5. Relationship with Venus Express
Website of Venus Express: http://sci.esa.int/science-e/www/area/index.cfm?fareaid=64
The sciences of VCO are strongly related to those of Venus Express, a Venus orbiter of ESA, which was launched in November 2005 and arrived at Venus in April 2006. Venus Express' science objectives are to study the atmosphere, the plasma environment, and the surface of Venus. The onboard science instruments are: Planetary Fourier Spectrometer (PFS); Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV); Visible and Infrared Thermal Imaging Spectrometer (VIRTIS); Venus Monitoring Camera (VMC); Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4); Magnetometer (MAG); and USO for Venus Radio Science (VeRa). The spacecraft was injected into a 24-hour polar orbit with the apoapsis altitude of 66000 km (11 Rv) and the periapsis altitude of 250-350 km. The periapsis latitude is 78 N, enabling close-up observation in the northern high latitude and global imaging over the southern hemisphere. The nominal mission lasted till October 2007, and the extended mission is still continuing and planed till December 2012. Regarding atmospheric and surface observations, the striking difference between VCO and Venus Express is that the former focuses on continuous global imaging from equatorial orbit, while the latter puts emphasis on spectroscopy from polar orbit, although VMC on Venus Express takes global images of the southern hemisphere and VIRTIS has a capability to map the planetary disk in mosaic mode. Such different approaches of the two missions are complementary with each other in many aspects of the Venus science.
The information on the vertical distributions of clouds and minor constituents is indispensable for the accurate interpretation of the image data taken by VCO; however, VCO hardly obtains such information, although the multiple-band imaging observations will resolve several different altitude levels. The vertical profiles provided by the Venus Express spectroscopy (PFS, SPICAV, VIRTIS) should offer the basis for the investigation by VCO.
Due to the orbital constraints, VCO cameras take global images of both hemispheres with emphasis on the low to mid-latitude, while VMC and VIRTIS on Venus Express get global views of the southern hemisphere especially in the high latitude region. The former is suitable for detecting planetary-scale symmetric or asymmetric wave modes such as the horizontal Y feature (Del Genio and Rossow, 1990) and characterizing the hemispheric difference. The latter will reveal more about the polar vortex and the so-called polar dipole whose origin is unclear (Taylor et al., 1980).
The way of surface sounding utilizing the near-infrared windows is also slightly different between VCO and Venus Express. IR1 on VCO maps the surface globally and continuously, together with the overlying clouds, exclusively at 1.01 micron wavelength. Such observations allow us to discriminate surface features from cloud features rather easily since clouds move and change their shapes from time to time. Venus Express, having more wavelength bands for surface sounding, yields spectroscopic data on the surface material. Latitudinal coverage is also different between VCO and Venus Express, in the same way as the atmospheric observation.