  SARA experiment aboard the first Indian lunar mission Chandrayaan-1 had the
objective to explore the solar wind-lunar interaction using energetic neutral
atoms (ENA) from the lunar surface as diagnostic tool. SARA consisted of an ENA
imaging mass analyzer CENA (Chandrayaan-1 Energetic Neutral Analyzer) and an
ion mass analyser SWIM (Solar Wind Monitor), along with a digital processing
unit (DPU) which commands and controls the sensors and provides the interface
to the spacecraft. Both sensors have provided excellent observational data.
CENA has observed ENAs from the lunar surface and found that ~20% of the
incident solar wind ions get backscattered as ENAs from the lunar surface. This
is contrary to the previous assumptions of almost complete absorption of solar
wind by the lunar surface. The observation is relevant for other airless bodies
in the solar system.
