Experiments showing a seasonal variation of the nuclear decay rates of a
number of different nuclei, and decay anomalies apparently related to solar
flares and solar rotation, have suggested that the Sun may somehow be
influencing nuclear decay processes. Recently, Cooper searched for such an
effect in 238Pu nuclei contained in the radioisotope thermoelectric
generators (RTGs) on board the Cassini spacecraft. In this paper we modify and
extend Cooper's analysis to obtain constraints on anomalous decays of
238Pu over a wider range of models, but these limits cannot be applied to
other nuclei if the anomaly is composition-dependent. We also show that it may
require very high sensitivity for terrestrial experiments to discriminate among
some models if such a decay anomaly exists, motivating the consideration of
future spacecraft experiments which would require less precision.Comment: 8 pages, 4 figures (to appear in Astroparticle Physics