  We have analyzed Cs-137 decay data, obtained from a small sample onboard the
MESSENGER spacecraft en route to Mercury, with the aim of setting limits on a
possible correlation between nuclear decay rates and solar activity. Such a
correlation has been suggested recently on the basis of data from Mn-54 decay
during the solar flare of 13 December 2006, and by indications of an annual and
other periodic variations in the decay rates of Si-32, Cl-36, and Ra-226. Data
from five measurements of the Cs-137 count rate over a period of approximately
5.4 years have been fit to a formula which accounts for the usual exponential
decrease in count rate over time, along with the addition of a theoretical
solar contribution varying with MESSENGER-Sun separation. The indication of
solar influence is then characterized by a non-zero value of the calculated
parameter \xi, and we find \xi=(2.8+/-8.1)x10^{-3} for Cs-137. A simulation of
the increased data that can hypothetically be expected following Mercury orbit
insertion on 18 March 2011 suggests that the anticipated improvement in the
determination of \xi could reveal a non-zero value of \xi if present at a level
consistent with other data.
