PLUTO

Pluto is the farthest planet from the Sun and the smallest. Pluto is smaller than seven of the solar system's moons. Its orbit is 5,913,520,000 km (39.5 AU) from the Sun, diameter is 2274 km and its mass is 1.27e22 kg.

Although Pluto was discovered in 1930, limited information has delayed a realistic understanding of its characteristics. Today Pluto remains the only planet that has not been visited by a spacecraft, yet an increasing amount of information is unfolding about this planet. The uniqueness of Pluto's orbit, rotational relationship with its satellite, spin axis, and light variations all give the planet a certain appeal.

Pluto is usually farther from the Sun than any of the nine planets; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249 year orbit.

As Pluto approaches perihelion it reaches its maximum distance from the eclpitic due to its 17-degree inclination. Thus, it is far above or below the plane of Neptune's orbit. Under these conditions, Pluto and Neptune will not collide and do not approach closer than 18 A.U. to one another.

Pluto's rotation period is 6.387 days, the same as its satellite Charon. Although it is common for a satellite to travel in a synchronous with its planet, Pluto is the only planet to rotate synchronously with the orbit of its satellite. Thus being tidally locked, Pluto and Charon continuously face each other as they travel through space.

Pluto's equator is now the view seen from Earth. Unlike most planets, Pluto rotates with its poles almost in its orbital plane. Pluto's rotational axis is tipped 122 degrees. When Pluto was first discovered, its relatively bright south polar region was the view seen from the Earth. Pluto appeared to grow dim as our viewpoint gradually shifted from nearly pole-on in 1954 to nearly equator-on in 1973.

During the period from 1985 through 1990, Earth was aligned with the orbit of Charon around Pluto such that an eclipse could be observed every Pluto day. This provided opportunity to collect significant data which led to albedo maps defining surface reflectivity, and to the first accurate determination of the sizes of Pluto and Charon.

The first eclipses began blocking the north polar region. Later eclipses blocked the equatorial region, and final eclipses blocked Pluto's south polar region. By carefully measuring the brightness over time, it was possible to determine surface features. It was found that Pluto has a highly reflective south polar cap, a dimmer north polar cap, and both bright and dark features in the equatorial region. Pluto's geometric albedo is is much brighter than Charon's. Pluto's average density lies between 1.8 and 2.1 grams per cubic centimeter. It is concluded that Pluto is 50% to 75% rock mixed with ices. Pluto's icy surface is 98% nitrogen (N2). Methane (CH4) and traces of carbon monoxide (CO) are also present. The solid methane indicates that Pluto is colder than 70 Kelvin. Pluto's temperature varies widely during the course of its orbit since Pluto can be as close to the sun as 30 AU and as far away as 50 AU. There is a thin atmosphere that freezes and falls to the surface as the planet moves away from the Sun. The atmospheric pressure deduced for Pluto's surface is 1/100,000 that of Earth's surface pressure.

Pluto can be seen with an amateur telescope but it is not easy.

CHARON

Charon is Pluto's only known satellite. Its orbit is 19,640 km from Pluto, its diameter 1172 km. and its mass 1.90e21 kg.

Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.

The 1978 discovery of Pluto’s satellite, Charon, was a breakthrough in gaining understanding of Pluto. By measuring the distance between the two and knowing the orbital period, it was possible to calculate for their mass. Charon’s mass is about 9.6 x 10-10 solar masses. This is about 1/7th the mass of Pluto.

During the 1980’s, Earth crossed the orbital plane of Charon and a series of mutual events were observed that significantly enhanced the study of the Pluto-Charon system. The mutual events led to the first accurate determination of the sizes of Pluto and Charon. Charon’s diameter is 1,172 kilometers (728 miles), just over half the size of Pluto. Charon is the largest satellite relative to its planet in the solar system. Their average separation is 19,640 km (12,200 miles). Charon’s orbit is gravitationally locked with Pluto, so both bodies continually keep the same hemisphere facing each other.

Charon’s surface composition is different from Pluto's. It appears to be covered with water-ice rather than nitrogen ice. Charon’s density is 1.2 to 1.3 g/cm3, indicating it contains little rock. Pluto’s density is 1.8 to 2.1 g/cm3. The differences in density tell us that Pluto and Charon formed independently. This rules out the idea that they accreted together as a double planet. The accuracy of Charon’s density is still being challenged by observers from ground based telescopes. Some theorize that Charon was formed when a planetesimal slammed into Pluto long ago, similar to the idea considered as the origin event of Earth’s moon.

It is thought possible that Charon, Neptunes’ moon Triton and Pluto are icy worlds of the outer solar system that were not swept up by Uranus and Neptune or ejected from the solar system. These three appear to be relatives. Charon and Triton could have become satellites and the largest could have become Pluto.

Although there is yet much to be learned, observation of mutual events and other studies provided new data that has greatly enhanced our understanding of Charon, Pluto, and the outer solar system.