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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 a planet.

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.

1/28/99 JP