3. Mission overview

3.1 Spacecraft design and orbit

VCO aims to unveil the mysteries described above with 5 cameras and radio science. The configuration of the spacecraft is shown in Fig. 1. The spacecraft, which is three-axis stabilized by momentum wheels, directs cameras toward Venus via the attitude control of the main body. The spacecraft surfaces on which solar array paddles are attached always face to the north or the south, and are used for radiative cooling. The solar array paddles have one freedom of rotation about the north-south axis, and their orientations are controlled to face to the sun independent of the orientation of the main body. The slot-array high gain antenna will be oriented toward the Earth by attitude control when communicating with the ground station. The telemetry rate will be higher than 4 kbps at 1.5 AU, 8 kbps at 1.1 AU, 16 kbps at 0.7 AU and 32 kbps at 0.5 AU. The mass of the spacecraft is 480 kg including fuel, and the science payload weighs 34 kg. The spacecraft will be launched in May 2010 and arrive at Venus in December 2010. During the cruise to Venus, VCO will observe the zodiacal light from various viewing points in the solar system without any contaminations of the sky light, and will map out the spatial distribution of the IPD cloud. The mission life at Venus will be 2 Earth years or more; the duration is limited by the degradation of the onboard batteries. The details of the spacecraft design are given in Ishii et al. (2004).

The orbit around Venus is a long elliptical one near the ecliptic plane (172-degrees inclination) with 30-hours orbital period. The direction of orbital motion is westward, which is the direction of atmospheric super-rotation. The apoapsis altitude is chosen to be 79000 km, or 13 Venus radii (Rv), so that the angular velocity of the spacecraft is roughly synchronized with the 60-m s^-1 super-rotational flow near the cloud base (50 km) for ~20 hours centered at the apoapsis (Fig. 2). The periapsis altitude is 300 km. Global images of the atmosphere and the ground surface will be obtained every 2 hours successively and continuously from such a ‘quasi-synchronized’ orbit. In order to transmit a quantity of image data to the ground station without serious degradation, the Sensor Digital Electronics Unit (DE) will be used for onboard calibration and data compression.

The systematic imaging sequence of VCO is advantageous for detecting meteorological phenomena with various temporal and spatial scales. The quasi-synchronized orbit is suitable for obtaining cloud-tracked wind vectors, especially the small deviation of local wind vectors from the background super-rotation. With such wind vectors the characterizations of the meridional circulation, mid-latitude jets and various wave activities are anticipated. Although the orbit is near equatorial, polar phenomena such as the polar dipole can also be monitored to some extent from the apoapsis by virtue of the 8-degrees inclination of the orbit with respect to the equatorial plane, which is for suppressing the shadow duration along the orbit. Close-up images of meso-scale features and limb images will also be obtained near the periapsis. The shadow region along the orbit is utilized for observing faint light such as lightning and airglow. Radio occultation experiment will also be performed when the spacecraft is hidden by Venus as viewed from the ground station.

3.2 Sounding regions

The onboard scientific instruments altogether sense different levels of the atmosphere (Fig. 3). Night airglows at visible wavelengths in the lower thermosphere will be studied by the Lightning and Airglow Camera (LAC). LAC will also detect yet-to-confirm lightning in the clouds. The cloud top level is covered by the Ultraviolet Imager (UVI), which maps SO₂ and unknown absorbers at wavelengths 283 and 365 nm on the illuminated (day) side. The meso- to global-scale structures in the cloud top height will be determined by the Longwave Infrared Camera (LIR) at 10 micron wavelength both on the dayside and the un-illuminated (night) side. UVI and LIR will yield wind vectors via the tracking of small-scale features. Variations in the cloud top height will be studied also by the 2-micron Camera (IR2) with its 2.02-micron filter (a CO₂ absorption band) applied to the dayside.

The main target of IR2 is the motions of the middle and lower atmosphere. Such observations will be done at 1.73, 2.26 and 2.32 micron wavelengths, which are known to be relatively absorption free (so-called atmospheric windows), enabling us to see the deep atmosphere through the clouds on the nightside (Taylor et al., 1997). The distribution of CO will be studied by differentiating 2.26 and 2.32 micron images to understand the production, circulation and dissociation processes of this molecule.

Finally, the deepest level (almost reaching the surface) will be investigated by the 1-micron Camera (IR1) at 0.90, 0.97 and 1.01 micron wavelengths. In addition to the studies of cloud properties and the H₂O vapor distribution below the cloud, suspected volcanic activities will be searched for and the surface emissivity distribution will be mapped with IR1. IR1 will also observe lower clouds on the dayside.

In addition to the imaging-camera suite above, Radio Science (RS) technique will be used to observe the vertical profiles of atmospheric temperature, sub-cloud H₂SO₄ vapor, and ionospheric plasma. The combination of these different types of observations will provide a new view of the three-dimensional structure and dynamics of the Venusian atmosphere.