Venus is our nearest planetary neighbour and orbits closer around the sun than the Earth. The mean mass densities and radii of Earth and Venus are very similar and therefore Venus is often regarded as Earth′s “sister planet”. However, the near-surface conditions on both planets differ considerably; while Earth′s mean surface temperature is 15°C and the main constituents of the Atmosphere are nitrogen (78%) and oxygen (21%), the mean temperature on Venus is 477°C and its atmosphere is almost exclusively composed of carbon dioxide (95%). The geological and geophysical exploration of Venus began in the 1960s and since then a large number of missions (flybys, orbiter and lander) built and operated by the Soviet Union and the United States have investigated the surface structure, the composition and dynamics of the atmosphere as well as the physics of the plasma environment surrounding Venus. Venus Express is the first European mission to the planet Venus and surveys the dynamics and chemistry of the Venusian atmosphere, the interaction processes between the atmosphere and the surface as well as the interaction of the interplanetary medium (solar wind) with Venus′ upper atmosphere.
A Soyuz-Fregat launcher set ESA′s spacecraft on its course to Venus from the Russian Baikonur Cosmodrome in Kazakhstan in November 2005. In April 2006 the spacecraft was inserted into its polar orbit around Venus with a pericentre altitude of ~250 km and an apocentre altitude of ~66000 km about the planet′s surface. Venus Express is equipped with seven scientific instruments which gather data about the surface, the atmosphere and the space plasma environment during the different observation phases of each orbit.
The mission name “Venus Express” originated from the project′s fast realisation within only four years from the planning phase to launch as well as the use of the already existing concept and design of the Mars Express spacecraft.
The magnetometer experiment (MAG) which measures the magnetic field was developed and lead managed by the Space Research Institute of the Austrian Academy of Science in Graz in collaboration with the Imperial College London. The Institute for Geophysics and extraterrestrial Physics contributed significantly to the sensors, the magnetometer electronics and the boom. Furthermore, it was responsible for the pre-flight calibration of the instrument in “Magnetsrode”.
Venus does not posses an intrinsic global magnetic field like the Earth. Instead, the solar magnetic field carried by the solar wind piles up at Venus and forms an “induced” magnetosphere. The determination of the magnetic field properties is therefore important to understand the solar wind interaction with the dense atmosphere of Venus. By means of the magnetic field data various interaction regions resulting from the deflection of the solar wind around the planetary obstacle can be identified and characterised. The analysis of these measurements will improve the knowledge about e.g. energy transfer processes via waves in the plasma. The orbit of Venus Express is very suitable to close gaps in the spatial coverage of the measurements of previous missions and thus allows to complement our understanding of the physical processes in Venus′ space plasma environment.
The magnetometer on board Venus Express is based on the same design as the magnetometer ROMAP which was already built for the Rosetta Lander “Philae”. It measures the magnetic field vector with a sampling rate up to 128 Hz and consists of two triaxial fluxgate sensors, an electronics box and a boom (length 1m). One sensor is mounted close to the body of the spacecraft while the other is mounted at the remote end of the boom which has been deployed after launch. Hence, both sensors have a different distance to the spacecraft which allows to identify and remove disturbances of the magnetic field caused by the spacecraft. The default sampling rate is 1 Hz. In the solar wind interaction region of the planet the sampling rate can be increased to 32 Hz or 128 Hz.
