|Venus as imaged by the European Space Agency's Venus Express spacecraft. Image credit: ESA|
Two days later, after launch on an 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 to provide a home for Venusian life. They also suggested that living things - most likely, microorganisms - might float high above Venus's surface in cool moist cloud layers.
Venera 4 reached Venus on a collision course, as planned, 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. The capsule was designed to float if it splashed down in water.
|Venera 4-type Venus landing capsule. Image credit: NASA|
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 October 1967, it began to play back data to Earth. The U.S. spacecraft found no radiation belts akin to the Van Allen Belts that girdle Earth; this was not surprising, since it also measured 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 - became occulted - by the solid body of the planet. The occultation experiment revealed that, at the point where it contacts the surface, Venus's atmosphere has a temperature of almost 1000° F. The planet's surface atmospheric pressure, it showed, is from 75 to 100 times greater than Earth sea-level pressure.
As Venera 4 and Mariner 5 explored Venus, D. Cassidy, C. Davis, and M. Skeer, engineers at Bellcomm, NASA's Washington, DC-based Apollo planning contractor, put the finishing touches on a report for the Office of Manned Space Flight at NASA Headquarters. 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 and 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 February 1977, almost a decade after the Venera 4 and Mariner 5 missions. The piloted flyby spacecraft would fly past Venus in June 1977, pass Mars in December 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 January 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 the planet's atmosphere, though they also described armored Venus landers, impactors, and large orbiters.
|1977 Venus-Mars-Venus piloted flyby mission first (dayside) Venus encounter geometry. Image credit: Bellcomm/NASA|
The Triple Planet Flyby crew 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 released, 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 it would enter Venus's atmosphere at the steepest angle of the six DSAPs, the terminator-equator DSAP would need to withstand 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 orbital motion to chart any Venusian gravity anomalies.
|Venus Meteorological Balloon deployment sequence. Image credit: Bellcomm/NASA|
The Bellcomm team targeted the twin "survivable type" Landers to Venus's north pole and mid-light regions. 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 the dense 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 flyby spacecraft 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 would each 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 surface.
|1977 Venus-Mars-Venus piloted flyby mission second (nightside) Venus encounter geometry. Image credit: Bellcomm/NASA|
Bellcomm recommended that the third Venus flyby of the series, the 1978 Dual Planet Flyby mission's May 1979 flyby, should emphasize "the search for life and extended surface operations." The astronauts would release 19,000 pounds of probes including a pair of 3100-pound Buoyant Venus Devices (BVDs), twin 3400-pound Near Surface Floaters (NSFs), and a 6000-pound Orbiter. Moving at 14.1 kilometers per second, the flyby spacecraft would attain periapsis 1170 kilometers above a point on the terminator near Venus's north pole.
|1978 Venus-Mars piloted flyby mission Venus encounter geometry. Image credit: Bellcomm/NASA|
Meanwhile, the 30-foot-diameter NSFs would image the gloomy surface from an altitude of a few hundred feet using floodlights and flares to light the scene as required. The Bellcomm engineers recommended that one NSF seek life in the relatively cool polar region. The other NSF might explore a site on the equator.
|Near Surface Floater in sample collection mode. Image credit: Bellcomm/NASA|
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 expected that one day astronauts might explore the Venusian atmosphere 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 might 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 almost entirely by the beginning of 1972 as NASA's piloted spaceflight program focused most of its future-directed energies on the Earth-orbital, semi-reusable 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 about Venusian 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 were also 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 passed by Venus en route to Comet Halley 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 a Venus gravity assist 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 and momentum 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 using a low-resolution imaging radar. 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 near-polar orbit around Venus. Launched from the cargo bay of the Shuttle Orbiter Atlantis in early May 1989, the spacecraft reached Venus in August 1990. Using a high-resolution imaging radar, Magellan imaged nearly the entire surface of the planet in unprecedented detail by September 1992, enabling detailed geological mapping. After a series of Venus gravity, radio science, and aerobraking experiments, Magellan descended into the Venusian atmosphere and burned up on 13 October 1994.
|Artist's impression of the Venus Express spacecraft in orbit over the double vortex at Venus's south pole. Image credit: European Space Agency|
In November 2007, scientists participating in the mission reported results from the 500-day Venus Express primary mission in the journal Nature. In addition to evidence for water oceans in the ancient past, they presented images of a strange double vortex in the atmosphere over the planet's south pole. In August 2011, they reported that Venus has an ozone layer.
Venus Express ceased transmitting data to Earth in November 2014 as it ran low on fuel. It is thought to have entered the Venusian atmosphere and burned up in January-February 2015. Scientists studying Venus Express data announced in June 2015 that they had found new evidence for present-day volcanism on Venus.
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Venus Space Probes, Novosti Press Agency Publishing House, 1979
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The Voyage of Mariner 10, NASA SP-424, NASA, 1978
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NATURE Web Focus: Venus Express - http://www.nature.com/nature/focus/venusexpress/ (Accessed 5 April 2017)
ESA Venus Express - http://www.esa.int/Our_Activities/Space_Science/Venus_Express (Accessed 5 April 2017)
The Challenge of the Planets, Part Three: Gravity
Centaurs, Soviets, and Seltzer Seas: Mariner II's Venusian Adventure (1962)
Triple Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)
Things to Do During a Venus-Mars-Venus Piloted Flyby Mission (1968)