The STEREO Encounter with the Tail of Comet Elenin and Expectations for ISON

The longest encounter to date of a space probe with a comet occurred in late July through mid August 2011 when the STEREO B (STB) observatory was immersed in the plasma environment of comet C/2010 X1 (Elenin). Closest approach was reached on July 31 at a distance of 7.4 million kilometers, with the nominal radial tail alignment occurring on August 12. Continuous observations of cometary ions are available for over three weeks. These ions were created over a broad range of solar wind conditions, including slow and fast solar wind, stream interaction regions, and an interplanetary coronal mass ejection. In the mass-per-charge range of water-group ions, the STB/PLASTIC instrument found that the major ion constituent was O+, with significant contributions from O+2 and C+. In the mass-per-charge range 23-36 amu/e there was a clear contribution by molecular ions. The composition is tracked on a daily basis and shows variations. There are also temporal variations in the speed distribution profiles. An abrupt decrease in the water-group ion intensity coincided with the passage of an interplanetary coronal mass ejection (ICME). STB/SECCHI imaged this same ICME as it interacted with the comet, observing a comet tail disconnection event. An opportunity for another comet tail passage by STEREO will occur with comet ISON in late November 2013. This comet's orbital plane is not near the ecliptic, hence this encounter, if it occurs, will be a shorter duration. Using the tools developed for the Elenin encounter, we will search for the ISON signatures in the in-situ data set and present any preliminary results available.