Chandrayaan-1 was India's first lunar probe. The mission lasted from October 2008 until the end of August 2009. The spacecraft (see Fig 1) orbited the Moon initially in a 100 km circular orbit, which was later raised to 200 km. The Sub-keV Atom Reflecting Analyser (SARA) is one of the ten scientific instruments of Chandrayaan-1. SARA is an instrument designed to investigate the interaction between the solar wind plasma and the lunar surface. SARA was the result of a collaboration between the Swedish Institute of Space Physics (IRF), the University of Bern (UBe), the Japanese Aerospace Exploration Agency (JAXA), and the Indian Space Research Organisation (ISRO).
SARA consists of two sensors (mass spectrometers): the Solar Wind Ion Monitor (SWIM) and the Chandrayaan-1 Energetic Neutral Analyzer (CENA) (see Fig 2). Whereas SWIM measures ions of solar wind origin in the energy range [10 eV– 15 keV], CENA measures neutral atoms coming from the lunar surface in the energy range [10 eV–3.3 keV]. Both sensors have similar designs: 1) a rejection part ensures that only particles with certain characteristics can enter the system, 2) an ionization subsystem converts neutrals to ions where necessary, 3) an electrostatic analyzer is responsible for energy analysis, and 4) a time-of-flight system providing mass analysis. The Space Research & Planetary Sciences division was not only involved in the design of the instrument, but also delivered part of the hardware of the instrument. Amongst these parts are the technology responsible of ionizing neutrals (see Fig 3) and part of the time-of-flight mass analyser (see Fig 4).
All data collected during SARA's lifetime have been analysed by the University of Bern and the other before-mentioned institutes to investigate the solar wind – lunar surface interaction from various viewpoints. The main scientific discoveries include (1) the detection of a high lunar surface reflection rate of impinging solar wind ions as neutrals, (2) the dependence of the hydrogen reflection ratio on the local crustal magnetic fields (see Fig 5), (3) the determination of the energy spectra of backscattered neutralized solar wind protons, (4) the use of the spectral shape to remotely define an electric potential above a lunar magnetic anomaly, (5) the favouring of backscattering over forward-scattering of impinging solar wind hydrogen particles, (6) the first-ever measurements of sputtered lunar oxygen, (7) the first-ever observation of backscattered solar wind helium, (8) the determination of the scattering properties of backscattered solar wind hydrogen measured when the Moon transversed Earth's magneto-sheath, and (9) the discovery of protons in the near-lunar wake.
