At certain orbital inclinations, Venus Express goes through the shadow of the planet and experiences an eclipse. The Sun-Venus-spacecraft orbit geometry is such that these eclipses occur approximately every three months, and the duration of each of these ‘seasons’ is about one month. Eclipses present a unique science opportunity. As the spacecraft enters and leaves the eclipse, its instruments can observe sunlight passing through the venusian atmosphere, making it possible to indirectly study the composition and structure of the atmosphere itself.
Full article published on the ESA Science and Technology website on 6 November 2012.