American probe Mariner 5 was originally built as a backup for Mariner 4 to Mars, but later it was refurbished and modified to go to Venus. The spacecraft was launched on June 14, 1967, and flew by Venus on October 19, 1967 (a day after Venera 4), at a distance of
3,990 kilometers (2,480 miles). With its more sensitive instruments than aboard Mariner 2, it revealed new information about Venus' atmosphere, including its composition of 85-99% carbon dioxide. Mariner 5 also studied the interplanetary space in the vicinity of Venus and advanced science and the art of building and operating interplanetary spacecraft.
As a consequence, in 1968, Soviet and American scientists met in Tucson, Arizona, to discuss and compare results from both the Venera series and Mariner. This meeting led to a new estimate of 427°C for the surface temperature and an estimate of 75 bars for the pressure, levels that exceeded the design limits of the Venera 4, 5, and 6 spacecraft family.
Mariner 10 was launched in 1973, and it was the seventh successful launch in the Mariner series and the first spacecraft to visit Mercury. It was also the first spacecraft to use the gravitational pull of one planet (Venus) to reach another (Mercury), and the first spacecraft mission to visit two planets. The spacecraft flew by Mercury three times in a retrograde heliocentric orbit and returned images and data on the planet. Mariner 10 returned the first-ever close-up images of Venus and Mercury. The primary scientific objectives of the mission were to measure Mercury's environment, atmosphere, surface, and body characteristics and to make similar investigations of Venus. Secondary objectives were to perform experiments in the interplanetary medium and to obtain experience with a dual-planet gravity-assist mission.
For further details, see: http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1967-060A (for Mariner 5), and http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1973-085A (for Mariner 10)
Galileo Venus Flyby
Galileo was launched on October 18, 1989, aboard space shuttle Atlantis. To achieve sufficient velocity to reach Jupiter directly, the trajectory design included Venus-Earth-Earth gravity assist (swingby) scheme. The Venus gravity assist flyby occurred on February 9-10, 1990, at a distance of 16,000 km above the cloudtops. Although the sole objective of this flyby was to pump up Galileo's orbit, scientists used this opportunity to turn the spacecraft's planetary sensors on the cloud-shrouded planet, in order to study its atmosphere and environment. The scientific data was stored on the spacecraft tape recorder until late October, 1990, when Galileo was close enough to Earth to play them back over its low-gain antenna. The Venus observations were selected on the basis of (1) not risking Jupiter observations in the 1995-1997 Jovian orbital phase, (2) not exceeding the capacity of the tape, and (3) getting the best new scientific information about Venus. Details on the Galileo mission can be found at http://www2.jpl.nasa.gov/galileo/
During the 1990 Venus flyby of the Galileo spacecraft, the Near-Infrared Mapping Spectrometer investigated the night-side atmosphere of Venus in the spectral range 0.7 to 5.2 micrometers. Multispectral images at high spatial resolution indicated substantial cloud opacity variations in the lower cloud levels, centered at 50 kilometers altitude. Zonal and meridional winds were derived for this level and are consistent with motion of the upper branch of a Hadley cell. Northern and southern hemisphere clouds appear to be markedly different. Spectral profiles were used to derive lower atmosphere abundances of water vapor and other species. (Ref: Carlson et al., "Galileo Infrared Imaging Spectroscopy Measurements at Venus", Science 27 September 1991: Vol. 253. no. 5027, pp. 1541 - 1548, DOI: 10.1126/science.253.5027.1541)
Following the footsteps of Mariner 10 over 30 years earlier (1975), the MESSENGER spacecraft (MErcury Surface, Space ENvironment, GEochemistry and Ranging) was launched on August 3, 2004, to study the characteristics and environment of Mercury from orbit. It was designed to characterize the chemical composition of Mercury's surface, the geologic history, the nature of the magnetic field, the size and state of the core, the volatile inventory at the poles, and the nature of Mercury's exosphere and magnetosphere over a nominal orbital mission of one Earth year. MESSENGER will perform significantly improved science investigations over Mariner, with vastly improved scanning capability, and cameras capable of resolving surface features to 18 m (59 ft), providing global imaging of Mercury. (In comparison, Mariner 10 was a flyby mission, which only observed the sunlit hemisphere with a resolution of 1.6 km (0.99 mi).)
MESSENGER made two Venus flybys. Its first flyby of Venus on October 24, 2006 was made at an altitude of 2,992 kilometers (1,859 mi). A second flyby of Venus was made on June 5, 2007 at an altitude of 338 kilometers (210 mi). During the flybys, a comprehensive set of science observations was obtained with the instrument suite onboard. Details on the mission can be found at:http://messenger.jhuapl.edu/index.php
Flyby missions (Mariner, Galileo, MESSENGER)