Pluto Revealed

      

      As it turned out, Pluto was within 6 degrees 
      of the position predicted by both Lowell and by William Pickering's 1919 
      estimation. (Pickering revised this in 1928, getting much further off the 
      mark than his earlier calculation.) It looked as if, once again, a solution 
      to a nearly unsolvable problem had been mathematically determined and subsequently 
      proven by an actual discovery. But that was far from clear.
      
 Lowell 
        had predicted that Pluto's mass would be 6.6 times that of the Earth, 
        while Pickering believed it to be twice the mass of the Earth. As more 
        observations were made of the diminutive planet it became evident that 
        Pluto had to have a mass much less than Lowell's or Pickering's predictions. 
        This was a dilemma: Pluto's mass could not account for the gravitational 
        disturbances in Uranus's motion, yet those very disturbances had been 
        used to predict Pluto's position. Had Tombaugh really found Lowell's Planet 
        X? 
      
 Little was discovered about Pluto over the next forty years. Telescopes 
        improved, but the planet remained a mystery. In 1955, Merle Walker and 
        Robert Hardie at the Lowell Observatory measured Pluto's brightness, which 
        varied regularly, to determine that Pluto's period of rotation was 6.4 
        Earth days. But what of its size? 
      
 In 1950, Gerard Kupier used the 200-inch telescope at Mount Palomar 
        and for the first time was able to see Pluto as a globe. He concluded 
        that Pluto's diameter was less than 5,900 kilometers (the Earth's diameter 
        is 12,756 kilometers). Then, in 1965, Pluto was observed as its orbit 
        passed near a star. If Pluto was large enough, it would cover the star 
        -- an occultation. It missed. This near miss confirmed that Pluto's diameter 
        had to be less than 6,800 kilometers, independently verifying Kupier's 
        measurements. Pluto was even smaller than Mars. 
      
 Nothing was known of Pluto's composition until 1976, when astronomers 
        at the Kitt Peak Observatory made the first near-infrared photometric 
        measurements of the planet. The spectroscopic analysis showed Pluto to 
        be covered with frozen methane, which is highly reflective. But with so 
        reflective a surface, Pluto should appear brighter to us, if it was of 
        a certain size. To appear so dim, Pluto had to be smaller still than previously 
        thought.
      
 In 1988 Pluto passed in front of a star, the first true occultation 
        observed since Pluto had been discovered. But rather than winking out 
        suddenly, the star gradually disappeared and reappeared. Pluto had an 
        atmosphere. This was assumed to be a thin layer of methane gas, but observations 
        in 1992 revealed it to contain mostly nitrogen, with trace amounts of 
        methane and carbon monoxide. 
      
 
      

        Next: Pluto's Companion
        
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