Pluto

Pluto is the furthest planet from the Sun and the smallest, having smaller radius than both Neptune's largest moon Triton and Earth's Moon.

Pluto is the God of the Underworld, like Hades in Greek mythology.

Pluto's orbit is highly eccentric, meaning its path is elliptical. This ellipse crosses inside the orbit of Neptune, which means Neptune is sometimes the outermost planet. They will never collide because Pluto's orbit is inclined at 17.1 degrees to the ecliptic.

Semimajor axis (km) 5906.38x106 Perihelion (km) 4436.82x106 Aphelion (km) 7375.93x106 Mean orbital speed (km s-1) 4.72 Sidereal orbit period (days) 90465 Tropical orbit period (days) 90588 Synodic period (days) 366.73 Length of day (hours) 153.2820 Sidereal rotation period (hours) -153.2928 Obliquity to orbit (deg) 122.53

Semimajor axis (AU) 39.48168677 Eccentricity 0.24880766 Inclination (deg) 17.14175 Longitude of ascending node (deg) 110.30347 Longitude of perihelion (deg) 224.06676 Mean longitude (deg) 238.92881 Discoverer Clyde Tombaugh Date of Discovery 18 Feb 1930 J2000 Data from NSSDC

Pluto has never been visited by spacecraft and is the only planet left to go to. NASA launched New Horizons in January 2006, which will arrive at Pluto in 2015 and then go on to the Kuiper Belt.

Pluto has three moons; Charon, Nix and Hydra. Charon is roughly 0.1 of the mass of Pluto, but New Horizons will obtain more accurate measurements. Nix and Hydra are thought much smaller.

Surface temperature varies between 38K and 63K, with next to no atmosphere. The atmosphere may freeze around aphelion, but the composition is unknown, so we can't tell. The composition of Pluto is thought to be 2:1 rock to ice.

Mass (kg)	1.25x1022
Equatorial radius (km)	1195
Polar radius (km)	1195
Mean density (kg m-3)	1750
Equatorial surface gravity (m s-2)	0.58
Escape Velocity (km s-1)	1.2
Black-body temp (K)	37.5
Solar intensity (W m2)	0.89
Natural satellites	3

Pluto's existence was predicted by the discrepancy of predicted and actual movements of Neptune and Uranus, which turned out to be erroneous as Pluto is too small to have significant effect. Tombaugh did a sweep search at Lowell Observatory and found Pluto.