  We present an analytic model to estimate the capabilities of space missions
dedicated to the search for biosignatures in the atmosphere of rocky planets
located in the habitable zone of nearby stars. Relations between performance
and mission parameters such as mirror diameter, distance to targets, and radius
of planets, are obtained. Two types of instruments are considered: coronagraphs
observing in the visible, and nulling interferometers in the thermal infrared.
Missions considered are: single-pupil coronagraphs with a 2.4 m primary mirror,
and formation flying interferometers with 4 x 0.75 m collecting mirrors. The
numbers of accessible planets are calculated as a function of {\eta}earth. When
Kepler gives its final estimation for {\eta}earth, the model will permit a
precise assessment of the potential of each instrument. Based on current
estimations, {\eta}earth = 10% around FGK stars and 50% around M stars, the
coronagraph could study in spectroscopy only ~1.5 relevant planets, and the
interferometer ~14.0. These numbers are obtained under the major hypothesis
that the exozodiacal light around the target stars is low enough for each
instrument. In both cases, a prior detection of planets is assumed and a target
list established. For the long-term future, building both types of
spectroscopic instruments, and using them on the same targets, will be the
optimal solution because they provide complementary information. But as a first
affordable space mission, the interferometer looks the more promising in term
of biosignature harvest.
