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On July 14, NASA’s New Horizon’s spacecraft passed within 8,000 miles of Pluto, the first spacecraft to take a close look at our system’s most distant member.  Before now, most information regarding Pluto came from Earth-based telescopes. Nobody even knew what color it was until New Horizon came close enough. In fact, until recently we knew more about the far reaches of the universe than the outer edges of our own solar system.

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Pluto was not discovered until 1930, when Clyde Tombaugh examined distortions in the orbits of Neptune and Uranus that were first noticed by Percival Lowell and tracked the source to Pluto.

As is often the case, many aspects of Tombaugh’s description were completely wrong. For example, he estimated Pluto’s diameter as 14,000 km (nearly six times it’s actual size), described its appearance as “black as coal” (its reddish-grey) and its density as “twice as dense as Earth’s densest surface rocks” (completely false).

Naming the new body provoked additional debate. One suggestion was to name it Lowell after its predictor. Kronos, the Roman variant of Saturn, was proposed, so having two different Saturns at once was seriously considered. Even Hoover was considered (and quickly dismissed).

Tombaugh may have been way off-base, but there were good reasons for his confusion. Part of the reason Tombaugh’s size estimate was so large is the presence of Pluto’s main satellite, Charon. Charon is nearly half the size of Pluto itself and was not discovered until 1978. Charon is so large relative to Pluto, in fact, that it behaves more like a co-planet than a moon. The two objects routinely eclipse each other, an extremely rare occurrence. Pluto is covered in Methane ice, which made Pluto appear brighter—and larger—than it really is. Pluto now has five known satellites, the most recent of which was not discovered until 2012.

Pluto’s darkest hour was actually triggered by a completely different discovery, Eris. In 2005, astronomers discovered an icy body orbiting at a slightly greater distance than Pluto. Closer inspection revealed that Eris might be slightly larger than Pluto, causing astronomers to rethink the definition of planet. After much soul searching, the decision was made to downgrade Pluto from planet to dwarf planet, a new category created for the occasion. The decision was met with fury and tears from Pluto’s supporters.

The New Horizon mission is already filling in many gaps. In addition to color, new information about Pluto’s atmosphere, geography, and geologic history are coming in daily. Pluto’s exact size has been calculated, and it turns out to be slightly larger than Eris after all. (Pluto’s dwarf status is nevertheless unchanged.)  In a final tribute, the probe also contains a portion of Clyde Tombaugh’s ashes. Safe journey, Dr. Tombaugh.


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The Science News-Letter, Vol. 17, No. 467 (Mar. 22, 1930), p. 178
Society for Science & the Public
Science News, Vol. 132, No. 13 (Sep. 26, 1987), p. 207
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Science News, Vol. 170, No. 15 (Oct. 7, 2006), p. 237
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