The stratosphere accumulates the bulk of meteoric and cometary materia
l that impacts the Earth and provides a measure of that flux. Recent m
odels and measurements of the effective age of stratospheric air demon
strate our ability to simulate this buildup. Current observations of H
2O in the stratosphere and mesosphere are consistent with an extraterr
estrial source of H2O, but limit its influx to less than 2 Tg/yr. Such
a flux is consistent with standard estimates but is 100 times less th
an the small-comet hypothesis.