More than 25 spacecraft from the USA and the Soviet Union visited Venus in the 20th century, but in spite of the many successful measurements they made, a great number of fundamental problems in the physics of the planet remained unsolved. In particular, a systematic and long-term survey of the atmosphere was missing, and most aspects of atmospheric behaviour remained puzzling. After the Magellan radar mapping mission ended in 1994, there followed a hiatus of more than a decade in Venus research, until the European Space Agency (ESA) took up the challenge and sent its own spacecraft to our planetary neighbour. The goal of this mission, Venus Express, is to carry out a global, long-term remote and in-situ investigation of the atmosphere, the plasma environment, and some aspects of the surface of Venus from orbit.

Venus Express continues and extends the investigations of earlier missions by providing detailed monitoring of processes and phenomena in the atmosphere and near-space environment of Venus. Radio, solar and stellar occultation, together with thermal emission spectroscopy, sound the atmospheric structure in the altitude range from 150 to 40 km with vertical resolution of few hundred meters, revealing strong temperature variations driven by radiation and dynamical processes. In particular, by stellar occultation SPICAV (Spectroscopy for Investigation of the Characteristics of the Atmosphere of Venus) discovered a warm layer at the mesopause (~100 km) apparently caused by adiabatic heating of descending air. Observations by VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) and radio occultation sounding by VeRa (Venus Radio Science) indicate remarkable latitudinal temperature variations at the cloud tops, as well as the presence of a convective region within the cloud deck all over the planet. The temperature field in the mesosphere of the southern hemisphere is similar to that observed in the northern hemisphere by earlier missions, thus indicating global north/south symmetry.

Multi-spectral imaging is performed to investigate the cloud morphology at different levels and its latitudinal and temporal changes. Venus Monitoring Camera (VMC) shows evidence of convection in the cloud layer at low latitudes. Streaky cloud features indicate that the convection ceases in the middle and high latitudes. Sequences of images are used to track motions of the cloud features and to derive wind speeds in the 50-70 km altitude range. The polar orbit of Venus Express provides near-nadir viewing of middle and high latitudes in the southern hemisphere, enabling the first detailed observations of the cloud morphology and dynamics, leading to the discovery and characterisation of the complex 'eye' of the southern polar vortex by VIRTIS. The imaging instruments also perform thermal mapping of the surface in the near IR spectral windows on the night side, searching for surface emissivity anomalies and signs of active volcanism.

The chemistry and dynamics of the mesopause region (~100 km altitude) is being studied by observing non-LTE emission from O₂ and NO molecules in the UV and near-IR. This airglow peaks at the equator, close to midnight and approximately at the mesopause level, indicating a thermospheric solar-antisolar component in the atmospheric circulation. Composition measurements over a wide range of altitudes are providing vertical profiles of CO, H₂O, HDO, HCl, HF, and SO₂ in the mesosphere (70-100 km), and global mapping of CO, COS, H₂O, and SO₂ in the lower atmosphere at heights around 35 km. These results provide powerful tests of dynamical and chemical models of the Venusian atmosphere.

The magnetometer (MAG) and ASPERA (Analyser of Space Plasmas and Energetic Atoms) measure the magnetic field and densities of neutral atoms, ions and electrons in-situ. These observations determine the structure and properties of the circumplanetary plasma and characterize escape processes at Venus. They cover the time of solar minimum, and thus complement Pioneer Venus investigations at solar maximum. Interestingly, the H/O ratio was found to be ~2 thus suggesting that these ions are escaping in the stoichiometric ratio of water. The magnetometer is detecting whistler signals in ~10% of the pericentre passes. This is interpreted as evidence of lightning and the rate is estimated to be at least as frequent as on Earth.