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Pluto is actually smaller
than one of Neptune's moons, Triton. Pluto is usually the farthest known planet
from the Sun. It has a very unusual
orbit. Once every 248 Earth years, Pluto swings inside
the orbit of Neptune. It stays there for twenty years. During those twenty
years, Pluto is closer to the Sun than Neptune. While it is closer to the Sun,
Pluto has an atmosphere. The methane and nitrogen frozen at the poles thaw out, rise, and temporarily form an
atmosphere. As it moves toward its farthest point from the Sun, Pluto's
atmosphere freezes and falls back on the surface of the planet. Pluto has only
one known moon. Pluto's moon, Charon, is half the size of Pluto. Some
astronomers call Pluto and Charon a double planet
because they are so close in size. Other astronomers do not think Pluto is a
real planet at all. They think it may be a moon which escaped from Neptune's
gravitational pull
.
Facts:
(1) Pluto is too faint to
be seen with the naked eye.
(2) When viewed through a telescope, it looks like a star.
(3) Stellar occultation's have revealed a tenuous atmosphere on Pluto composed
primarily of nitrogen and methane gas. Pluto is cold: -233° C (-390 ° F), just
40° C (72 ° F) above absolute zero. At this temperature, all elements would be
frozen but neon, hydrogen, and helium.
(4) Measurements
indicate Pluto is the smallest planet, 2320 kilometres (1440 miles) in diameter.
It is smaller than Earth's Moon.
Discoverer:
Clyde Tombaugh - January 23,
1930Spacecraft Encounter: none
Average distance from Sun 39.48 AU 5.906 billion km
(3.670 billion mi):
Length of year:
248.5 years
Rotation period:
6.4 days
Average orbital velocity: 4.7
km/sec (3 mi/sec)
Inclination of axis:
122.46 degrees
Diameter:
2340 km (1454 mi)
Number of satellites:
1
Comparisons
with Earth:
Average distance from Sun: 39.5 x Earth
Diameter:
0.18 x Earth
Mass:
0.002 x Earth
Density:
0.36 x Earth
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