  Recent discoveries imply that Early Mars was habitable for
life-as-we-know-it; that Enceladus might be habitable; and that many stars have
Earth-sized exoplanets whose insolation favors surface liquid water. These
exciting discoveries make it more likely that spacecraft now under construction
- Mars 2020, ExoMars rover, JWST, Europa Clipper - will find habitable, or
formerly habitable, environments. Did these environments see life? Given finite
resources (\$10bn/decade for the US ), how could we best test the hypothesis of
a second origin of life? Here, we first state the case for and against flying
life detection missions soon. Next, we assume that life detection missions will
happen soon, and propose a framework for comparing the value of different life
detection missions:
  Scientific value = (Reach x grasp x certainty x payoff) / \$
  After discussing each term in this framework, we conclude that scientific
value is maximized if life detection missions are flown as hypothesis tests.
With hypothesis testing, even a nondetection is scientifically valuable.
