Similar to Earth in size, mass and distance from the Sun, Venus was expected to be very similar to our planet when the first Russian and American space probes approached it in the early 1960s and started returning data about its atmosphere. Observers soon realised that Venus is an entirely different, exotic and inhospitable world, hidden behind dense clouds of noxious gases. It has an atmosphere mainly composed of carbon dioxide, with crushing surface pressure and burning-hot temperatures. The question quickly arose: why did a planet apparently so similar to Earth evolve in a way so radically different over the last four thousand million years?
By the mid 1990s, many ground observatories and more than 20 spacecraft had attempted to explore Venus. Orbiters and descent probes - including the Soviet Venera-series and the Vega balloons and landers, and the US Mariner, Pioneer Venus and the 1990-94 Magellan missions - tried to penetrate this hostile world to find solutions to the giant puzzle that Venus represented for scientists.
Many missions were lost and many landers were destroyed under the high pressures and temperatures of the Venusian atmosphere; nonetheless, past missions provided solid initial information about the planet, lifting the veil of mystery. For years, scientists were prevented from seeing what lays below the dense atmosphere and the thick cloud cover blocked any view below, hindering attempts to understand the nature of the Venusian surface.
Venus - false colour image
The atmospheric pressure at the surface of Venus was found to be over 90 times greater than that on Earth. Further, the composition and dynamics of the two planets' atmospheres are completely different. On Venus, almost no wind occurs at the surface, but higher up - in clouds of sulphuric acid - winds blow at hurricane-force speeds, circling the planet in just four days - much faster than Venus turns on its axis.
The atmosphere comprises a mixture of carbon dioxide, sulphur and traces of water vapour, with the carbon dioxide causing an intense 'greenhouse' effect, trapping the strong solar energy and helping generate a surface temperature of 480 degrees Celsius, hot enough to melt lead. Any water on the surface would immediately boil away and as a result the planet is a bone-dry desert.
The mission aims to gather data that will help scientists create an overall characterisation of the planet, and will look at Venus's structure, composition and dynamics. In addition to studying the immediate environment of Venus and its interaction with the intense solar wind, Venus Express is particularly designed to study the planet's atmosphere.
Scientists have many questions, including:
- What is the chemical composition of individual layers of the atmosphere?
- What physical characteristics do these layers have, and what drives global wind circulation -- particularly in the fast upper cloud layers?
- How, exactly, does the greenhouse effect occur on Venus? And how has this developed over billions of years?
- Was Venus already quite hot, or has the greenhouse effect made it hotter yet?
- Did the strong greenhouse effect develop only later in planetary evolution, and were conditions on Venus perhaps less extreme in earlier phases?
- Was there ever water on the surface?
- What types of rocks are on the surface and what do they tell us about development of volcanism?
- Are there still active volcanoes on the Venus?
Ground-breaking areful technology developed by DLR, partners and institutes
Of the seven experiments onboard, six share a common heritage with the two previous ESA missions, Mars Express and Rosetta. Only one instrument, the Venus Monitoring Camera, had to be newly developed to help answer the scientific questions above. All the highly sensitive instruments, as well as the spacecraft itself, had to be specially adapted to operate in the harsh environment around Venus, where the Sun's radiation and the solar wind are much stronger than by Earth or Mars. Excessive irradiation will be avoided by carefully selecting the spacecraft's orbit, while ensuring robust opportunities for science data gathering.
The Venus Monitoring Camera (VMC) was developed by the Max Planck Institute for Solar System Research in Lindau, Germany. The VMC is a wide-angle multichannel camera that will gather light in the visible, ultraviolet and near-infrared spectrum; data will be analysed by DLR's Institute for Planetary Research and will be used to generate images of Venus's dynamic atmosphere. VMC images will also help in the identification of phenomena seen by other instruments.
The VMC may also be able to gather images of the Venusian surface. The VMC principal investigator (PI) is Dr Wojciech Markiewicz of the Max Planck Institute.
To investigate the chemical composition of the Venusian atmosphere, including temperature and dynamics, scientists will use three onboard spectrometers.
First is the Planetary Fourier Spectrometer (PFS), led by an Italian team. The PFS will be able to measure the temperature of the atmosphere between altitudes of 55-100 kilometres at a very high resolution, and also obtain measurements of the surface temperature and thus, by implication, gather evidence for volcanic activity.
Next is the Ultraviolet and Infrared Atmospheric Spectrometer (SPICAV/SOIR), operated by scientists from France, Belgium and Russia. SPICAV will assist in the analysis of Venus's atmosphere. In particular, it will search for the small quantities of water expected to exist in the atmosphere. It will also look for sulphur compounds and molecular oxygen in the atmosphere. It will determine the density and temperature of the atmosphere at 80-180 kilometres altitude.
The third spectrometer is the VIRTIS (Visible and Infrared Thermal Imaging Spectrometer), which will also study the atmosphere. VIRTIS is an imaging spectrometer (VIRTIS-M) for the visual (0.25-1µm) and the near-infrared range (1-5µm) combined with a high resolution IR-spectrometer (VIRTIS-H) for the spectral range from 2-5µm. The VIRTIS instrument was developed by DLR Adlershof, together with Kayser Threde GmbH.
VIRTIS can also use the so-called 'spectral windows' that exist at certain wavelengths in the atmosphere to look through to the surface of the planet. VIRTIS scientists plan to create a chemical and mineralogical map of the surface using this technique. "It would be even more exciting," says DLR scientist Dr Joern Helbert, who is part of the investigation team, "if VIRTIS succeeds in discovering active volcanoes on Venus by virtue of their thermal radiation or due to gases ejected from volcanoes being detected in the atmosphere." DLR Adlershof, in Berlin, is also responsible for evaluation and analysis of VIRTIS data.
Venus Express also carries:
- ASPERA (Analyser of Space Plasma and Energetic Atoms) experiment, led by the Institute of Space Physics in Kiruna, Sweden ASPERA aims to investigate the interaction between the solar wind and the atmosphere of Venus.
- MAG (Venus Express Magnetometer), led by the Space Research Institute in Graz, Austria MAG will examine the interaction between the solar wind and Venus's atmosphere.
- VeRa (Venus Radio Science Experiment), led by the University of the Federal Armed Forces in Munich, Germany VeRa will use the powerful radio link between the spacecraft and Earth to investigate the upper levels of the atmosphere of Venus by analysing radio signals that transit through the ionosphere.
German involvement in the development of the engine
In addition to key onboard instruments, DLR has been intimately involved in testing one of the spacecraft's most important components: the main engine.
Long before this week's crucial live firing that will initiate Venus orbit entry, the main engine was proven on the test stands at DLR Lampoldshausen.
In June 2003, engineers from DLR's Institute of Propulsion Technology successfully operated the engine in acceptance tests under vacuum conditions, including engine re-ignition with a running time of 20 minutes. The testing at DLR Lampoldshausen's Test Stand P1.0 was conducted in cooperation with EADS Space Transportation, manufacturer of the engines for Venus Express.
The engine operates using nitrogen tetroxide (N2O4) and monomethylhydrazine (MMH) as fuel. These require extremely careful handling as they are hypergolic reactants, i.e. they will spontaneously catch fire upon contact with each other. The engine, about as big as a shoebox, developed a thrust of some 400 Newtons (N), the force of which was transferred to the test mountings which had to be anchored against this pull, equal to approximately 850 Horsepower (HP).