International audienceThe current dry state of Venus indicates that most of its water has been lost to space by a combination of early hydrodynamic and ongoing neutral and ion loss processes. The abundance and HDO/H2O ratio of the remaining water is regulated by hydrogen and deuterium escape to space, but there remains substantial uncertainty in the rate of present-day H loss, the relative importance of neutral H vs. H+ ion escape, and the fractionation factor of this escape. The interpretation of planned lower atmosphere measurements depends critically on these unconstrained upper atmosphere processes. For O and other heavier species, ion loss has likely played a major role, but existing measurements of ion escape are hopelessly separated from measurements of the ion source region by a poorly constrained transport and acceleration region, so that the processes controlling ion loss remain obscure. Further, ion supply, energization and transport, and finally escape are heavily influenced by the incoming flux of extreme ultraviolet photons and related changes in Venus’ external space environment. The substantial variations of these inputs over both solar cycle and geologic timescales, and their effects on key atmospheric processes, have not been sufficiently constrained. These unknowns frustrate the interpretation of lower and middle atmosphere observations, including those planned by future NASA and ESA missions, and inhibit the formulation of a self-consistent history for our sister planet. Here we call for new measurements of the Venus upper atmosphere and ionosphere to resolve these uncertainties and extend the scope of currently funded efforts to cover the entire atmosphere, from surface to space. The required remote sensing and in situ instruments are high-heritage, low mass, and low power, and the required altitude sampling and spatial coverage could be provided by a small spacecraft, yielding a transformative science return at relatively modest cost. A flotilla of space missions brought forth by an international community of scientists will soon visit Venus: upper atmosphere measurements, and the critical contributions they enable, must not be left behind