![]() Pluto's Companion![]() Several images of Pluto were elongated, and the pictures were thought to be defective. However, Christy found other photographs in which the bump on Pluto was in different positions. In fact, he found a series of photographs from 1970 that showed the bump progressing around Pluto in a 6-day period. Christy reasoned that Pluto must have a satellite. Using the information given to him by Christy, Robert Harrington made an estimate of the satellite's orbit , then used this to compute Pluto's mass. It was the first time astronomers could make a direct calculation like this -- Charon had been the breakthrough they needed. The mass of Pluto was estimated to be just two-tenths of one percent of the Earth. This was significant -- there was no way Pluto could have disturbed Uranus's motion to cause the observed discrepancies. Charon orbits Pluto in a North-South motion, revealing that Pluto's axis is tipped on its side, in the plane of its orbit. (Earth's axis is tilted 23 degrees to its orbital plane, which is why we have seasons.) In 1980, Charon passed in front of a star, which disappeared for fifty seconds and showed that Charon's diameter is about 1,200 kilometers. In 1985, Charon began a series of eclipses of Pluto that lasted five years. From these, astronomers measured the time it takes Charon to pass in front of and behind Pluto. Result: Pluto's diameter is 2,300 kilometers. These eclipses began just seven years after Charon was discovered, which was extremely fortunate. The next series of eclipses won't occur until the 22nd century. Had Charon been discovered twelve years later, astronomers would have missed this rare opportunity, and we might still be in the dark about Pluto.
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