Venus is one of the most attractive targets in the solar system when we seek to understand the formation of the terrestrial environment. Venus is our nearest neighbor, and has a size very similar to the Earth$B!G(Bs; however, previous spacecraft missions discovered an extremely dense (~92 bar) and dry CO2 atmosphere with H2SO4-H2O clouds floating at high altitudes, and exotic volcanic features covering the whole planet. The abundant gaseous CO2 brings about a high atmospheric temperature (~740 K) near the surface via greenhouse effect. The atmospheric circulation is also much different from the Earth$B!G(Bs. The mechanisms which sustain such conditions are unclear and considered to be the keys to understand the origin of the terrestrial environment. In spite of the many previous missions that explored Venus, such as the Venera, Pioneer Venus, Vega and Magellan, most of the fundamental questions raised so far still remain unsolved.
The Venus Climate Orbiter (VCO) of Japan aims to elucidate the mechanism of the mysterious atmospheric circulation of Venus, with secondary targets being the exploration of the ground surface and the zodiacal light observation during the cruise to Venus. The exploration of the Venusian meteorology is given a high priority not only for understanding the climate of Venus but also for the general understanding of planetary fluid dynamics. VCO is the 24th science spacecraft of the Institute of the Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), and has a code name PLANET-C. The phase B study of VCO has started in April, 2004, and the flight model integration will start in the middle of 2006. The spacecraft will be launched and arrive at Venus in 2010, and will perform 2 Earth years of operation.
VCO will explore Venus using a set of sophisticated cameras dedicated to meteorological study and radio science technique. Such a VCO$B!G(Bs approach complements the Venus Express mission of the European Space Agency (ESA), which aims at totally understanding the Venusian environment using spectrometers, a multi-band camera, a plasma analyzer, a magnetometer, and radio science. The orbits of these two spacecrafts are also different from each other. The present paper describes the science goals of VCO, the mission overview, the specifications of the science instruments, and the relationship with Venus Express.