who took dead souls to Pluto for judgment, receive 1/900 as much sunlight per unit area as Earth when the two are at perihelion. At aphelion, the system receives only 1/2500 as much sunlight per unit area as Earth.
Figure 7-10 illustrates the orbits of Pluto-Charon, Neptune, and Earth to scale. You might wonder if either Pluto or Charon will ever crash into Neptune. The answer is no because of a phenomenon called orbital resonance . The Pluto-Charon system makes exactly two solar orbits for every three orbits of Neptune; as a result, the two systems can never get any closer than 17 AU to each other. Unless some other celestial object intervenes and gravitationally upsets the orbit of Neptune or the orbit of Pluto-Charon, a cosmic collision will never take place.
The Year And The Day
The equatorial plane of Pluto intersects its orbital plane at an angle of 58 degrees, but the rotation of Pluto is retrograde. There are pronounced seasonal changes in the path of the Sun across the sky at any particular location; the Sun always appears to rise in the west and set in the east.
Pluto takes about 6 days and 9½ Earth hours to rotate once on its axis. Charon follows a prograde orbit (in the same direction as Pluto rotates) over Pluto’s equator and completes one orbit every Plutonian day, so Charon always stays over the same spot on Pluto. An observer on Pluto would see Charon hanging almost perfectly still in the sky. In addition, Charon, like most planetary moons, keeps the same side toward Pluto constantly.
The Pluto-Charon system takes 248 Earth years to make one complete journey around the Sun. Its orbit is a pronounced ellipse. Thus the variations in this system’s distance from the Sun, as well as its extreme axial tilt, affect the seasons. The maximum-to-minimum ratio of solar irradiation is about 2.8:1.
Pluto is approximately 2400 kilometers (1500 miles) in diameter; this is smaller than the Earth’s moon. Charon is about half the diameter of Pluto. The centers of the two objects are about 20,000 kilometers (12,500 miles) apart. If the Pluto-Charon system could be brought close to Earth for size comparison, the result would look like Fig. 7-11.
Both Pluto and Charon have about twice the density of water. This implies that they consist of a combination of ices and rocky materials. They may in fact be huge “dirty snowballs,” consisting of primordial matter that never accreted into an object large enough to properly be called a planet. Controversy has arisen here; an outspoken group of astronomers has expressed their belief that Pluto would not be called a planet if it were discovered today.
Pluto has a thin atmosphere consisting largely of nitrogen. However, astronomers think that this atmosphere, which is on the order of one-millionth the density of Earth’s atmosphere at the surface, exists only when Pluto is near perihelion. When the system moves farther from the Sun, the atmosphere is believed to freeze onto the surface. Because Pluto’s gravitation is weak, the atmosphere extends to considerable distances from the planet, enveloping Charon. The atmosphere might be blown into a teardrop shape by the solar wind, in much the same way as a comet’s tail is blown away from the Sun.
Although no probe has yet flown near Pluto, images of the planet have been obtained through the Hubble Space Telescope. The surface is pinkish red; this is thought to be caused by the presence of methane ice. There are bright and dark regions, with the south polar region being especially reflective. High-resolution images of Pluto and Charon will be obtained when and if a close flyby is made. Some astronomers believe that when this happens, Pluto and Charon might be reclassified as a double comet.
What Makes A Planet?
Astronomers have been searching for a large planet beyond Neptune ever since Neptune itself was discovered. Pluto is not massive enough to account for observed aberrations in the orbits of Uranus and Neptune. Perhaps such a “Planet X” does not exist, and the so-called perturbations in the orbits of Uranus and Neptune are caused by some unseen (or unseeable) object or effect. Maybe “Planet X” is a large, massive object with albedo (reflectivity) so low that we cannot see it even with the largest Earth-based telescopes.
Astronomers are almost certain that there are thousands or millions of asteroids and dormant comets in solar orbits beyond the orbit of Neptune. This Kuiper Belt is a disk-shaped swarm of primordial rocks and “dirty snowballs”; the Oort Cloud is a larger, spherical congregation of such objects that encloses the Solar System like a bubble. Every once in a while, an object from one of these swarms undergoes a gravitational interaction or collision with another object and is hurled into the main part of the Solar System. If the object passes near Neptune or Uranus, the gravitation of the large planet can send it diving toward the Sun. A few decades later, we on Earth discover a new asteroid or comet.
We might say that in order to be a planet, a celestial object must be spherical, must orbit the Sun (and not some other planet), and must be larger than a certain diameter (say, 500 kilometers) or have more than a certain amount of gravitation (say, 5 percent that of the Earth). However, no official standard yet exists. Depending on the set of criteria adopted, assuming scientists ever agree on one, Pluto-Charon may be “demoted” to the status of a double comet or else hundreds, maybe thousands, of objects now considered primordial matter will be reclassified as planets.
Practice problems of this concept can be found at: The Outer Planets Practice Problems
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