  LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a
small, robust spectro-polarimeter with a mission to observe the Earth as an
exoplanet. Detecting Earth-like planets in stellar habitable zones is one of
the key challenges of modern exoplanetary science. Characterising such planets
and searching for traces of life requires the direct detection of their
signals. LOUPE provides unique spectral flux and polarisation data of sunlight
reflected by the Earth, the only planet known to harbor life. This data will be
used to test numerical codes to predict signals of Earth-like exoplanets, to
test algorithms that retrieve planet properties, and to fine-tune the design
and observational strategies of future space observatories. From the Moon,
LOUPE will continuously see the entire Earth, enabling it to monitor the signal
changes due to the planet's daily rotation, weather patterns, and seasons,
across all phase angles. Here, we present both the science case and the
technology behind LOUPE's instrumental and mission design.
