The Venus Express mission was selected by ESA in 2002 following a call for ideas concerning the potential use of the Mars Express satellite spare model. Spare models and parts from instruments developed for Mars Express and Rosetta were also available for Venus Express.
The mission was designed to try and answer the following questions:
- What are the mechanisms and dominant forces responsible for the super-rotation of the atmosphere?
- What are the fundamental processes responsible for the atmosphere’s general circulation?
- What is the current state of equilibrium for water vapour, and what was it in the past?
- What impact has the greenhouse effect had on planetary evolution in the past, and what role will it play in the future?
- Is there any tectonic or volcanic activity on Venus?
- And finally, the fundamental question: why is Venus so different from Earth, given their similarities in size and composition?
Here is a more detailed list of the primary science objectives for this orbital mission:
- General study of the atmospheric temperature profile, from the surface up to an altitude of 200 km;
- Detailed study of atmospheric circulation (super-rotation, thermospheric winds) and waves, from 50 to 150 km above the surface;
- Complete study of chemical composition (H2O, SO2, SO, COS, HCl, HF, etc.) and evolution of the lower atmosphere (0-40 km) and of the atmosphere within and above the clouds (55-150 km);
- Study of the structure, clouds, and distribution of the unknown UV radiation absorber; study of the clouds’ formation and evolution processes;
- Study of the energy balance sheet and the greenhouse effect;
- Search for atmospheric lightning;
- Study of the plasma environment (energetic neutral atoms, ions, and electrons), escape processes and solar wind interaction;
- Measure the magnetic field resulting from the interaction between the solar wind and the ionosphere;
- Infrared mapping of the surface, measuring infrared emission variations;
- Search for volcanic and seismic activity and study of volcanic activity’s impact on the formation of the current climate;
- Study of the Sun’s corona by radio sounding during Venus’s conjunctions.
A 153-DAY JOURNEY TO VENUS
Venus Express was successfully launched on 9th November 2005, from the Baikonur space centre in Kazakhstan. The launch window was open from 26th October to 25th November 2005. The Soyuz launcher placed the Fregat upper stage (on which the Venus Express spacecraft was installed) on a sub-orbital trajectory and then on an escape trajectory before the spacecraft finally separated from the Fregat stage.
During its 153-day journey to Venus, the satellite was monitored by the New Norcia ground station in Australia and the Cebreros ground station in Spain, the latter of which was operational for the first time for the Venus Express mission. The spacecraft received instructions to adjust its trajectory using its boosters. At least one trajectory adjustment was planned 60 days after launch. Once it approached Venus, the spacecraft used its primary engine so slow down and let itself be captured by the planet’s gravitational pull.
Insertion into orbit
Venus Express was initially inserted into a very elliptical polar orbit with a pericentre of about 250 km and a 5-day orbital period. Auxiliary boosters were then used to reduce the apocentre and reach operational orbit.
The operational orbit was determined so that every longitude could be covered during one Venus sidereal day (243 Earth days). Venus’s sidereal revolution period (a Venus year) is 225 days, longer than the planet’s (retrograde) rotation. The mission’s nominal lifetime was set for 2 Venus sidereal days. Planet coverage was obtained at different spatial resolutions. Unlike Mars Express, which aimed to cover the entire surface of Mars, Venus Express didn’t cover the entire surface in high resolution but instead studied the spatial and temporal variability of the atmosphere and the surface on different scales. The chosen operational orbit was an elliptical polar orbit, with a pericentre of about 250 km and a pericentre of about 66,000 km, and an orbital period of 1 Earth day to facilitate ground monitoring operations.