International audienceThe MAVEN spacecraft launched in November 2013, arrived at Mars in September 2014, and completed commissioning and began its one-Earth-year primary science mission in November 2014. The orbiter’s science objectives are to explore the interactions of the Sun and the solar wind with the Mars magnetosphere and upper atmosphere, to determine the structure of the upper atmosphere and ionosphere and the processes controlling it, to determine the escape rates from the upper atmosphere to space at the present epoch, and to measure properties that allow us to extrapolate these escape rates into the past to determine the total loss of atmospheric gas to space through time. These results will allow us to determine the importance of loss to space in changing the Mars climate and atmosphere through time, thereby providing important boundary conditions on the history of the habitability of Mars. The MAVEN spacecraft contains eight science instruments (with nine sensors) that measure the energy and particle input from the Sun into the Mars upper atmosphere, the response of the upper atmosphere to that input, and the resulting escape of gas to space. In addition, it contains an Electra relay that will allow it to relay commands and data between spacecraft on the surface and Earth

[1] Observed electron distribution functions of the solar wind permanently exhibit three different components: a thermal core and a suprathermal halo, which are always present at all pitch angles, and a sharply magnetic field aligned ''strahl'' which is usually antisunward moving. Whereas Coulomb collisions can explain the relative isotropy of the core population, the origin of the halo population, and more specifically the origin of its sunward directed part, remains unknown. In this study we present the radial evolution of the electron velocity distribution functions in the fast solar wind between 0.3 and 1.5 AU. For this purpose we combine data measured separately by the Helios, Wind, and Ulysses spacecraft. We compute average distributions over distance and normalize them to 1 AU to remove the effects of the solar wind expansion. Then we model separately the core, halo, and strahl components to compute their relative number density or fraction of the total electron density. We observe that, while the core fractional density remains roughly constant with radial distance, the halo and strahl fractional densities vary in an opposite way. The relative number of halo electrons is increasing, while the relative number of strahl electrons is decreasing with distance. Therefore we provide, for the first time, strong evidences for a scenario that is commonly assumed: the heliospheric electron halo population consists partly of electrons that have been scattered out of the strahl.