  Several concepts now exist for small, space-based missions to directly
characterize exoplanets in reflected light. Here, we develop an instrument
noise model suitable for studying the spectral characterization potential of a
coronagraph-equipped, space-based telescope. We adopt a baseline set of
telescope and instrument parameters appropriate for near-future planned
missions like WFIRST-AFTA, including a 2 m diameter primary aperture, an
operational wavelength range of 0.4-1.0 um, and an instrument spectral
resolution of 70, and apply our baseline model to a variety of spectral models
of different planet types, including Earth twins, Jupiter twins, and warm and
cool Jupiters and Neptunes. With our exoplanet spectral models, we explore
wavelength-dependent planet-star flux ratios for main sequence stars of various
effective temperatures, and discuss how coronagraph inner and outer working
angle constraints will influence the potential to study different types of
planets. For planets most favorable to spectroscopic characterization, we study
the integration times required to achieve moderate signal-to-noise ratio
spectra. We also explore the sensitivity of the integration times required to
either detect the bottom or presence of key absorption bands to coronagraph raw
contrast performance, exozodiacal light levels, and the distance to the
planetary system. Decreasing detector quantum efficiency at longer visible
wavelengths makes the detection of water vapor in the atmospheres of Earth-like
planets extremely challenging, and also hinders detections of the 0.89 um
methane band. Additionally, most modeled observations have noise dominated by
dark current, indicating that improving CCD performance could substantially
drive down requisite integration times. Finally, we briefly discuss the
extension of our models to a more distant future Large UV-Optical-InfraRed
(LUVOIR) mission.
