Comet 21P/Giacobini-Zinner: Water Production Activity over 20 Years with SOHO/SWAN

TL;DR

This study analyzes 20 years of water production activity of Comet 21P/Giacobini-Zinner using SOHO/SWAN data, revealing consistent activity levels over time with some outbursts, and provides a detailed power-law model of water production rates.

Contribution

It offers a comprehensive comparison of water production rates across four apparitions, extending previous observations and refining the power-law model for comet activity over time.

Findings

Water production rates remained relatively stable over 20 years.

Outbursts were observed in 2012 but did not significantly alter overall activity.

Power-law fits describe water production as a function of heliocentric distance.

Abstract

In 1985 Comet 21P/Giacobini-Zinner was the first comet visited by a spacecraft, the International Cometary Explorer (ICE) satellite, several months before the armada of Halley spacecraft had their encounters in 1986. ICE was originally the ISEE-3 satellite, designed for magnetospheric measurements near the Earth, and was diverted via a lunar gravity assist to pass through the plasma tail of the comet. The comet has been observed by the all-sky hydrogen Lyman-alpha Solar Wind Anisotropies (SWAN) camera on the SOlar and Heliospheric Observatory satellite during its last four apparitions in 1998, 2005, 2012 and 2018. This paper compares water production rates calculated from the hydrogen images from the 1998 and 2005 results, published by Combi et al. (2011), with new observations from 2012 and 2018. Unlike some comets that have faded over time, except for 2 outbursts seen in the 2012 results, the activity levels for Comet 21P have not changed consistently over time. A power-law fit to the pre-perihelion water production rate vs. heliocentric distance for all four apparitions gives a result of 3.75x10^28 x r^-0.28+/-0.4, and that for the post-perihelion production rate, discounting 2012 data dominated by outbursts, gives 4.15 x 10^28 x r^-10.2+/-0.9. The production rates are in s^-1 and heliocentric distances are in AU.