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Not so Distant Pluto

Week of September 22, 1997

image of Pluto Another quick quiz: which planet is currently farthest from the Sun?

If you said Pluto, sorry, that's incorrect. Right now, Neptune is farther from the Sun than Pluto (so Neptune will have to wait until next week to be snacked upon). Pluto's orbit is very elliptical, so much so that although it spends most of the time farther from the Sun than Neptune, it does spend the odd decade or two every orbit closer to the Sun. Pluto gave up its title of farthest back in 1979, and will reclaim it in 1999. At first thought, you might conclude that its orbit must intersect Neptune's, causing an eventual collision. However, that's two-dimensional thinking; actually, the orbits are tilted and never intersect.

Even though Pluto may be a hair warmer than usual these days, it's not exactly a balmy vacation spot. At a distance of 6 billion kilometers, its surface temperature is only a few degrees above absolute zero. With a diameter of 2300 kilometers, Pluto looks so tiny from Earth that even the Hubble Space Telescope barely sees it as more than a point of light.

However, a few years back an extraordinary occurrence took place. Pluto has a very large moon named Charon, which revolves around Pluto in a very tight orbit (actually, the two both circle a common center of gravity). Over the course of a few months, as seen from the Earth the two objects actually passed in front of each other as they revolved around each other. By measuring the brightness of the eclipsing events, astronomers Buie, Tholen and Horne were actually able to make a map of the surface of Pluto and Charon. Think of it this way: let's say we know in advance (from very careful measurements) that part of Pluto will be blocked by Charon. When we make a measurement, we see Pluto's light drops dramatically. We can guess that a bright spot was covered. If the amount of light only dips a bit, then we conclude a dark spot was covered. This method may be a bit crude, but by doing this over and over again, a map of Pluto's and Charon's surfaces was made that actually revealed more detail than we could otherwise ever see.

The reason I picked this topic is a personal one. I am delighted by clever people. I love it when someone comes along and says, "Hey, I have an idea..." and it works. It simply makes me happy that such people exist.


Want to know more about Pluto? The Nine Planets is loaded with info. You can skip right to the Pluto pages too.

I have a small bit about Pluto in my Bad News section too!



©2008 Phil Plait. All Rights Reserved.

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