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JET PROPULSION LABORATORY
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Contact: Jane Platt (818) 354-0880
PLUTO TO BECOME MOST DISTANT PLANET
What's the significance of February 11, 1999 for Pluto?
On February 11, Pluto will move farther from the Sun than
Neptune, regaining its status as the most distant planet in the
solar system. JPL astronomers calculate that it will take place
at 2:08 am Pacific Time. Pluto will maintain its title of "most
distant planet" for the next 228 years. Neptune has been the
farthest planet for the past 20 years (since February 7, 1979).
Why is Pluto sometimes the farthest planet from the Sun, and
other times the second-farthest planet from the Sun?
Unlike the other planets in our solar system, Pluto has a highly
elliptical orbit, completing its journey around the Sun every 248
years. Thus, Pluto's distance from the Sun varies. Most of the
time, Pluto is the farthest planet from the Sun, but for a short
time during its orbit, Pluto is closer to the Sun than Neptune.
Any chance Pluto and Neptune will collide when their orbits cross
on Feb. 11?
No chance at all. Pluto goes around the Sun twice for every
three times Neptune orbits the Sun. Because of this fact, Pluto
and Neptune's positions relative to each other repeat every 497
years. They will never be close to each other when Pluto is
crossing the same distance from the Sun as Neptune is, and
therefore, a collision can't happen. The high inclination of
Pluto's orbit relative to the other planets also contributes to
keeping them apart.
Why is there controversy about Pluto?
In some ways, Pluto is different. It's much smaller than the
four inner planets (Mercury, Venus, Earth and Mars) and it
doesn't fit in with the four gas giant outer planets (Jupiter,
Saturn, Uranus, and Neptune). Pluto's diameter is 1, 430 miles,
making it less than half the size of any other planet, and only
two-thirds as big as Earth's Moon. Pluto's orbit is much more
tilted and elliptical than the other planets. Some scientists
believe Pluto should not be called a planet at all. They feel it
should be put in the same category as Kuiper Disk objects, icy
worlds smaller than Pluto that lie in the "same neighborhood" as
Pluto and Neptune, and even beyond. These objects may be
leftover debris from our solar system's early formation. But
Pluto is spherical and it does orbit the Sun. Although this
controversy has come up on occasion, Pluto is still classified as
What else do we know about Pluto?
Pluto was discovered on February 18, 1930 by Clyde Tombaugh, who
studied photographic plates taken of the night sky through a
Lowell Observatory telescope. Pluto's moon Charon was first
found in 1978. Pluto is made from a mixture of rocky and icy
material, and it appears to have seasonal changes, but we don't
know much else about it.
Does NASA plan further studies of Pluto?
Observations from ground and orbiting telescopes will continue to
glean some information from 4.5 billion kilometers' distance
(about 2.8 billion miles). Detailed study requires a close-up
look. A mission called Pluto-Kuiper Express, managed by JPL,
would fly past Pluto and its moon Charon, and study Kuiper Disk
objects. The mission would launch in 2004, and would take about
ten years to reach its destination.