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The Planetary Society Blog
By Emily Lakdawalla
Charon's Diameter, Redux
Jan. 6, 2006 | 11:44 PST | 19:44 UTC
When I went to DPS last year I reported on a new diameter for Charon that had been calculated based upon careful measurements performed during a stellar occultation by Charon on July 10-11, 2005. That new diameter came from a talk given by Bruno Sicardy and was 1,205 +/- 2 kilometers. On the night of July 10-11, the 6.5-meter Clay Telescope at Las Campanas Observatory in Chile spotted Pluto, Charon, and a faint star. At left, Pluto (top) and the star (bottom) are visible; the star overwhelms the dim light from Charon. At center, Charon has crossed in front of the star, casting a shadow across the Clay telescope; only the light of Pluto (top) and Charon (bottom) are visible. At right, after less than a minute, Charon has completed its transit, and the star is again visible. Images: James Elliot, Jay Pasachoff, and others |
Talks given at scientific conferences provide the world with a first look at the latest research, but these numbers aren't really considered official until they are published in a peer-reviewed scientific journal. That publication has finally happened, and guess what? The number they published isn't quite the same. In fact, two papers appeared in Nature last week dealing with this question, and they report similar but not identical numbers based on simultaneous observations of the event from different sets of telescopes in South America.So if someone asked me what Charon's diameter is, what would I say? 1,205? 1,207? 1,212? Well, don't ignore those little numbers after the "+/-" symbols on these size measurements. Those numbers don't mean that Charon's diameter varies by that much (although, who knows? it probably does vary by a couple of kilometers from one place to another). Those "+/-" numbers are error bars on the measurements. That is to say, Sicardy's measurements pinpoint the actual diameter to be somewhere between 1,204.4 and 1,210.0 kilometers. Gulbis' measurements put the diameter somewhere between 1,196 and 1,228 kilometers. It is extremely good news that Sicardy's and Gulbis' ranges of possibilities overlap. The fact that they overlap helps to reinforce their results, suggesting that they both probably did a good job analyzing their data, and that they both probably correctly bracketed Charon's size.
So how big is Charon? It's safe to say it's "about 1,210 kilometers" in diameter -- and also to say that New Horizons will really help finally to pin this number down for good! Pluto's diameter isn't known that precisely either, for that matter.
By the way, both papers also looked for any trace of a detection of any kind of atmosphere around Charon -- the atmosphere would have dimmed the star just a bit before Charon occulted it completely. And neither one detected an atmosphere. That's interesting, because Charon is more than half the size of Pluto, and Pluto has an atmosphere, but we've never detected one at Charon. Looking for Charon's atmosphere, if it exists at all, is one of the objectives of the New Horizons mission.
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