Pluto has been demoted from planethood, but before that determination and after, it has been carefully studied. As much as a point of light on a photographic plate can be studied. Clyde Tombaugh really picked a needle out of a celestial haystack:
Would you catch that without the arrows? Neither would I.
Seth Barnes Nicholson, discoverer of four satellites of Jupiter, studied that faint dot after Clyde Tombaugh’s discovery. His analysis suggested that faraway Pluto had about the mass of the Earth. A respectable planet, to be sure.
As the years passed, astronomers were able to discern a bit more about that white spot. Gerard Kuiper, another satellite discoverer, whittled Pluto’s size estimate down to about a tenth that of Earth–about the size of Mars. No idea how he did that. Other astronomers saw enough of a shift in the light to determine that Pluto spun on its axis giving it a day of about 153 hours. How they got that from a small white dot on a photographic plate, I have no idea on that either.
Two things happened by the mid-1970’s. First, NASA was looking at targeting Pluto for one of its Grand Tour missions to the outer planets. Second, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii determined that Pluto’s surface was largely bright methane ice. And because a bright faraway object was likely smaller than a dark body at the same distance, Pluto’s size estimate was further reduced to about one-hundredth of the Earth. A bit smaller than the moon.
Enter James Christy. In 1978, the astronomer, a double star specialist, was looking at photographic plates snapped by the 61-inch telescope at the US Naval Observatory in Flagstaff, Arizona (USNOF). He found a bulge in the grainy image of planet Pluto. Since his specialty was observing distant stars revolve around each other, his best judgment was that Pluto had a moon. It orbited once every 6.39 days, the same as what astronomers thought Pluto’s day was.
Do you see the satellite in the discovery plate? It’s the lump appearing on different sides of the planet:
Christy checked some of the photographic plates in storage at the USNOF. Way back in 1965, someone marked an image, “Pluto image elongated.” But astronomers just assumed that it was a smudge or some error, human or instrument. Guess that might teach some people to jump to conclusions …
Most scientists agreed with Christy. But a few thought it might be a big mountain on the planet. After all, a lump is a lump, right? All skepticism was silenced in 1985, when the tilt of orbits permitted astronomers to verify, through changing light levels, that Pluto actually had a moon. And more, very rough maps were produced. By the time the Hubble Space Telescope was in orbit, there was no doubt, as you can see from this 1994 image:
No mountain, that.
Once scientists had verified a body in orbit around Pluto, it’s an easy matter to “weigh” both planet and satellite. Alas, Pluto came out on the short end again. The final determination was that it would take five-hundred Plutos to balance the scales with Earth. That new satellite was pretty hefty, relatively speaking. Twelfth largest in the solar system.
Let’s get to the name. Astronomers who discover things often get to name the object. It must pass muster with the IAU (International Astronomical Union). But as long as one keeps to the conventions, it’s likely to get approved. Christy’s colleagues at the USNOF were pushing for Persephone, the consort of the Roman god Pluto. But the discoverer wanted to honor his wife Charlene, familiarly known as Char. As it turns out, Charon is the name of the ferry operator in the Greek myths about the underworld. Greek pronunciation, however, is the hard “k.” Charon was approved. Though Mrs Christy is honored by many English-speaking astronomers who have picked up the soft “sh” sound. I pronounce it “sh.” I can appreciate honoring a wife.
I will likely not ever be a satellite discoverer. Asteroid, possibly. The list of asteroid names is ample. My wife’s is not on the list, though my daughter’s name is attached to minor planet number 51599. I don’t know how James Christy thought of his wife. If I were to indulge my imagination, my wife would be my star around which I spin. Maybe my wife and daughter a double star, and I would be a planet revolving around both. It doesn’t take much imagination to ponder that. I think you need more to see a lump on a black spot and think “satellite” rather than “mountain” or “mistake.”
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