29 April 2017
Piloted flybys in the 1970s were intended to clear a path to piloted "stopover" missions in the 1980s. Stopovers - a category which included Mars and Venus orbiters and Mars landings - almost always emphasized a single objective. That is, each mission would travel to a single world, then return to Earth. The closest stopovers came to visiting more than a one planet was when a Mars stopover mission performed a Venus "swingby" to bend its course, slow its approach to Earth to enable a safe direct Earth-atmosphere reentry, or accelerate toward Mars.
During a Venus swingby, a Mars stopover spacecraft might explore the cloudy planet much as piloted Venus flybys were meant to do. That is, it might drop off probes insulated and armored against Venusian temperatures and pressures and scan the hidden Venusian surface with radar.
That a piloted spacecraft might stop at both Mars and Venus during a single mission was unthinkable. It was widely accepted that such a mission would demand enormous quantities of propellants, all of which would need to be launched into Earth orbit atop costly heavy-lift rockets.
In a brief September 1969 NASA Technical Memorandum, E. Willis and J. Padrutt, mathematicians at NASA's Lewis Research Center (LeRC) in Cleveland, Ohio, sought to overturn the prevailing view of what would be possible during stopover missions. Lead author Willis was no stranger to NASA piloted Mars mission planning: he had designed interplanetary trajectories at LeRC at least since early 1963.
Willis and Padrutt's mission design would see a piloted spacecraft depart a circular low-Earth orbit and capture into an loosely bound high-apogee elliptical orbit around Mars or Venus. It would then transfer to a loosely bound high-apogee elliptical orbit around Mars (if the first stopover was at Venus) or Venus (if the first stopover was at Mars). From there, the spacecraft would transfer back to Earth, where the crew would reenter the atmosphere directly in a small capsule. Its usefulness ended, the dual-stopover spacecraft would, meanwhile, swing past Earth into a disposal orbit around the Sun.
The mission plan was designed to reduce the amount of energy required to move between worlds, thus conserving propellant. The piloted dual-stopover spacecraft would travel between planets only when an opportunity for a minimum-energy transfer occurred; that is, only when the planets moved into positions relative to each other necessary for a minimum-energy transfer. Loosely bound orbits would reduce energy needed to capture into and escape from orbit. Direct reentry into Earth's atmosphere would ideally require only enough energy to deflect the capsule's course so that it would intercept Earth after it separated from the dual-stopover spacecraft.
The LeRC mathematicians calculated the total "propulsive effort" necessary to carry out the seven dual-stopover missions in the 1979-1986 cycle. They measured propulsive effort in terms of the total velocity change firing the dual-stopover spacecraft's rocket motor would produce. Firing the motor would expend precious propellant, so most of the time small velocity changes were to be preferred over large ones.
They explained that they had discovered a repeating 6.4-year cycle of seven potentially useful dual-stopover mission opportunities. The seven opportunities varied only slightly from one 6.4-year cycle to the next. The first, fourth, and sixth opportunities would begin with an Earth-Mars transfer, while the second, third, fifth, and seventh would begin with an Earth-Venus transfer. In most cases, the minimum propulsive effort needed to perform Earth-Venus-Mars-Earth dual-stopovers would be less than that needed for Earth-Mars-Venus-Earth dual-stopovers. In their paper, Willis and Padrutt emphasized the 6.4-year cycle that would begin in late 1979.
The second opportunity of the 6.4-year cycle would occur in the first half of 1980. The dual-stopover spacecraft would depart Earth on a minimum-energy path to Venus. It would spend 180 days at Venus, 10 days at Mars, and 669 days between worlds, for a total mission duration of 860 days (two and a third years). This made it the shortest dual-stopover mission of the seven-mission cycle.
Because short missions limited the time available for hardware breakdowns and the crew biomedical problems, they were to be preferred to long ones. Willis and Padrutt acknowledged, however, that the opportunity's short stopover at Mars would provide little time for exploration. Total propulsive effort would amount to 8.738 kps.
The third opportunity would occur in late 1981. The dual-stopover spacecraft would leave Earth for Venus, where it would spend about 265 days. It would stop over for 133 days at Mars, and spend 629 days between worlds, yielding a total mission duration of 1027 days (nearly three years). Total effort would equal 8.7 kps.
The fourth opportunity would occur at the end of 1981. The dual-stopover spacecraft would leave Earth for Mars. It would spend about 274 days at Mars, 340 days at Venus, and 680 days between planets, for a total duration of about 1294 days (a little more than three and a half years). Total propulsive effort would equal 9.252 kps.
The fifth opportunity would occur in the first half of 1983. The dual-stopover spacecraft would leave Earth for elliptical Venus orbit, where it would spend just 10 days. It would spend 601 days at Mars and 619 days between worlds, yielding a mission duration of 1230 days (a little less than three and a half years). Total propulsive effort would total 8.896 kps. The short stopover at Venus might make the opportunity undesirable; on the other hand, the mission's Mars stopover would be the lengthiest in the 6.4-year cycle, enabling a long period of exploration.
The sixth opportunity would see the dual-stopover spacecraft depart Earth for elliptical Mars orbit in early 1984. The spacecraft would spend 200 days at Mars, 250 days at Venus, and 639 days between planets, for a total mission duration of 1089 days (a little less than three years). Total propulsive effort would amount to 9.339 kps.
The seventh and last opportunity of the 6.4-year cycle would occur in mid-1985. The dual-stopover spacecraft would spend 767 days in elliptical Venus orbit before voyaging to Mars for a 78-day stopover. It would spend 599 days between worlds - the shortest travel time of the seven opportunities. The long Venus stopover would, however, result in a mission duration of 1444 days (about four years), making it the lengthiest of the seven dual-stopover missions. Total propulsive effort would amount to 9.321 kps.
The 6.4-year-cycle Willis and Padrutt studied in detail would end just before the first dual-stopover opportunity of the next 6.4-year cycle. That opportunity, very similar to the late 1979 Earth-Mars-Venus-Earth opportunity, would occur in the first half of 1986.
Willis and Padrutt compared the total propulsive effort necessary to accomplish four of the dual-stopover missions in the 1979-1986 period with that needed to carry out four Mars stopover/Venus swingby missions. They sought to reduce dual-stopover mission duration, however, so permitted increased propulsive effort. This would enable shorter stays at planets and shorter transfers between planets. The Mars stopover/Venus swingby missions - all of which would include a 30-day Mars stopover - were assumed to leave Earth on approximately the same dates as the dual-stopover missions.
They found that the first dual-stopover mission, the December 1979 Earth-Mars-Venus-Earth mission, would need a total propulsive effort of about 13 kps to reduce its duration to 700 days. A Mars stopover/Venus swingby mission launched at about the same time could be performed in 700 days with a total propulsive effort of only eight kps. The same missions could be carried out in 575 days with propulsive efforts of 20 kps and a little less than 11 kps, respectively. These numbers indicated that the first opportunity in the 6.4-year dual-stopover cycle was not a favorable one for dual-stopover missions of reduced duration.
Dual-stopover missions launched in the other three opportunities compared more favorably with Mars stopover/Venus swingby missions. The fourth mission of the 1980s dual-stopover cycle - another Earth-Mars-Venus-Earth mission - could be shortened to 700 days if a total propulsive effort of about 12 kps were permitted, while a 700-day Mars stopover/Venus swingby mission departing Earth at about the same time would need a propulsive effort of about 10 kps.
The sixth dual-stopover mission (Earth-Mars-Venus-Earth) could be accomplished in just 625 days with a total propulsive effort of a little more than 10 kps. Willis and Padrutt calculated that a 625-day Mars stopover/Venus swingby mission launched at the same time would actually need a greater total propulsive effort: a little less than 12 kps.
The seventh dual-stopover mission in the cycle - an Earth-Venus-Mars-Earth mission - could be shortened to 675 days with a total velocity change of about 10 kps. A 675-day Venus swingby/Mars stopover mission launched at the same time would need a velocity change of eight kps.
Willis and Padrutt conceded that the minimum propulsive effort required to carry out a dual-stopover mission would almost always exceed that of a single mission that traveled from Earth to either Venus or Mars and back to Earth. They noted, however, that the minimum propulsive effort of a separately launched Earth-Venus-Earth stopover mission and a separately launched Earth-Mars-Earth stopover mission combined would always exceed that of a single dual-stopover mission. The two separate missions would together need a minimum propulsive effort of at least 17 kps; that is, nearly double the minimum propulsive effort of a typical dual-stopover mission.
Round Trip Trajectories With Stopovers At Both Mars and Venus, NASA TM X-52680, E. Willis and J. Padrutt, NASA Lewis Research Center, September 1969
Planetary Exploration Utilizing a Manned Flight System, NASA Office of Manned Space Flight, 3 October 1966
After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)
NASA Marshall's 1966 NERVA-Electric Piloted Mars Mission
To Mars by Way of Eros (1966)
Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)
Humans on Mars in 1995! (1980-1981)
Footsteps to Mars (1993)
14 April 2017
|Apollo 17 launch, 7 December 1972. Image credit: NASA|
"The mission into geosynchronous orbit," Ehricke declared, would provide "additional return on America's investment in Apollo" by dramatizing "the usefulness of manned orbital activities." He added that his proposal, which he dubbed Destination Mankind, "would inspire many, as did the lunar missions before it, but in a different, perhaps more direct manner, because of its greater relevance to some of the most pressing problems of our time."
Ehricke's emphasis on practical benefits over lunar exploration reflected a significant shift in the public perception of spaceflight - one which had gained momentum throughout the 1960s. President Richard Nixon had articulated this shift in his "Statement About the Future of the United States Space Program" on 7 March 1970. The 37th President stated that he believed that the U.S. space program should proceed at a measured pace (not on "a crash timetable") and should be devoted to scientific exploration (mainly using interplanetary robots, but with man on Mars as a "longer-range goal").
In addition, NASA should emphasize international cooperation, cost reduction, and, crucially, "practical application - turning the lessons we learned in space to the early benefit of life on Earth." Nixon declared that results of space research should be "used to the maximum advantage of the human community." He listed among the practical applications of spaceflight "surveying crops, locating mineral deposits, and measuring water resources."
Ehricke described a representative 12-day Destination Mankind mission. Reaching GSO would require about as much propulsive energy as reaching lunar orbit, he noted. The three-stage Destination Mankind Apollo Saturn V rocket would lift off from Launch Complex 39 at Kennedy Space Center, Florida, at about 8:30 p.m. local time. Following first and second stage operation, the S-IVB third stage would fire briefly to place itself, the Apollo Command and Service Module (CSM), and a Payload Module (PM) into 100-nautical-mile parking orbit. Ehricke did not describe the PM design.
One orbital revolution (about 90 minutes) later, the S-IVB would ignite again to perform Transynchronous Injection (TSI). After S-IVB shutdown, the astronauts would separate their CSM and turn it 180° to dock with the PM, which would be attached to the top of the S-IVB in place of the Apollo Lunar Module (LM). They would then extract the PM, maneuver away from the S-IVB, and settle in for the 5.2-hour coast to GSO.
Ehricke emphasized the technology objectives of his Destination Mankind mission. He was particularly enamored of a solar illumination experiment that would see a circular reflector assembled by spacewalking astronauts. The experiment would provide reference data for design and operation of future space-based reflectors, he explained. He calculated that a 100-meter reflector in GSO could light Earth's surface one-tenth as brightly as a full moon in a selected area. This level of illumination, though "subvisual," would be useful for night meteorology and surveillance of border and coastal areas, Ehricke wrote.
The astronauts would also erect "Manstar," a 500-to-700-foot-diameter reflective balloon visible over a wide area of Earth's surface as a modestly bright star. Ehricke called Manstar "a visible manifestation for all mankind of the potential value of space."
Ehricke called public relations "Public Exposure." Destination Mankind astronauts would become television stars. They would describe their Earth observations - "especially aspects useful and of interest to regional populations" - via TV broadcasts from GSO. Their spacewalks would also make for good TV fare, Ehricke judged.
|DSCOVR image of North America, South America, and Central America with adjacent oceans and seas. New Orleans, near the northern limit of the Destination Mankind Panamerican-Pacific Station, is located near the center of the image. Image credit: NASA|
The CSM and PM would oscillate between 28.5° south (over the Pacific off northern Chile) and 28.5° north (over the Gulf of Mexico south of New Orleans), again reaching the southern limit at 10 a.m. local time and the northern limit at 10 p.m. local time. Equatorial crossing would occur above the Galapagos Islands. The astronauts would spend their time much as they did at the Afro-Eurasian Station, then would fire the SPS again to drift westward across the Pacific.
The Destination Mankind crew would return to Earth from the Australo-Asian Station. Using the SPS, they would perform a Trans-Earth Injection burn as their CSM crossed the equator near Sumatra moving north at 4 p.m. local time. Fall to Earth would last 5.2 hours, and splashdown would occur in the Pacific west of Hawaii at just after 6 a.m. local time.
"Destination Mankind: Proposal for a Saturn V - Apollo Mission into Geosynchronous Orbit," K. Ehricke, North American Rockwell, 10 May 1972
The American Presidency Project, "Statement About the Future of the United States Space Program," Richard Nixon, 7 March 1970 - http://www.presidency.ucsb.edu/ws/index.php?pid=2903&st=Future+of+the+United+States+Space+Program&st1= (Accessed 14 April 2017)
"A Continuing Aspect of Human Endeavor": Bellcomm's January 1968 Lunar Exploration Program
Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (1970)
A Bridge from Skylab to Station/Shuttle: Interim Space Station Program (1971)
Reviving & Reusing Skylab in the Shuttle Era: NASA Marshall's November 1977 Pitch to NASA Headquarters
05 April 2017
Floaters, Armored Landers, Radar Orbiters, and Drop Sondes: Automated Probes For Piloted Venus Flybys (1967)
|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.
"Preliminary Considerations of Venus Exploration via Manned Flyby," TR-67-730-1, D. Cassidy, C. Davis, and M. Skeer, Bellcomm, Inc., 30 November 1967
"Experiment Payloads for Manned Encounter Missions to Mars and Venus," W. Thompson, et al., Bellcomm, 21 February 1968
Venus Space Probes, Novosti Press Agency Publishing House, 1979
NASA Facts: Mariner Spacecraft, Planetary Trailblazers, NF-39, NASA, February 1968
The Voyage of Mariner 10, NASA SP-424, NASA, 1978
Pioneer Venus, NASA SP-461, NASA, 1983
Science and Space, Novosti Press Agency Publishing House, Moscow, 1985
Soviet Space Programs 1980-1985, Nicholas L. Johnson, American Astronautical Society/Univelt, 1987, pp. 179-189
"Magellan Loss of Contact Caps Venus Mission," NASA Release 94-170, D. Isbell and J. Doyle, NASA/JPL, 12 October 1994
The Face of Venus: The Magellan Radar Mapping Mission, NASA SP-520, L. Roth & S. Wall, NASA, June 1995
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)