In recent years it seems that Mars has dominated NASA’s planetary exploration program while proposals to study our twin-planet-gone-bad, Venus, are being repeatedly rejected. Something similar was also happening during the 1960s as NASA’s original plans to explore Venus and Mars were descoped as Apollo-era budgets began to tighten leaving increasingly less ambitious missions to Mars on the long term schedule. The one exception was NASA’s underappreciated Mariner 5 mission to Venus in 1967. Using spare hardware leftover from the successful Mariner 4 mission to Mars launched in 1964, American engineers and scientists were able to quickly assemble an inexpensive Venus mission. This low-budget mission was finally able to answer some basic questions about our neighbor and the conditions on its hostile surface.


The Mariner-Venus 1967 Project

After the Jet Propulsion Laboratory’s (JPL’s) Mariner-Mars 1964 project was officially approved by NASA in early November 1962, authorization was given to build three flight-ready spacecraft. Two of them would be launched towards Mars on missions designated Mariner C and D. In order to maximize the chances of performing a pair of launches to Mars, the third was to serve as a spare which could be substituted for a malfunctioning spacecraft before launch. Spacecraft MC-2 was launched as Mariner 3 on November 5, 1964 but it failed when its newly designed launch shroud did not separate properly during ascent (see “50 years Ago Today: The Launch of Mariner 3”). After a crash program to fabricate a new shroud, MC-3 was successfully launched on November 28 to become Mariner 4. After Mariner 4 completed the transmission of the historic images it had acquired during its encounter with Mars on July 15, 1965, the Mariner-Mars 1964 project had officially met its objectives (see “Mariner 4 to Mars”).

Diagram indicating the major components of the Mariner-Mars 1964 spacecraft. Click on image to enlarge. (NASA/JPL)

No longer needed for a Mars mission, the spare MC-4 Mariner-Mars 1964 spacecraft was now surplus. But instead of consigning this hardware to a museum, scientists and engineers began considering options to reuse it for an inexpensive, one-off interplanetary mission. One mission considered was a 1969 flyby of Comet 7P/Pons-Winnecke while another called for a 1967 mission to the brighter Comet 10P/Temple (better known as Temple 2). Unfortunately the state of interplanetary navigation was not really up to the task at that time and there were still too many unknowns involved in mounting a successful comet mission. Very quickly, another target which could be reached in 1967 came into favor: Earth’s mysterious cloud-shrouded neighbor, Venus.

By the mid-1960s, the general consensus among planetary scientists was that Venus had an atmospheric surface pressure of somewhere between 5 to 300 bars (where one bar is roughly equal to Earth’s atmospheric surface pressure) with temperatures from as low as 267° C up to 480° C or more. Unfortunately the true values were not known with any certainty despite observations from increasingly sophisticated ground-based instruments and a distant flyby performed by JPL’s Mariner 2 in December 1962. Subsequent missions launched by the Soviet Union culminating with the Venera 2 and 3 flights launched in November 1964 (see “Venera 2 & 3: Touching the Face of Venus”), which could have provided new data including in situ measurements from landers, had all failed due to launch vehicle or spacecraft malfunctions. With the prospects of learning new vital information about our neighbor at a bargain price, on December 22, 1965 NASA authorized the Mariner-Venus 1967 mission. The goal of this mission was to learn as much as possible about Venus within the stipulated mission constrains including an aggressive 18-month development schedule and a tight budget equivalent to about $240 million in today’s money.

At the heart of the spare Mariner-Mars 1964 spacecraft which would fly the new mission to Venus was an octagonal magnesium frame 1.38 meters across and 0.46 meters tall. Seven of the bays supported by this framework housed electronics and power supplies for the spacecraft’s various systems while the eighth held a monopropellant, 220-newton course correction engine. Capable of making two burns with a total delta-v of 92 meters/second, the hydrazine propellant for this engine was stored in a tank in the center of the spacecraft. Two redundant sets of a half dozen nitrogen gas thrusters mounted on the ends of the four solar “wings” provided attitude control for the three-axis stabilized probe using a 2.35 kilogram supply of nitrogen gas. Attitude references were provided by Sun and star sensors along with gyroscopes to be used during mid-course maneuvers when these references were temporarily unavailable.

A drawing showing the major components of the Mariner-Venus 1967 spacecraft as viewed from above. Click on image to enlarge. (NASA)

In addition to defective hardware being replaced and selected systems upgrades based on actual flight experience with Mariner 4, there were a range of modifications and design changes made to the spare Mariner-Mars spacecraft in order to accommodate the new mission to Venus. Mounted on top of the spacecraft bus was an elliptical 1.17-by-0.53 meter parabolic high gain antenna. When Mariner’s star sensor was locked onto the bright star Canopus, which is conveniently located near the south ecliptic pole, the fixed-position high gain antenna was originally oriented to produce a long and narrow beam pattern for the S-band transmissions focused along the ecliptic plane and centered where Earth would be during the July 1965 encounter with Mars and subsequent post-encounter data transmission. Because of the new mission to Venus, the orientation of this high gain antenna had to be modified to keep it pointing towards the Earth at a different location in the sky as viewed from the spacecraft. In addition, provisions were made to change its pointing by 17.6° in flight because the Earth would be in a different position in the sky after Mariner’s trajectory was changed drastically by the close encounter with Venus.

Because Mariner would now be travelling inside of the orbit of the Earth in order to reach Venus, the orientation of the spacecraft with respect to the Sun had to be flipped by 180° to keep the high gain antenna pointing out towards the Earth. This necessitated a number of changes in the arrangement of Sun and star sensors needed to provide references for attitude control. The solar panels, which charged the bank of silver-zinc batteries that supplied power to the spacecraft systems, also had to be remounted on the underside of the frames of the four solar wings on the spacecraft. Because the sunlight is more intense at Venus than at Mars, the total area of the solar panels was also reduced from 6.54 to 4.03 square meters. With each solar panel now down to a stubbier 1.12 by 0.9 meters, the total of 17,640 solar cells would now produce 370 watts of power at the Earth and 550 watts when closer to the Sun at Venus.

A view of the Sun-facing underside of the Mariner-Venus 1967 spacecraft. (JPL/NASA)

The Mariner would also have to deal with a much higher heat load than the 1,430 watts per square meter maximum experienced during the Mariner 4 mission. With a heat load now expected to hit 2,670 watts per square meter at Venus and 4,160 watts per square meter at perihelion after encounter, the Mariner spacecraft was fitted with a Sun shade on its underside. Deployed by lanyards attached to the solar panels, this 1.23 square meter shield was made of aluminized Teflon with a thickness of 25 microns. These and other minor modifications would help the existing thermal control system maintain proper operating temperatures.


The Mariner E Mission

In order to maximize the science return from Venus, the goal of what was now called “Mariner E” before its launch was to fly within 3,200 kilometers of the cloud shrouded planet – less than one tenth of the minimum distance of 34,800 kilometers during the Mariner 2 encounter with Venus on December 14, 1962. After much deliberation, seven experiments were approved for Mariner E in February 1966. In a somewhat controversial decision, the digital television camera that was carried by the original Mariner-Mars spacecraft and its scan platform were deleted. Although it was known at that time that Venus’ otherwise featureless cloud deck displayed structure in the blue and ultraviolet (UV) parts of the spectrum, it was felt that the volume of data, the cost, complexity and effort required to test and integrate a camera working in the near-UV could not be accommodated within the project’s tight funding and schedule constraints.

A diagram showing the major components of the Mariner-Venus 1967 spacecraft as viewed from above. Click on image to enlarge. (JPL/NASA)

Three experiments that were selected to fly on Mariner E would use instruments which were essentially identical to those flown earlier on Mariner 4. The Solar Plasma Probe would monitor the properties of the solar wind, the Helium Magnetometer would measure the direction and strength of the magnetic field during the cruise and encounter with Venus while the Trapped-Radiation Detector would monitor the flux of energetic particles. New to this suite of instruments was an Ultraviolet Photometer which would be used to monitor specific UV wavelength bands to detect atomic hydrogen and oxygen in the upper atmosphere of Venus. This 4.2-kilogram instrument was originally suppose to fly on Mariner 4 but it was deleted at the last minute when difficulties were encountered with the hardware late in testing.

The Ultraviolet Photometer, which was removed from Mariner 4 at the last minute, was modified to fly on the Mariner-Venus 1967 mission. (JPL/NASA)

Another pair of Mariner-Mars investigations that Mariner E would carry out would employ the S-band transmitter on the spacecraft. The Celestial Mechanics investigation would use information from tracking the S-band transmissions to help refine the orbits and masses of the Earth and Venus as well as the value of the astronomical unit or AU (which was a derived quantity until the International Astronomical Union officially defined it as being 149,597,870,700 meters in 2012). This same transmitter would also be employed for the S-Band Radio Occultation Experiment in which special equipment at NASA’s Goldstone tracking station would be used to detect the changes in the signal’s intensity, frequency and phase when the radio transmissions passed through the atmosphere of Venus as Mariner E flew behind the planet as viewed from the Earth. These observations could be used to provide information on the temperature, pressure and density of the Venusian atmosphere and how they changed with altitude. This technique was first employed during the Mariner 4 mission to probe the atmosphere of Mars for the first time confirming new ground observations that it was much less dense than had been earlier believed (see “Zond 2: Old Mysteries Solved & New Questions Raised”)

A diagram showing the major components of the Mariner-Venus 1967 spacecraft as viewed from below. Click on image to enlarge. (JPL/NASA)

One final experiment included on Mariner E to supplement the other investigations of the atmosphere of Venus was the Dual-Frequency Propagation Experiment which was similar to that flown on NASA’s Pioneer 6 solar orbiter which had been launched on December 16, 1965. To support this experiment, Mariner was fitted with a set of antennas mounted on two of its solar wing frames connected to receivers sensitive to frequencies of 49.8 and 423.3 MHz in the VHF and UHF bands, respectively. These receivers would monitor continuous wave, phase-locked transmissions with a power of 350 kilowatts in the VHF and 30 kilowatts in the UHF from the steerable 46-meter parabolic antenna at Stanford University’s Center for Radar Astronomy (which today is a landmark known simply as “The Dish”). Measurements of the phase and strength of the signals would provide information on the electron density of interplanetary space and the ionosphere of Venus as well as supplement measurements of the neutral atmosphere of Venus being made by the S-Band Radio Occultation Experiment.

The digital recorder employed by Mariner 4 was modified to record close encounter data from the Mariner-Venus 1967 mission. (NASA

Because of the deletion of the camera and the new sequence of events during the Venus encounter, the systems which handled the acquisition and recording of data needed to be modified. Most notable were changes made to the digital data recorder. As was the case with the encounter with Mars, Mariner E would be transmitting data at a rate of only 8⅓ bits per second around the time of its encounter with Venus – only about an eighth of the rate at which it was to be acquired. The digital tape recorder was modified to now handle 129-minutes of continuous data recording at a rate of 66⅔ bits per second instead of 25 minutes of intermittent image data recording at a rate of 10,700 bits per second. With the total data volume now only about one fifth of that of the Mariner 4 mission, the length of magnetic tape was decreased from 100 meters down to only 15 meters. After all of its modifications, the Mariner E spacecraft had a launch mass of 244.8 kilograms – 16 kilograms lighter than Mariner 4.

The Mariner-Venus 1967 flyby trajectory. (NASA)

In order to meet the science objectives and simplify the design of the spacecraft and its encounter with Venus, it was decided to fix the encounter time with Venus at 18:00 GMT on October 19, 1967 regardless of the actual launch date. This fixed encounter time meant the positions of the Earth, Sun and Venus with respect to Mariner E would also be fixed. This allowed the antennas, body-mounted sensors for the Sun, stars and Venus as well as instruments like the UV photometer to be fixed into optimum positions to observe their target during the flyby. In addition to striking a balance between launch energy requirements, transit time and communication distance, this encounter time was also chosen to be in the middle of the visibility window from NASA’s primary deep space tracking facility at Goldstone.

This diagram shows the configuration of the Atlas-Agena D to be used for the Mariner-Venus 1967 mission. Click on image to enlarge (NASA)

While from a launch energy point of view Mariner E could lift off on its Atlas-Agena D launch vehicle in early June 1967 in order to reach Venus on the desired date, the resulting encounter geometry would be less than optimal for the first part of the month. The best launch window for the Mariner E mission which would allow all of the science objectives to be achieved was eventually selected to run from June 14 to 24. In order to ensure this mission got off the ground for Venus with only a single spacecraft and launch vehicle available, a spare launch shroud similar to those used on NASA”s Lunar Orbiters was made ready and provisions were in place to use the maximum allotted performance margin of the Atlas-Agena D as well as reduce its weight in order to extend the launch window to as late as July 1 if necessary. If Mariner E missed its June 1967 launch window, it would be almost 19 months before another launch attempt could be made (assuming that NASA decided to authorize the extra funding for a mission with a new launch date).


Getting Mariner 5 Underway

On March 17, 1967 the first piece of mission hardware, Atlas SLV-3 number 5401, arrived at Cape Kennedy followed a week later by its Agena D upper stage. On April 14, the Atlas was erected at Launch Complex 12 for the beginning of testing on the pad. The Agena was added to the stack on May 29 followed by an initial mating with the Mariner spacecraft encapsulated in its launch fairing two days later. The spacecraft was removed on June 2 for some additional work and mated with launch vehicle for the final time six days later. After completing a simulated countdown exercise on June 9, Mariner E was ready for launch.

The Mariner-Venus 1967 spacecraft is shown being weighed on May 15, 1967 at Kennedy Space Center before it was encapsulated for launch. (NASA)

But as Mariner was waiting on the pad, the Soviet Union launched their own Venus-bound spacecraft on June 12, 1967. Called Venera 4, this 1,106-kilogram spacecraft was the first to be built by the Soviet design bureau known as NPO Lavochkin. Based on the earlier troubled 3MV interplanetary probes designed and built by OKB-1 (the ancestor of today’s Russian aerospace company, RKK Energia), this new spacecraft had been significantly redesigned and built to much higher standards to improve the chances of it successfully reaching Venus. Unlike NASA’s comparatively modest flyby mission, the primary objective of Venera 4 was the ambitious goal of landing a 383-kilogram capsule on the surface of Venus returning information about its surface and atmosphere during the long parachute descent (see “Venera 4: Probing the Atmosphere of Venus“).

A Russian diagram of the Venera 4 lander launched on June 12, 1967. Click on image to enlarge.

The window for Mariner’s first launch opportunity ran from 1:16 AM to 3:30 AM EDT on June 14, 1967. The countdown for Mariner E proceeded as planned until a hold at T-7 minutes. At this point, the hold was extended by 14 minutes so that the flight plan could be updated to provide better radar and telemetry coverage downrange. At 2:01:00.176 AM EDT (06:01:00.176 GMT), the Atlas-Agena D with Mariner E on board lifted off from LC-12. After 296.7 seconds of powered flight, the Atlas sustainer engine shutdown 0.8 seconds earlier than predicted. Because the ascending rocket had been placed into a slightly lower than expected trajectory, the ignition of the main engine on the Agena D was delayed by 9.6 seconds to 380.4 seconds after launch to ensure that the parking orbit would be high enough. After a 143.7-second burn by the Agena D, the upper stage and its payload had been successfully placed into their temporary 181.5 by 194.5 kilometer parking orbit with an inclination of 29.9°.

The launch of Mariner 5 from LC-12 at Cape Kennedy on June 14, 1967. (KSC/NASA)

After a coasting for 13 minutes and 14.5 seconds, the main engine of the Agena D ignited for a second time to send its payload towards a point 75,000 kilometer from Venus – an intentional wide miss to ensure that a malfunctioning Mariner or the Agena D would not accidentally impact Venus and break planetary quarantine. Despite an unexplained momentary drop in thrust during the 95.5-second burn (a phenomenon observed in earlier NASA and USAF Agena D missions), what was now called Mariner 5 was on its way. Just over 26 minutes after launch, Mariner 5 separated from the Agena D then proceeded to unfold its solar panels and begin its search for the Sun. Five minutes later, the spent upper stage performed a posigrade maneuver after turning to one side to safely move away from Mariner 5 and miss Venus by 231,100 kilometers.

For the next 16 hours, Mariner 5 was placed into a slow roll of two revolutions each hour with its solar panels locked onto the Sun. Part of this was to allow the receding probe’s star sensor to map the stars in its field of view so that ground controllers would know where Canopus was located. Other reasons had to do with the science objectives. The slow roll allowed the UV photometer to map a swath across the celestial sphere as well as observe the Earth’s geocorona. In addition, the slow roll allowed a careful calibration of the sensitive magnetometer to help characterize any interference from the spacecraft (a problem which compromised the accuracy of Mariner 2 magnetic field measurements). At a distance of 241,000 kilometers from the Earth, Mariner 5 locked onto Canopus and settled into its three-axis stabilized cruise mode at 01:12:26 GMT the day after launch.

A plot of the raw intensity values from the UV photometer as Mariner 5 rolled at a rate of twice per hour during the first 16 hours of flight. The peaks show when the UV photometer was observing hydrogen in Earth’s geocorona which decreased in brightness as Mariner moved away from the Earth. Click on image to enlarge. (NASA)

With Mariner 5 now safely on its way to Venus, the Soviet Union attempted one more launch to Venus to accompany its Venera 4 lander. On June 17 a second lander was launched but the engine of the final stage failed to ignite stranding what was now designated Kosmos 167 in a 201 by 286 kilometer parking orbit which decayed eight days later. It would be just Venera 4 and Mariner 5 making their way to Venus – the first time a Soviet and an American spacecraft would be simultaneously heading towards Venus.


The Mariner 5 Encounter and Results

As the receding Mariner 5 settled into its cruise routine of acquiring data and monitoring the performance of its various systems, preparations were made to correct its course to ensure the desired arrival time and miss distance at Venus. At 21:23:57 GMT on June 19, Mariner 5 started turning to the proper attitude for its first scheduled midcourse maneuver. At 23:08:11 GMT, Mariner’s engine ignited for 17.65 seconds to change the spacecraft’s velocity by 15.4 meters per second. Although this delta-v was 4.5% lower than planned, it changed the calculated arrival time of Mariner 5 at Venus from 03:53 to 17:59 GMT on October 19 and altered the original 75,781 kilometer miss distance to a calculated distance of just 3,990 kilometers. While this aim point was slightly farther from the planet than originally hoped, it was still sufficient to meet the mission’s objectives so the second course correction was deemed unnecessary. Mariner 5 would continue to gather data during its 127-day cruise and, during September and October, even coordinated observations of the interplanetary environment with measurements made by Marnier 4 then on its extended mission.

This diagram shows the positions of Mariner 4 and 5 as they made coordinated observations of the interplanetary environment during September and October 1967. Click on image to enlarge. (JPL/NASA)

As Mariner 5 continued with its routine, Soviet controllers were busy monitoring the progress of Venera 4 as it used its instruments to collect data on the interplanetary environment as well. On July 29, Venera 4 was commanded to perform its only midcourse maneuver at a distance of 12 million kilometers from the Earth. The burn successfully changed the trajectory from a miss of 60,000 kilometers to an impact near the equator on the night side of Venus.

After another 81 days in transit, Venera 4 finally released its lander at 04:34 GMT on October 18, 1967 while 44,800 kilometers from Venus. The lander hit the atmosphere over the night side at a speed of 10.7 kilometers per second and quickly slowed to 300 meters per second after experiencing peak braking loads of 350 g. When the atmospheric pressure hit 0.6 bars, a parachute was deployed and, at 04:39 GMT, Venera 4 began transmitting data back to Earth. Venera 4 continued transmitting data for 93 minutes until the measured temperature had climbed to 262° C and the pressure had reached an estimated 18 bars – far above the 7.3 bar upper limit of the lander’s barometer. Based on Venera’s radar altimeter measurements, Soviet authorities claimed that Venera 4 had stopped transmitting when it reached the surface at about 19° N, 38° E roughly between what we know today as Eistla Regio and Bell Regio. After seven years of frustrating failures, a Soviet spacecraft had finally succeeded in returning data from another planet – and all the way to the surface of another planet for the first time, no less.

Here we see the Mariner 5 control room at JPL during its close encounter with Venus on October 19, 1967. (JPL/NASA)

As the Soviet propaganda machine hailed the accomplishment, controllers at JPL were busy preparing for the Mariner 5 flyby encounter with Venus after completing its uneventful cruise. At 02:49:00 GMT on October 19, commands were sent from the Deep Space Station 41 (DSS-41) in Womera, Australia to Mariner 5 for it to prepare for the encounter sequence. The tape recorder started storing science data at 16:46:46 GMT as Mariner 5 picked up the pace of observations. Mariner 5 reached its minimum encounter distance of 4,094 kilometers above the Venusian surface at 17:34:56 GMT. Some 252 seconds later, Mariner 5 entered the “occultation zone” and was observed using the then-new 64-meter dish of DSS-14 at Goldstone until lock was lost at 17:42:05 GMT as Mariner 5 passed behind Venus as seen from the Earth.

This is a plot of S-band signal strength as a function of time as Mariner 5 passed behind Venus. (JPL/NASA)

At 17:46:51 GMT while Mariner 5 was still behind Venus, it shifted the pointing of its high gain antenna by 17.6° to optimize it for exit from occultation and subsequent data transmission. Goldstone’s new DSS-14 antenna locked onto Mariner’s signal at 17:59:59 GMT as it began to reemerge from behind Venus. At 18:34:18 GMT the command was given to stop recording near encounter data and the instruments were placed back into their low data-rate modes. The close encounter with Venus was now completed. At 07:25:23 GMT on October 20 commands were initiated to begin playback of the data on the tape recorder. The first playback, followed by two more to correct for missed bits, required 72½ hours to return all the data with the specified bit error rate.

This diagram illustrates the major milestones during the Mariner 5 encounter with Venus on October 19, 1967. Click on image to enlarge. (NASA)

With the new data in hand, planetary scientists began the long task of analysis. The tracking of Mariner 5 and the 101.5° change in its trajectory with respect to Venus showed that planet has a mass of 0.815 times that of the Earth – essentially identical to the earlier result from Mariner 2 but with a lower uncertainty. During the encounter, no trapped radiation like the Van Allen Belts of Earth were found and the maximum magnetic field strength was placed at 1% of Earth’s. The upper bound of the planet’s ionosphere was found to be at an altitude of 500 kilometers while the UV photometer detected an Earth-like hydrogen corona around Venus but with no trace of atomic oxygen. It also detected a faint UV glow on the night side of Venus.

A plot of Lyman-alpha emissions measured by the UV photometer during the Mariner 5 flyby of Venus. Click on image to enlarge. (JPL/NASA)

By far the most interesting results came from the radio occultation observations which probed the atmosphere of Venus independently of Venera 4. Because of the density of the atmosphere, S-band transmissions could only probe down to a point 6,085 kilometers from the planet’s center or about 32 kilometers above the radius of Venus derived from Earth-based radar observations. At lower altitudes, the atmosphere becomes super-refractive and radio signals are bent around the planet. Extrapolating down to the surface, the temperature was expected to be between 377° C and 527° C with a pressure somewhere between 75 and 100 bars, depending on the assumptions made about how much carbon dioxide and nitrogen were in the atmosphere. These values were far in excess of what Venera 4 had recorded when it was allegedly near the surface of Venus. So what happened?

Here we see a plot of the pressure versus temperature of the atmosphere of Venus based on observations by Venera 4 and Mariner 5. (JPL/NASA)

A reexamination of the measurements of Venera’s results showed that the radar altimeter data had been misinterpreted. Instead of transmitting data all the way to the surface, Venera 4 had actually stopped transmitting data at an altitude of 26 kilometers either because of structural failure caused by the higher-than-expected atmospheric pressure or the exhaustion of its battery (which was nominally rated at 100 minutes) by the longer-than-expected descent. While Venera 4 was the first Soviet spacecraft to reach another planet in operational condition, it was not the first spacecraft to transmit data from the surface of another planet. Although disappointing, the in situ measurements returned by Venera 4 were of immense value. Combining the data on the atmosphere and its composition as measured by Venera 4, by 1969 Soviet planetary scientists had extrapolated their data to show that the surface temperature as about 442° C with a pressure of 90 bars – very close to today’s accepted values and a bit closer to the mark than those of Mariner 5. The complimentary data sets of Mariner 5 and Venera 4 had finally resolved one of the long standing mysteries of Venus.


The Extended Mission

After its encounter with Venus, Mariner 5 was now in a 0.580 by 0.735 AU orbit with a period of 194.6 days. This was much tighter than the 0.716 by 1.018 AU solar orbit with a period of 295.0 days just before the encounter and provided Mariner 5 with a unique position in the solar system. Assuming that it would continue to operate well past its primary mission like its predecessor Mariner 4 had done, scientists had already planned for an extended mission for Mariner 5 which was still fully operational and in good health after departing Venus. The primary objectives for this phase of the mission were to continue gathering data on the interplanetary environment, map uncharted regions of the celestial sphere using the UV photometer and continue refinement of important astronomical constants like the AU.

Because of the increasing distance to the spacecraft, however, controllers at JPL placed Mariner 5 into hibernation on November 21, 1967 with the spacecraft at a distance of 97.9 million kilometers from the Earth. The spacecraft had switched to its low gain antenna and was now transmitting a simple carrier wave. The beacon from Mariner 5 was last detected by DSS-14 on December 4 at a range of 137.5 million kilometers with the spacecraft reaching its first perihelion on January 4, 1968.

Diagram showing the orbits of Mariner 4 and 5 up to the beginning of 1968. Click on image to enlarge. (JPL/NASA)

Attempts to reacquire Mariner 5 began on April 26, 1968 but with no success. Additional attempts were made on May 3, 23, 31 and June 18 using 64-meter DSS-14 with no results. A failure mode analysis was then begun in an effort to determine what may have happened to Mariner 5 and devise an alternate recovery plan. Another 17 attempts between June 28 to October 14 using a variety of strategies to recontact Mariner 5 all failed to detect any signals. Finally, another attempt on October 14 detected Mariner’s carrier signal at 16:35 GMT but controllers were unable to establish two-way communications. The first two-way lock with Mariner 5 was established on October 20 but no telemetry could be detected. Additional attempts to command Mariner 5 failed as it reached its closest point to Earth of 39.0 million kilometers on October 27. As the spacecraft began to recede from the Earth once more, the final attempt to regain control of Mariner 5 took place on November 5 again with no success. While the nature of the signals from Mariner 5 suggested that it was rolling at a rate of about 4 RPM, the lack of any telemetry effectively ended attempts to diagnose the nature of the apparent spacecraft failure.

With the failure to regain control of Mariner 5, all efforts to save the wayward spacecraft ended on November 5, 1968. Although there was certainly disappointment over the failure to perform an extended mission, Mariner 5 had succeeded brilliantly at its primary mission of characterizing the atmosphere of Venus. For the next several years, JPL’s efforts would focus on the Mariner missions to Mars with only the last mission of the series, Mariner 10 launched on November 3, 1973, passing by Venus on its way to the mission’s primary destination, Mercury. While NASA had its eyes on other planetary targets, the Soviet Union used its increasingly sophisticated (and successful!) Venera spacecraft to continue the detailed study of our near neighbor.


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Related Reading

“Venera 4: Probing the Atmosphere of Venus”, Drew Ex Machina, October 21, 2017 [Post]

“Mariner 4 to Mars”, Drew Ex Machina, July 14, 2015 [Post]


General References

Wesley T. Huntress, Jr. and Mikhail Ya. Marov, Soviet Robots in the Solar System: Mission Technologies and Discoveries, Springer-Praxis, 2011

L.R. Koenig, F.W. Murray, C.M. Michaux and H.A. Hyatt, Handbook of the Physical Properties of the Planet Venus, NASA SP-3029, 1967

Paolo Ulivi with David M. Harland, Robotic Exploration of the Solar System: Part 1 – The Golden Age 1957-1982, Springer-Praxis, 2007

Mariner/Venus Set for June Launch at Cape, NASA Press Release 67-124, June 4, 1967

Mariner V Passes Venus, NASA Press Release 67-267, October 17, 1967

Mariner Venus 67 Final Project Report: Volume I. Launch Through Midcourse Maneuver, JPL Technical Report 32-1230, June 15, 1968

Mariner Venus 67 Final Project Report: Volume II. Midcourse Maneuver Through End of Mission, JPL Technical Report 32-1230, May 1, 1969

Atlas-Agena Performance for the 1967 Mariner Venus Mission, NASA TM X-1826, June 1969

Mariner-Venus 1967 Final Project Report, NASA SP-190, 1971