Galileo Venus flybys

In the 1980s, the discovery of spectral windows allowed Earth-based IR observations of the subcloud atmosphere on Venus nightside (Allen and Crawford, 1984). High-resolution IR spectroscopy in these windows led to the discovery of carbonyl sulfide (OCS) and the first measurements of HCl and HF below the clouds (Bezard et al., 1990). The Galileo and CASSINI flybys of Venus (see Table 1) utilized these spectral windows to image the surface at near IR wavelengths. 

Venus atmosphere is so dry that Earth-based and spacecraft measurements of the water-vapor abundance are extremely difficult. Historically, many of the in situ water-vapor measurements gave values much higher than the actual water-vapor content. However, reliable values are now available from several sources including the Pioneer Venus mass spectrometer, spectrophotometer experiments on Venera 11-14, Earth-based FTIR spectroscopy of Venus lower atmosphere on the nightside, and IR observations during the Galileo and Cassini flybys of Venus. 

Venus. Reports of the chemical composition of the surface and nearsurface regions of the planet, as measured by Earth-based instruments, were reported as early as the mid-1990s. The Galileo and Cassini spacecraft collected similar data in the 1 mm region of the EM spectrum during their flybys of the planet in 1990 and 1998, respectively. 

The Galileo mission to Jupiter and beyond obtained nearly 5 of the required 9 kilometers per second delta-v from one flyby of Venus and two flybys of Earth. This is only slighdy higher than the delta-v required to reach lunar orbit. 

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