PLUTO

NASA Image of Pluto and Moons

Pluto and its moons

     Pluto lies in the outer reaches of our solar system in a region classified as the Kuiper belt.  The Kuiper Belt is composed of thousands of icy, solid objects extending from near the orbit of Neptune to over 5 billion miles from the Sun.  Pluto is one of the largest bodies in the Kuiper Belt with enough mass to exhibit a spherical shape.  Even though Pluto has enough mass to give it a spherical shape, Pluto is in fact smaller in physical size than seven of the solar system's moons, including the Earth's.  Pluto rotates on its axis every 6.39 days and takes 247.8 years to revolve around the Sun.  It has three known satellites named Charon, Hydra and Nix.  Charon is the most interesting as it is about half the size of Pluto.  Charon orbits Pluto every 6.39 days and also rotates once during this time frame, matching Pluto's rotation.  Therefore, Pluto and Charon are acting like a dumbbell in space, rotating around each other in a near perfect lock-step.  This is the closest thing to a binary-planet system that we know about.

Atmosphere and Weather:   Since there have been no spacecraft probes to fly-by Pluto, not much is known about this icy planet (the NASA probe New Horizons will reach Pluto in 2015).  Much of what scientists know are observations made from Charon eclipsing Pluto and Pluto's chance encounter in occulting (covering up) a star.  When Pluto occulted a star, a haze was revealed suggesting that Pluto likely has an extremely thin atmosphere.  This atmosphere is probably made up of a blend of nitrogen and methane.  These elements may exist as a gas when Pluto is closest to the Sun (at perihelion), but would then freeze as Pluto moves further away.  Analysis indicates that the surface of Pluto is composed of about 98% nitrogen with small amounts of methane and carbon monoxide.  There is some evidence that Pluto's tenuous atmosphere may be slowly escaping into space, possibly moving to Charon.  When Pluto occulted the above mentioned star, the haze layer was also theorized to indicate that Pluto has a temperature inversion, possibly indicating that Pluto has some weather.  The surface temperature of Pluto has been estimated in a range from -378 to -396 degrees F. 

Like Uranus, Pluto is highly tilted at 122.5 degrees.  This would give one side of the planet extremely long periods of darkness or light, depending on that side's orientation to the Sun.  However it is likely that the planet's temperature is very uniform around the globe.  This is due to the fact that the Sun casts such feeble light at that distance and Pluto's atmosphere is almost non-existent. 

QUICK FACTS
(
Data is from NASA Goddard)

Average distance from Sun 3.67 billion miles
Perihelion 2.75 billion miles 
Aphelion 4.53 billion miles
Sidereal Rotation 6.39 Earth days
Length of Day 6.39 Earth days
Sidereal Revolution 247.8 Earth years
Diameter at Equator 1,482 miles (smallest planet)
Tilt of axis 122.5 degrees
Moons 3 known
Atmosphere Probably nitrogen and methane  
Discoverer Clyde Tombaugh
Discovery Date February 18, 1930

DEFINITIONS:

Average distance from Sun:  Average distance from the center of a planet to the center of the Sun. 
Perihelion:  The point in a planet's orbit closest to the Sun.
Aphelion:  The point in a planet's orbit furthest from the Sun. 
Sidereal Rotation:  The time for a body to complete one rotation on its axis relative to the fixed stars such as our Sun.  Earth's sidereal rotation is 23 hours, 57 minutes.
Length of Day:  The average time for the Sun to move from the Noon position in the sky at a point on the equator back to the same position.  Earth's length of day = 24 hours
Sidereal Revolution:  The time it takes to make one complete revolution around the Sun.
Axis tilt:  Imagining that a body's orbital plane is perfectly horizontal, the axis tilt is the amount of tilt of the body's equator relative to the body's orbital plane.  Earth is tilted an average of 23.45 degrees on its axis.


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