http://spaceflightnow.com/news/n0601/04charon/ - Rare opportunity seized to measure Pluto's large moon
--- In a paper released in the January 5, 2006, edition of Nature, the MIT-Williams collaboration determined Charon's radius to be 606 ± 8 km. For perspective, this radius is roughly twice the width of Massachusetts with an error of only 5 miles. The size was combined with mass measurements from Hubble Space Telescope data to establish a density for Charon of 1.72 g/cm3. This density, roughly 1/3 that of the Earth, reflects Charon's rocky-icy composition.
"We also find that Charon contains roughly 10% less rock by mass than Pluto. This difference suggests that either, or both, objects involved in a Charon-forming collision had concentrations of heavier materials in their cores." A collisional formation like this has a parallel in theories for the formation of the Earth-Moon system.---
VLT press release:
http://www.eso.org/outreach/press-rel/pr-2006/pr-02-06.html - Measuring the Size of a Small, Frost World
--- This accuracy now allows astronomers to pin Charon's density down to 1.71 that of water, indicative of an icy body with about slightly more than half of rocks. Quite remarkably, Charon's density is now measured with much more precision than Pluto's. ---
In a http://www.sciam.com/article.cfm?chanID=sa003&articleID=000B6146-4A5A-13BC-8A5A83414B7F0000 on the this, the author states
Similar occultations could provide data on Sedna and 2003UB313. That, in turn, could decide whether they are planets, because the IAU is working on a definition based on minimum size.
I wonder if he has inside information about this -- last time there were rumors the IAU committee on the definition of a planet was nearing a decision, it came to naught. Hmmm...
Astrophysics, abstract
astro-ph/0602082
From: Michael Person [view email]
Date: Fri, 3 Feb 2006 17:35:45 GMT (651kb)
Charon's radius and density from the combined data sets of the 2005 July 11 occultation
Authors: M. J. Person, J. L. Elliot, A. A. S. Gulbis, J. M. Pasachoff, B. A. Babcock, S. P. Souza, J. Gangestad
Comments: 25 pages including 4 tables and 2 figures. Submitted to the Astronomical Journal on 2006 Feb 03
The 2005 July 11 C313.2 stellar occultation by Charon was observed by three separate research groups, including our own, at observatories throughout South America. Here, the published timings from the three data sets have been combined to more accurately determine the mean radius of Charon: 606.0 +/- 1.5 km. Our analysis indicates that a slight oblateness in the body (0.006 +/- 0.003) best matches the data, with a confidence level of 86%. The oblateness has a pole position angle of 71.4 deg +/- 10.4 deg and is consistent with Charon's pole position angle of 67 deg.
Charon's mean radius corresponds to a bulk density of 1.63 +/- 0.07 g/cm3, which is significantly less than Pluto's (1.92 +/- 0.12 g/cm3). This density differential favors an impact formation scenario for the system in which at least one of the impactors was differentiated.
Finally, unexplained differences between chord timings measured at Cerro Pachon and the rest of the data set could be indicative of a depression as deep as 7 km on Charon's limb.
http://arxiv.org/abs/astro-ph/0602082
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