  In the search for life in the Universe, exoplanets represent numerous natural
experiments in planet formation, evolution, and the emergence of life. This
raises the fascinating prospect of evaluating cosmic life on a statistical
basis. One key statistic is the occurrence rate of life-bearing worlds, $f_{\rm
L}$, the 'frequency of life' term in the famous Drake Equation. Measuring
$f_{\rm L}$ would give profound insight into how common life is and may help to
constrain origin-of-life theories. I propose $f_{\rm L}$ as the goal for the
DRAKE mission (Dedicated Research for Advancing Knowledge of Exobiology): a
transit spectroscopy survey of M-dwarf habitable zone terrestrial planets. I
investigate how the uncertainty on the observed value of $f_{\rm L}$ scales
with sample size. I determine that sampling error dominates over observational
error and that the uncertainty is a function of the observed $f_{\rm L}$ value.
I show that even small sample sizes can provide significant constraints on
$f_{\rm L}$, boding well for the transit spectroscopy approach. I perform a
feasibility study of the DRAKE mission using a nominal instrument design and
mission plan. Due to low observing efficiencies, DRAKE may need to be
incorporated into a wider-ranging deep-space or lunar observatory. A 50-planet
survey could constrain $f_{\rm L}$ to $\leq$ 0.06 (at 95% confidence) if the
sample $f_{\rm L}$ = 0, or 0.03-0.2 if the sample $f_{\rm L}$ = 0.1. This can
be achieved (on average) in 10 years using a 17-m telescope with an
unrestricted field-of-regard. DRAKE is a viable approach to attempting the
first experimental measurement of $f_{\rm L}$.
