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Although Pluto was discovered in 1930, limited information on the distant object delayed a realistic understanding of its characteristics. Pluto is the second largest known dwarf planet and tenth largest orbiting the Sun. From its time of discovery in 1930 to 2006 it was considered to be the ninth planet in the solar system, but because additional objects have been discovered including Eris which is 27% more massive, the IAU reclassified Pluto and the other objects as dwarf planets. The New Horizons spacecraft was launched on January 16, 2006 and will make its closest approach to Pluto on July 14, 2015. This mission will provide an increased amount of information about this peculiar dwarf planet. The uniqueness of Pluto’s orbit, rotational relationship with its satellite, spin axis, and light variations all give it a certain appeal.

Pluto is usually farther from the Sun than any of the eight planets; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249 year orbit. Pluto crossed Neptune’s orbit January 21, 1979, made its closest approach September 5, 1989, and remained within the orbit of Neptune until February 11, 1999. This will not occur again until September 2226.

As Pluto approaches perihelion it reaches its maximum distance from the ecliptic 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 orbit with its planet, Pluto rotates synchronously with the orbit of its satellite. Thus being tidally locked, Pluto and Charon continuously face each other as they travel through space.

Unlike most planets, but similar to Uranus, 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. Pluto’s equator is now the view seen from Earth.

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, including all the numbers that could be calculated therefrom.

The first eclipses (mutual events) 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 0.49 to 0.66, which is much brighter than Charon. Charon’s albedo ranges from 0.36 to 0.39.

The eclipses lasted as much as four hours and by carefully timing their beginning and ending, measurements for their diameters were taken. The diameters can also be measured directly to within about 1 percent by more recent images provided by the Hubble Space Telescope. These images resolve the objects to clearly show two separate disks. The improved optics allow us to measure Pluto’s diameter as 2,274 kilometers (1413 miles) and Charon’s diameter as 1,172 kilometers (728 miles), just over half the size of Pluto. Their average separation is 19,640 km (12,200 miles). That’s roughly eight Pluto diameters.

Average separation and orbital period are used to calculate Pluto and Charon’s masses. Pluto’s mass is about 6.4 x 10-9 solar masses. This is close to 7 (was 12 x’s) times the mass of Charon and approximately 0.0021 Earth mass, or a fifth of our moon.

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. Charon’s density is 1.2 to 1.3 g/cm3, indicating it contains little rock. The differences in density tell us that Pluto and Charon formed independently, although Charon’s numbers derived from HST data are still being challenged by ground based observations. Pluto and Charon’s origin remains in the realm of theory.

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 was officially labeled the ninth planet by the International Astronomical Union in 1930 and named for the Roman god of the underworld. It was the first and only planet to be discovered by an American, Clyde W. Tombaugh. It has since been reclassified as a Dwarf Planet along with Eris and Ceres.

The path toward its discovery is credited to Percival Lowell who founded the Lowell Observatory in Flagstaff, Arizona and funded three separate searches for “Planet X.” Lowell made numerous unsuccessful calculations to find it, believing it could be detected from the effect it would have on Neptune’s orbit. Dr. Vesto Slipher, the observatory director, hired Clyde Tombaugh for the third search and Clyde took sets of photographs of the plane of the solar system (ecliptic) one to two weeks apart and looked for anything that shifted against the backdrop of stars. This systematic approach was successful and Pluto was discovered by this young (born 4 Feb 1906) 24 year old Kansas lab assistant on February 18, 1930. Pluto is actually too small to be the “Planet X” Percival Lowell had hoped to find. Pluto’s was a serendipitous discovery.

Pluto Statistics
Discovered byClyde W. Tombaugh
Date of discoveryFebruary 18, 1930
Mass (kg)1.27e+22
Mass (Earth = 1)2.125e-03
Equatorial radius (km)1,137
Equatorial radius (Earth = 1)0.1783
Mean density (gm/cm^3)2.05
Mean distance from the Sun (km)5,913,520,000
Mean distance from the Sun (Earth = 1))39.5294
Rotational period (days)-6.3872
Orbital period (years)248.54
Mean orbital velocity (km/sec)4.74
Orbital eccentricity0.2482
Tilt of axis (degrees)122.52
Orbital inclination (degrees)17.148
Equatorial surface gravity (m/sec^2)0.4
Equatorial escape velocity (km/sec)1.22
Visual geometric albedo0.3
Magnitude (Vo)15.12
Atmospheric compositionMethane