In the past decade NASA’s Kepler Mission has revealed that small potentially habitable exoplanets are common. The Transiting Exoplanet Survey Satellite has found many planets in our Solar neighborhood, and the James Webb Space Telescope, and other future missions, will be able to study using detailed spectra to understand the conditions and compositions of their atmospheres, and ultimately whether there are any detectable indicators of life. In order to interpret these observations, we need detailed models of what the atmosphere of an inhabited planet might look like. The best available source of “ground truth” for such models would be satellite observations taken of the whole Earth. While plenty of data from current Earth climate monitoring satellites exist, there are limited observations of the unresolved Earth seen as an exoplanet. Planetary missions like Voyager, Galileo LCROSS, and EPOXI have trained their instruments towards the Earth to take whole-Earth spectra, but due to their serendipitous nature they lack the full range of variability of the Earth.
We describe here the design of a small satellite mission dedicated to taking whole-Earth spectra and polarimetric images in order to measure the temporal variability of observations from an inhabited planet with illumination phase, rotation, cloud cover, and seasons. The data would help improve models of habitable planets, guide analysis of exoplanet atmospheric measurements, and inform instrumentation for future NASA missions to search for signs of life on exoplanets.
The mission will be composed of a SmallSat or CubeSat class spacecraft equipped with visible, near-infrared (NIR), and near-ultraviolet (NUV) spectrometers as well as a polarimeter with imaging capabilities. The spacecraft will be deployed beyond Low Earth Orbit and will collect data for a minimum of one year to ensure continuous coverage of the Earth from all phase angles and seasons. Current preliminary work is being done from a systems-level approach to determine top-level mission requirements such as operational orbit, subsystems, mass, power, and detailed instrument requirements for the established science goals. These include a science traceability study, trade studies and simulations.