Venera 4 left Baikonur Cosmodrome in the central Soviet Union early in the morning of 12 June 1967. The first two stages of its three-stage Molniya-M launch vehicle placed the 1106-kilogram automated spacecraft into a 173-by-212-kilometer parking orbit about the Earth, then the launcher's third stage boosted Venera 4 out of orbit onto a fast path Sunward toward the cloudy planet Venus.
Two days later, after launch on a Atlas-Agena D rocket from the Eastern Test Range-12 launch pad at Cape Kennedy, Florida, 244.8-kilogram Mariner 5 followed Venera 4 toward Venus. Mariner 5 had been built as the backup for Mariner IV, which flew successfully past Mars in July 1965. Hardware modifications for its new mission included a reflective solar shield, smaller solar panels, and deletion of the visual-spectrum TV system in favor of instruments better suited to exploring Venus's hidden surface.
When Mariner 5 and Venera 4 left Earth, the nature of Venus's surface was only beginning to be understood. Though the Mariner II Venus flyby (14 December 1962) had measured a surface temperature of at least 800° Fahrenheit (F) over the entire planet, some planetary scientists still held out hope for surface water. They believed that Venus's atmosphere was made up mostly of nitrogen, with traces of oxygen and water vapor. They supposed that, even if Venus was in general hotter than Earth, its polar regions had to be cooler than its equator and mid-latitudes; perhaps cool enough for Venusian life. They also suggested that life might float high above Venus's surface in cool moist cloud layers.
Venera 4 reached Venus on a collision course on 18 October 1967. Shortly before entering the atmosphere at a blazing speed of 10.7 kilometers per second, it split into a bus spacecraft and a one-meter-wide, cauldron-shaped atmosphere-entry capsule. Both parts had been sterilized to prevent contamination of Venus with Earth microbes, and the capsule was designed to float if it splashed down in water.
Radio signals from Venus ceased suddenly as the bus was destroyed as planned high in the Venusian atmosphere; then, after a brief pause, signals from the capsule reached Earth-based antennas in the Soviet Union. After its steep atmosphere entry, during which it experienced a deceleration of 350 Earth gravities, the capsule lowered on a single parachute for 94 minutes. It transmitted data on atmospheric composition, pressure, and temperature as it fell toward the surface. Twenty-five kilometers above Venus, at a pressure 20 times greater than Earth sea-level pressure and a temperature of more than 500° F, transmission abruptly ceased. Venera 4 confirmed that Venus's atmosphere is more than 90% carbon dioxide.
Mariner 5 flew by Venus the next day at a distance of 4100 kilometers. For nearly 16 hours it performed an automatic encounter sequence and stored data it collected on its tape recorder. On 20 Oct, it began to play back data to Earth. The U.S. spacecraft found no radiation belts; this was hardly surprising, since it also found a magnetic field only 1% as strong as Earth's.
As it flew behind Venus, Mariner 5 sent and received a steady stream of radio signals. The signals faded rapidly as they passed through the dense Venusian atmosphere, yielding temperature and pressure profiles before they were cut off by the solid body of the planet. Mariner 5 revealed that Venus's atmosphere at its surface has a temperature of almost 1000° F and a pressure 75 to 100 times greater than Earth's.
As Venera 4 and Mariner 5 explored Venus, D. Cassidy, C. Davis, and M. Skeer, engineers at Bellcomm, NASA's Washington, DC-based planning contractor, put the finishing touches on a report for NASA's Office of Manned Space Flight. In it, they described automated Venus probes meant to be released from piloted Venus/Mars flyby spacecraft. They based their plans on a sequence of piloted Mars/Venus flyby missions outlined in the October 1966 report of NASA's Planetary Joint Action Group (JAG).
In the Planetary JAG's plan, NASA's piloted flyby program would begin with a Mars flyby mission in 1975. The second mission in the program, the 1977 Triple Planet Flyby, would depart Earth in Feb. 1977, almost a decade after Venera 4 and Mariner 5. It would zip past Venus in June 1977, pass Mars in Dec. 1977, explore Venus again in August 1978, and return to Earth in December 1978. The third and final Planetary JAG piloted flyby mission, the 1978 Dual Planet Flyby, would leave Earth in December 1978, pass Venus in May 1979, pass Mars in Jan. 1980, and return to Earth in September 1980.
Cassidy, Davis, and Skeer presented a progressive plan of Venus exploration, with preliminary reconnaissance during the first Venus flyby and increasingly in-depth studies during the next two. Most of the Venus probes they proposed were designed to float in Venus's atmosphere, though they also described armored landers, impactors, and large orbiters.
The June 1977 Venus flyby would see a piloted flyby spacecraft pass the planet at a distance of 680 kilometers moving at 11.8 kilometers per second. Periapsis (the point of closest approach to the planet) would occur over a point just north of the equator in the middle of the dayside hemisphere. The astronauts on board the flyby spacecraft would study Venus with a 40-inch reflecting telescope and a cloud-penetrating mapping radar.
They would also release a total of 15 automated probes with a combined mass of 27,200 pounds. These would include six 200-pound Drop Sonde/Atmospheric Probes (DSAPs); four 2075-pound Meteorological Balloon Probes; two 700-pound Venus Landers; two 700-pound Photo-RF Probes; and one 8000-pound Orbiter. The crew would release all of the DSAPs, two Meteorological Balloons, one Lander, one Photo-RF Probe, and the Orbiter during approach to Venus. The other four probes (one Photo-RF probe, two Meteorological Balloons, and one Lander) they would release as the flyby spacecraft moved away from Venus and began its journey to Mars.
The DSAPs would be the first to go, separating from the piloted flyby spacecraft between 10 and 16 hours before periapsis passage. Following a fiery entry into the Venusian atmosphere, they would transmit temperature, density, and composition data as they fell toward the surface, much as had Venera 4.
The Bellcomm team recommended targeting one DSAP to the "sub-solar region" (that is, the middle of the dayside), one to the "anti-solar" region (the middle of the nightside), one to the terminator (the line between day and night) near the equator, one to the "mid-light" region (mid-latitude on the dayside), and one to the "mid-dark" region (mid-latitude on the nightside). Because of its steep atmosphere entry angle, the terminator-equator DSAP would undergo deceleration equal to 200 Earth gravities.
Following release from the flyby spacecraft, the large Orbiter would fire its rocket motors to place itself into a low near-polar orbit about Venus. It would pass over both the sub- and anti-solar regions during the piloted flyby, then would continue to orbit and explore the planet after the flyby, transmitting its findings directly to Earth. Using radar and a multispectral scanner, it would map Venus's entire surface in about 120 Earth days. Controllers on Earth would also track its motion to chart any Venusian gravity anomalies.
The four Meteorological Balloons would communicate with Earth via the Orbiter, not the flyby spacecraft; this, the Bellcomm team explained, would help to reduce the burden on the crew during the hectic flyby. The Orbiter would track the Balloons for weeks to chart circulation patterns in the Venusian atmosphere at different locations and altitudes.
The Bellcomm team targeted the twin "survivable type" Landers to Venus's north pole and mid-light region. The former would enter the atmosphere steeply about three hours before flyby spacecraft periapsis, experiencing up to 500 Earth gravities of deceleration. Both Landers would descend through Venus's atmosphere for up to an hour. After they impacted on the surface, they would transmit meteorological and surface composition data for up to an hour.
The first Photo-RF Probe would enter Venus's atmosphere over the sub-solar region one hour before flyby spacecraft periapsis. The second would enter over the mid-light Lander site 15 minutes after periapsis passage. The Bellcomm engineers explained that the Photo-RF probes, which they likened to the Block III Ranger moon probes, would transmit only while the flyby spacecraft was close enough to accommodate their one-million-bit-per-second data rate. They each would transmit one wide-angle image from their downward-pointing cameras every 10 seconds for up to an hour as they plummeted toward destructive impact on the Venusian surface.
The 1977 Triple Planet Flyby mission's second Venus pass in Aug. 1978, 14 months after the first, would build on knowledge gained in the first pass, enabling a greater emphasis on Venus surface exploration. The flyby spacecraft would reach periapsis 700 kilometers above a point near the equator at the center of Venus's nightside. In addition to performing observations using flyby spacecraft instruments, the astronauts would aim five Lander Probes and five Photo-RF probes at interesting surface features discovered during their first Venus flyby and subsequently by the Orbiter they left behind.
Bellcomm recommended that the third Venus flyby of the series, the 1978 Dual Planet Flyby mission's single Venus flyby in May 1979, focus on "the search for life and extended surface operations." The astronauts would release a pair of 3100-pound Buoyant Venus Devices (BVDs), a pair of 3400-pound Near Surface Floaters (NSFs), and an 6000-pound Orbiter, for a total probe mass of 19,000 pounds. Moving at 14.1 kilometers per second, the flyby spacecraft would reach periapsis 1170 kilometers above a point on the terminator near Venus's north pole.
As they drifted in the cool atmospheric layer some believed existed between 125,000 and 215,000 feet above Venus, the 82-foot-diameter BVDs would filter "very large quantities" of atmospheric gas in the hope of capturing high-flying Venusian "aerosol life." So hopeful were the Bellcomm planners that life might be found on or above Venus that they set aside 180 pounds of each BVD's 230-pound science payload for biology experiments.
Meanwhile, the 30-foot-diameter NSFs would image the gloomy surface from an altitude of a few hundred feet using floodlights and flares as needed. The Bellcomm engineers recommended that one NSF explore a polar region; the Venusian poles, they argued, would be relatively cool and thus hospitable to life. The other NSF might explore a site on the equator.
The four Floaters would transmit their data to the flyby spacecraft at a high bit rate as it passed periapsis. The astronauts would examine images from the polar NSF in hopes of finding a biologically interesting site to sample. If the NSF drifted over such a site, the crew would quickly command it to drop a claw-like anchor and lower a biological sampling device to the surface on a cable. After the flyby, control of the Floaters would pass to Earth, with radio signals relayed through the Orbiter at a reduced bit rate.
The Meteorological Balloons deployed during the 1977 Triple Planet Flyby mission and the 1978 Dual Planet Flyby mission's Floaters would share many features. All would include "superpressure" balloons filled with hydrogen. They would, however, be made of different materials because of their different operating temperatures. For those floating within 65,000 feet of the surface, the Bellcomm engineers proposed "super-alloy steel fiber weave (impregnated with silicon polymer filler)." Such fabric had been tested on Earth at temperatures of up to 1200° F, they explained. Kapton and Mylar films would probably be adequate at higher altitudes where the Venusian atmosphere would be cooler.
The Bellcomm engineers assumed that one day astronauts might explore the Venusian surface in person. They wrote that "the [manned] exploration mode could well employ a class of propeller driven cruising vehicles. . .employing nuclear power," and suggested that the NSF probes could constitute "a first step in achieving this design."
In August 1967, the U.S. Congress, eager to rein in spending in the face of increased expenditures in Vietnam, cut all funds for piloted planetary mission planning and most funds for robotic missions from NASA's Fiscal Year 1968 budget. NASA went to bat for its automated planetary program in September 1967, and succeeded in convincing lawmakers to fund automated Mars missions in the 1969, 1971, and 1973 Mars transfer opportunities. The agency did not, however, try to save piloted flybys. By the time the Bellcomm team submitted its Venus probe report, the piloted flyby concept was all but defunct. Planning for piloted planetary missions continued at a low level during 1968, enjoyed a resurgence in 1969-1970, and ceased entirely by the end of 1971 as NASA's piloted spaceflight program focused all of its efforts on the Space Shuttle.
Robotic Venus exploration continued, however; in fact, the Soviet Union made Venus its favorite target for planetary exploration. Each new mission confirmed that early optimism concerning Venus biology was unfounded. Veneras 5 through 8 were near-copies of Venera 4. In December 1970, Venera 7 crash-landed, yet managed to transmit data to Earth, making it the first spacecraft to return data from the surface of another planet. The Venera 9 through 14 landers were of a more complex and capable design. Venera 9 returned the first images of Venus's rocky surface in October 1975; these also were the first images returned from the surface of another planet. Veneras 15 and 16 included no landers; instead, they radar-mapped much of Venus's northern hemisphere between October 1983 and July 1984. The Vega 1 and 2 missions to Halley's Comet passed by Venus in June 1985; each released a balloon and a lander.
NASA's Mariner 10 spacecraft flew past Venus in February 1974. In addition to collecting data, it used Venus's gravity to shape its orbit so that it flew past the planet Mercury three times in 1974-1975. Other spacecraft have explored Venus while using its gravity to speed them toward some other destination: after the Vega twins, the next spacecraft to do so was the Galileo Jupiter orbiter, which flew by Venus in February 1990.
Pioneer Venus 1 captured into Venus orbit in May 1978 and explored the planet until August 1992, when its orbit at last decayed and it burned up in the atmosphere. It mapped most of the planet's surface at low resolution. In November 1978, Pioneer Venus 2 released one large and three small Venus atmosphere probes. Although not designed to survive landing, one of the small probes reached the surface intact and continued to transmit for more than an hour.
By the time Pioneer Venus 1 burned up, the Magellan spacecraft was in orbit around Venus. Launched from the cargo bay of the Space Shuttle Atlantis in early May 1989, the spacecraft reached Venus in August 1990. Using a high-resolution imaging radar, Magellan mapped nearly the entire surface of the planet in unprecedented detail.
On 5-6 June 2012, as Venus crossed the disk of the Sun as viewed from much of the Earth, the European Space Agency's Venus Express spacecraft was in orbit around the planet. Venus Express was launched on a Russian rocket in November 2005, and reached Venus polar orbit in May 2006. At this writing, it has operated continuously for more than six and a half years. In November 2007, scientists participating in the mission reported results from the 500-day Venus Express primary mission in the journal Nature. Besides evidence for past water oceans, they presented images of a strange double vortex in the atmosphere over the planet's south pole. They reported the existence of a Venus ozone layer in August 2011.
Reference:
Preliminary Considerations of Venus Exploration via Manned Flyby. TR-67-730-1, D. Cassidy, C. Davis, and M. Skeer, Bellcomm, Inc., 30 November 1967.
