ISSN 0038-0946, Solar System Research, 2009, Vol. 43, No. 3, pp. 185–209. © Pleiades Publishing, Inc., 2009. Original Russian Text © D.V. Titov, H. Svedhem, F.W. Taylor, S. Barabash, J.-L. Bertaux, P. Drossart, V. Formisano, B. Häusler, O. Korablev, W.J. Markiewicz, D. Nevejans, M. Pätzold, G. Piccioni, J.-A. Sauvaud, T.L. Zhang, O. Witasse, J.-C. Gerard, A. Fedorov, A. Sanchez-Lavega, J. Helbert, R. Hoofs, 2009, published in Astronomicheskii Vestnik, 2009, Vol. 43, No. 3, pp. 195–217. Venus Express: Highlights of the Nominal Mission1 D. V. Titova,b, H. Svedhemc, F. W. Taylord, S. Barabashe, J.-L. Bertauxf, P. Drossartg, V. Formisanoh, B. Häusleri, O. Korablevb, W. J. Markiewicza, D. Nevejansj, M. Pätzoldk, G. Piccionil, J.-A. Sauvaudm, T. L. Zhangn, O. Witassec, J.-C. Gerardo, A. Fedorovp, A. Sanchez-Lavegaq, J. Helbertr, and R. Hoofss a Max-Planck Institute for Solar System Research, Katlenburg-Lindau, Germany b Space Research Institute (IKI), Moscow, Russia c ESA/ESTEC, Noordwijk, The Netherlands d Oxford University, UK e IRF, Kiruna, Sweden f Service d’Aeronomie, CNRS, France g LESIA, Observatoire de Paris, Meudon, France h IFSI/INAF, Rome, Italy i Universitute der Bundeswehr, München, Germany j Institute for Space Aeronomy, Belgium k Rheinean Institute for Environmental research, Universitute zu Küln, Cologne, Germany l IASF/CNR, Rome, Italy m Centre d’Etude Spatiale des Rayonnements, Tolouse, France n Space Research Institute, Graz, Austria o LPAP, Universite de Liege, Belgium p CESR, Toulouse, France q Universidad del Pais Vasco, Bilbao, Spain r Institute for Planetary Research, DLR, Berlin, Germany s ESA/ESAC, Villanueva de la Canada, Madrid, Spain e-mail: [email protected] Received October 7, 2008 Abstract—Venus Express is the first European (ESA) mission to the planet Venus. Its main science goal is to carry out a global survey of the atmosphere, the plasma environment, and the surface of Venus from orbit. The payload consists of seven experiments. It includes a powerful suite of remote sensing imagers and spectrome- ters, instruments for in-situ investigation of the circumplanetary plasma and magnetic field, and a radio science experiment. The spacecraft, based on the Mars Express bus modified for the conditions at Venus, provides a versatile platform for nadir and limb observations as well as solar, stellar, and radio occultations. In April 2006 Venus Express was inserted in an elliptical polar orbit around Venus, with a pericentre height of ~250 km and apocentre distance of ~66000 km and an orbital period of 24 hours. The nominal mission lasted from June 4, 2006 till October 2, 2007, which corresponds to about two Venus sidereal days. Here we present an overview of the main results of the nominal mission, based on a set of papers recently published in Nature, Icarus, Plan- etary and Space Science, and Geophysical Research Letters. PACS numbers: 96.30.Ea, 96.12.Jt, 96.12.Kz, 96.12.Ma, 96.12.Wx DOI: 10.1134/S0038094609030010 1 1. INTRODUCTION none of the world space agencies planned another mis- sion to the planet, leading to that Venus was nicknamed: Despite the fact that about 25 spacecraft visited the “forgotten planet”. After more than a decade, the Venus in the 20th century, a great number of fundamen- European Space Agency (ESA) was the first to respond tal problems in its physics remained unsolved (Moroz, 2002; Titov et al., 2006a; Taylor et al., 2006). This was to this challenge by initiating Venus Express—the first already recognized by the end of the 1990's, however, European mission to Venus. The mission aims at a glo- bal remote and in-situ investigation of the atmosphere, 1 The article is published in the original. plasma environment, and some surface properties from 185 186 TITOV et al. orbit. In particular, fundamental problems in the fields 2. SPACECRAFT, PAYLOAD, ORBIT, MISSION of atmospheric structure and composition, morphology SCENARIO AND OBSERVATIONS and distribution of clouds and hazes, atmospheric dynamics, properties of plasma and magnetic field, and The Venus Express spacecraft design is based on the escape processes, are all addressed by the Venus successful Mars Express bus—a 600 kg 3-axis stabi- Express observations. These investigations will shed lized platform with two body-fixed telecommunication light on the current climate of Venus, will make it pos- antennas (Fig. 1). The spacecraft is described in detail sible to reconstruct the evolutionary path of our sister by Svedhem et al. (2007a, 2008). Most of the sub- planet, and will make significant contribution to the systems and payloads are mounted on its walls and on field of comparative planetology. panels inside the bus, while ASPERA and MAG are accommodated outside the spacecraft. The propulsion The Venus Express payload consists to a great extent system is located on the bottom of the bus on the –Z of the instruments inherited from the Mars Express and side. The +X wall is almost completely occupied by the Rosetta missions, which were found to be quite suitable 1.3 m dish of the main high-gain antenna (HGA1). The for making a breakthrough in Venus studies (Titov et al., opposite (–X) side of the bus carries payload cryogenic 2001). The payload core consists of a suite of imagers radiators. The Y walls carry the solar panels and radia- and spectrometers, combining wide spectral range from tors that cool the equipment and some of the instru- the UV to the thermal IR. Broad band context imaging ments. The small 0.3 m dish of the second high-gain and high spectral resolution makes the mission payload antenna (HGA2) and the MAG boom are mounted on the most capable remote sensing package ever flown to the top +Z floor. The apertures of the optical instru- Venus. This suite is complemented by a radio science ments are also located on this wall. The spacecraft can experiment to investigate the structure of the neutral withstand only limited solar illumination on all walls atmosphere and ionosphere. Two experiments monitor except for +Z and +X, which greatly adds to the com- the ambient magnetic field and study the plasma envi- plexity of planning the observations. ronment and escape processes. The Venus Express payload consists of seven instru- ments (Fig. 1, table), including three spectrometers The mission re-uses the Mars Express bus modified (SPICAV/SOIR, PFS, and VIRTIS) operating in broad for the conditions at Venus (Svedhem et al., 2007a). The spectral range from UV to thermal 1R to study the spacecraft provides a versatile 3-axis stabilized plat- structure, composition, and dynamics of the atmo- form to carry out various observations. These include sphere in nadir geometry. For the first time, spectral nadir observations along almost the entire orbit, track- imaging in the near-IR range routinely provides an ing the planetary limb and selected spots on the surface, orbital survey of the composition of the lower atmo- as well as solar, stellar and Earth radio occultations that sphere, the structure of the deep cloud, and the surface, require high precision and stability of pointing. The through night-side imaging in the spectral transparency broad spectrum of mission goals, complexity of the “windows” by VIRTIS and VMC. Also for the first payload, and operational limitations due to re-use of the time, SPICAV/SOIR uses stellar and solar occultation Mars Express spacecraft that had been originally technique at Venus for precise investigation of the designed for the colder environment at Mars, justified structure and composition of the mesosphere and upper the need for careful planning of observations in order to haze. These observations are complemented by the effectively use the spacecraft and payload capabilities limb imaging by VIRTIS and VMC. Venus Monitoring and to maximize overall science return (Titov et al., Camera takes wide angle context images in four chan- 2006c). Coordination of the observations between dif- nels to study morphology and dynamics at the cloud ferent experiments onboard the spacecraft is a particu- tops. Both VIRTIS and VMC provide thermal mapping lar feature of Venus Express science planning. This syn- of the surface. The ASPERA-4 experiment investigates ergistic approach makes it possible to effectively in-situ plasma and energetic neutral atoms (ENA) address the science objectives of the mission using sev- around the planet to characterize escape processes. It is eral experiments. supported by MAG measurements of the magnetic field. The standard radio system of the spacecraft, com- The nominal Venus Express mission lasted from plemented by an ultra-stable oscillator (USO), consti- June 4, 2006 till October 2, 2007, which corresponds to tutes the radio science experiment (VeRa) that studies about two Venus sidereal days. This paper gives an the fine structure of the atmosphere and ionosphere in overview of the results recently published in a special Earth radio occultation mode, investigates the surface section of Nature, in several papers in Geophysical with bi-static radar and gravity anomaly experiments, Research Letters, and in Icarus. The results are grouped and sounds the solar corona. Unfortunately, due to a in topics to demonstrate payload synergy in tackling failure in the scanning mechanism, the Planetary Fou- certain scientific problems. We also outline future rier Spectrometer (PFS) is currently not operational. investigations that are already being carried out or The programme of observations was adjusted so that planned for the extended mission (October 3, 2007– the other instruments could partially compensate for December 31, 2009). this loss. SOLAR SYSTEM RESEARCH Vol. 43 No. 3 2009 VENUS EXPRESS: HIGHLIGHTS OF THE NOMINAL MISSION 187 MAG VIRTIS PFS SPICAV/SOIR VMC Z VeRa X Y ASPERA Fig. 1. Semi-transparent view of the Venus Express spacecraft with science instruments shown.