  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.
