Close Pairings of Galilean Satellites Observed Using Speckle Interferometry

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During November-December 1998, a series of events occurred involving the Galilean satellites of Jupiter, where two satellites (usually Io and Europa, but sometimes Europa and Ganymede) passed within 5 arcsec of each other. Depending on the orbital geometry and closest separation (as close as 2.9 arcsec) the events lasted anywhere from 20 minutes to several hours. Since 5 arcsec roughly defines the atmospheric isoplanatic patch, attempts were made to observe these events using the speckle interferometry camera attached to the U.S. Naval Observatory (USNO) 26-inch refractor. The camera and associated software are normally used for precise measurements of the distance and position angle of binary star components. For the satellite events, the goal was to obtain very precise relative positions of the satellite pairs at specific times, as well as the time of apparent closest separation. Speckle observations of binary stars made from USNO typically yield positional accuracy of about 1 We successfully observed 4 out of a possible 8 events visible from USNO. Reduction of these observations is in progress. Despite the fact that the Galileans are resolved, not point sources, autocorrelations of the speckle patterns appear fairly strong. However, because of the relative motion of the satellites, only short integration times can be used, and it remains to be seen whether the signal-to-noise ratio will permit relative position measurements of useful precision. Close pairings of the Galilean satellites occur in series that are determined by the mutual resonances, within a geometric envelope defined by the apparent inclination of the orbital planes (i.e., Jupiter's equator) and distance. There is another series of events in May-June 1999, then again in January 2000. This technique may also be applicable to some of the Saturnian satellites near the time of ring-plane crossing. We invite other speckle interferometry groups to attempt observations of these events so that the usefulness of the technique can be better determined.

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