Reflecting back on the history of robotic space missions, Larry Klaes offers a look at the early missions to Venus and Mars, harbingers of the far more complex probes we would later send into the Solar System. The Pioneers, Veneras and Mariners were, in their day, on the forefront of planetary research, blazing the trail most recently followed by Curiosity on Mars. As a site focused on deep space issues, we often return to Voyager and Pioneer, but let’s not forget how planetary exploration got its start.

By Larry Klaes

Once upon a time, our Solar System was a very lively place.

In past centuries, most if not all of the known planets and their moons, along with the even smaller members of our celestial neighborhood, were imagined to have native life forms as numerous and diverse as those found on Earth. Otherwise, it seemed pointless for whole worlds to exist without any inhabitants.

Then along came the Twentieth Century. Improved knowledge about the Sol system caused the majority of contemporary astronomers and other scientists to realize that most of these distant bodies were just too hot or too cold or too airless to support complex organisms, especially intelligent ones capable of building civilizations.

There were some holdouts, however.

The fourth world from the Sun, the planet Mars, retained enough of its Earth-like characteristics to convince scientists that plant life and perhaps even some lower animal forms dwelled in the harsh environment. It was also considered that while the canal-building beings of the Red Planet were probably figments of the Nineteenth Century imagination of Percival Lowell and his followers, astronomers could not entirely rule out the possibility that an advanced society once did exist in the Martian past, when that alien globe was warmer and wetter. The crumbling ruins of their civilization might be awaiting discovery, buried by ages of unchecked planet-wide dust storms.

Image: A once enigmatic planet, Mars began to yield its secrets to our early probes. This HST image shows the Valles Marineris region (centered on roughly 60 degrees longitude). Credit: NASA/NSSDC.

Moving in the direction of our yellow dwarf star past Earth, the planet Venus was even more of an enigma than Mars. The primary mystery of our other celestial neighbor came from its being sheathed in an unbroken cover of vaguely yellowish clouds.

The rather high limitations on factual information about the second world from the Sun due to its global atmospheric conditions did very little to stem some rather high levels of speculation about what was going on far beneath the Venusian cloud deck.

Some assumed that because Venus was perpetually shrouded with clouds, it must be raining there everywhere all the time. Toss in the fact that Venus is closer to the Sun than every other planet but Mercury and it became easy to conclude that the whole world was a very tropical place bathed in a perpetual twilight. Perhaps Venus was reminiscent of the Carboniferous Period on Earth, with alien swamps full of strange plants and reptilian creatures crawling through the dankness.

Even when astronomers were able to do more than just stare at the nearly blank visage of Venus with their optical telescopes, conflicting data ruled the day. As a result, some scientists theorized that the planet was covered in a global ocean of either boiling oil or seltzer. Others concluded that Venus was a desert world where fast, hot winds whipped across the utterly dry surface and the clouds were made not of water droplets but of constantly elevated dust.

Image: Venus’ hidden surface would not be understood until Soviet and American probes confirmed its extreme temperatures and pressures. Credit: NASA/NSSDC.

Nor could the scientists come to an agreement on the average surface temperature of Venus: Some thought their measurements meant that the face of the planet was very hot, many hundreds of degrees warmer than on Earth; any water present would be turned to vapor and any known forms of native life would be out of the question. The other camp said the high temperatures came not from the planet’s surface but its surrounding ionosphere. Perhaps Venus was just hospitable enough to keep water in a liquid state, thus preserving the Venusian ferns and dinosaurs – at least in theory.

Enter the Space Age

Two major events of the Twentieth Century played important roles in helping science to discover the true nature of the planet Venus: World War Two and the subsequent Cold War. Both international conflicts provided the technology, engineering, and motivation to bring about the Space Age, which officially began in the fall of 1957.

On October 4, the Soviet Union lofted into Earth orbit a silvery sphere named Sputnik 1, its four swept-back whip antennae transmitting to the world far below a deceptively simple signal of its existence, along with the wider implications of what had been and could be achieved in this new era. Then, just one month later, the Soviets sent a dog named Laika into space aboard Sputnik 2, the first living creature to circle the globe.

In response to this geopolitical challenge and potential military threat, the United States ramped up its own space efforts, quickly turning the Space Age into the Space Race. Both superpowers began to use the rest of the Universe as a generally non-destructive means to show the rest of humanity that its capabilities, resources, and especially its ideologies were the best.

Science and the expansion of human knowledge into the Cosmos were often touted as the first and foremost reason for lofting these new machines into space: One of the goals of the International Geophysical Year (IGY) of 1957-1958 was to achieve the orbiting of an artificial satellite. While not a fabrication, it was clear to anyone who looked beyond the rhetoric and propaganda that without the swiftly growing military-industrial complex of both sides, most space science and exploration plans would have remained largely science fiction for many more years if not decades into the future.

The early years of the Space Age/Race were dominated by achieving a series of firsts in the Final Frontier. Once low Earth orbit, or LEO, had been reached (and won in the political sense by the USSR), the next obvious goals were the nearest worlds to Earth: Our celestial home’s singular natural satellite, the Moon, and the two closest planets in the Solar System, Venus and Mars.

By the end of the 1950s, the geopolitical winner in the robotic race to the Moon was again the Soviet Union. After several launch failures by both superpowers in 1958, the next year witnessed the USSR’s aptly named Luna series sending one probe past the Moon (and subsequently becoming the first artificial satellite to orbit the Sun), a second vessel impacting on the lunar surface, and a third robot revealing to humanity with its onboard cameras the face of the mysterious lunar farside.

The next obvious goal in this celestial competition was Venus. With a closest approach to Earth of 26 million miles during its 224.7-day dance around the Sun, an unmanned probe could reach that world in less than four months with the most ideal launch window. This relatively short trip time was a most important factor, for the environment of the inner Sol system held many potentially hazardous unknowns for an exploring spacecraft.

Another key milestone to conquer was the endurance of the prevailing space technology itself. If a vessel survived the initial launch and escape from its Earth parking orbit – for numerous rockets during that pioneering era had an alarming frequency of exploding or otherwise malfunctioning – the probe then had to last long enough to communicate its scientific findings to Earth across many millions of miles of near vacuum.

The farthest any vessel had reached into that celestial region at the time was the American probe named Pioneer 5. Originally intended to be part of an ambitious double mission to flyby and orbit Venus, the goals were scaled back until Pioneer 5’s primary focus became the interplanetary space between Venus and Earth. The probe spent over three months in 1960 returning precious scientific data and was tracked out to a distance of 22.5 million miles from home. Pioneer 5 paved the way for the flotilla of Venus probes that were soon to follow.

As with Mars in 1960, the Soviets were the first nation to attempt sending automated spacecraft to the second world from the Sun. Two probes were launched from their primary space center in a remote region of Kazakhstan just over one week apart in February of 1961.

The first machine never left Earth orbit due to a technical issue with one of its rocket stages. The second probe, later to be known as Venera 1 (the Russian word for Venus), was successfully injected into deep space but ceased communicating with its controllers before the month was out. Attempts were made to detect and resume contact with Venera 1 through its predicted arrival at Venus in May of 1961, but the probe was never heard from again.

Years later, the West learned that the Soviets had intended to place simple instrumented capsules on the surface of Venus with these 1961 missions and not just fly by the planet as originally thought, just as they had tried and failed with Mars one year earlier. As a number of the Soviet scientists had sided with the Venus model that included some kind of liquid ocean covering the planet, the capsules were designed to float.

The Soviets’ confidence in a watery Venus was demonstrated by the fact that the landers were not equipped with any mechanical means to touch down softly on the planet. Instead the capsules would have to rely on the thick Venusian atmosphere along with its presumed alien ocean for cushioning, which the Soviets estimated to be five times denser than on the surface of Earth.

The Mariners of a Vaster Sea

When the next launch window to Venus opened up in the late summer of 1962, the United States was finally ready to join the Soviets in sending robotic representatives to that alien world.

The first American Venus explorer, named Mariner, was based on the Ranger series of lunar probes made by the Jet Propulsion Laboratory (JPL). Designed to image the lunar surface until impact and place on the Moon a seismometer wrapped in a protective sphere of balsa wood to record any quakes, the Ranger craft were not performing as planned. Various technical issues kept the three Rangers launched throughout 1962 from returning any images or data about Earth’s natural satellite. National prestige took some small comfort when Ranger 4 impacted on the lunar farside, becoming the first American probe to touch the Moon.

The Soviets attempted to trump the planned American deep space efforts by sending three rather sophisticated probes toward Venus between August 25 and September 12 of that year. One vessel would conduct a photographic flyby of the planet, while its two siblings would place landers on the surface, analyzing the thick and cloudy atmosphere during their descents.

Unfortunately for Soviet prestige and planetary science, none of the three unnamed Venus explorers were able to get any further than temporary Earth orbits at best due to technical failures with their launch rockets. The Soviet Union officially downplayed the loss of their newest Venus probes. This was standard procedure for many of their space efforts, failed and otherwise depending on the mission, during the Cold War.

An Oil Rig to Venus

At first glance, the design of the first two planetary Mariners bears some resemblance to an old fashioned oil rig, albeit one with two rectangular wing-like solar panels and a parabolic high-gain antenna attached to its hexagonal base where the basic computer, electronics, batteries, and attitude control devices were stored. The lattice-work mast held the majority of the probes’ forty pounds of scientific instruments and was capped off by a cylindrical omni-directional antenna.

Of all the scientific devices on the Mariners, perhaps the most important ones were the microwave and infrared radiometers. They would determine the actual overall temperature of the planet’s surface and the makeup of the obscuring cloud deck, scanning Venus across its day and night hemispheres. The rest of the instruments would search for any magnetic and radiation fields generated from the planet, along with detecting high-energy cosmic radiation, interplanetary dust, and charged particles streaming from the Sun, also known as the solar wind.

Small sensors spread about the Mariners would constantly measure the state and health of the space probes while in flight. They would alert mission controllers to any technical problems during the long and potentially hazardous trek to Venus and provide the space agency with vital data for improving future vessels on interplanetary journeys.

An optical imaging system was not included among the scientific equipment of these pioneering robotic explorers, partly because it was felt there would be little or nothing to witness beyond the bland, yellowish cloud cover. Earth-based astronomers sometimes reported and photographed diffuse darker patches on the clouds of Venus (and a strange luminous glow on the night side called the Ashen Light) and even cloud breaks through the turgid atmosphere straight to the surface, but the last claims were always questionable at best.

One last item placed aboard each Mariner was neither a scientific instrument nor a sensor: A small American flag was sandwiched between some layers of thermal material at the top of the probes. The Soviets also placed commemorative mementos aboard their Venus explorers. Their tributes consisted of small metal Earth globes designed to survive the plunge to the planet’s surface with their Venera landers. Inside the hollow spheres were medallions with the Soviet Coat of Arms engraved on one side. The other face of the metallic disk displayed a diagram of the inner Sol system depicting the space probe’s (planned) journey from Earth to Venus.

Donald Mitchell offers a very nice collection of images of the Soviet space medallions and pendants which now rest on Venus and several other nearby worlds.

First Try

The first of the twin 447-pound robotic probes to be aimed for the second world from the Sun was naturally named Mariner 1. Encased atop an Atlas-Agena B launch vehicle, the vessel was sent on its way from Launch Complex 12 at Cape Canaveral in Florida into the predawn skies of July 22, 1962.

Oran W. Nicks, who was the Director of Lunar and Planetary Programs for NASA and present at the launch of Mariner 1, poignantly described what happened next in his wonderfully written book Far Travelers: The Exploring Machines (NASA SP-480, 1985):

“Our friend died violently at 4:26 A M. on a hot July night. Her finish was spectacular; she was trapped amid the flaming wreckage of an explosion that lit the night sky. Four of us watched helplessly, standing together at a site that gave us a perfect view. We had come there with a common interest in her adventuresome goal, though we came from different backgrounds, and each of us brought a different perspective and commitment to her tragic performance.

“We were soon to learn that she had been blown up intentionally by a man with no firsthand knowledge of her ability and promise. My emotion changed from disappointment to bitterness when I learned that she was destroyed barely seconds before flying beyond his reach [just six seconds, to be exact]. We had witnessed the first launch from Cape Canaveral of a spacecraft that was directed toward another planet. The target was Venus, and the spacecraft blown up by a range safety officer was Mariner 1, fated to ride aboard an Atlas/Agena that wobbled astray, potentially endangering shipping lanes and human lives.”

Guidance equipment for the rocket carrying Mariner 1 had become faulty, sending the entire vehicle off course. The Range Safety Officer (RSO) at Cape Canaveral sent a destruct signal to explosive devices on the booster to keep it from potentially causing harm or worse to people and property in the vicinity. Falling from the sky with the shattered remains of its rocket, Mariner 1 continued to transmit right until it slammed into the Atlantic Ocean. The first American attempt at Earth’s nearest planetary neighbor joined the ranks of the Soviet Venera probes which never left their home world.

Second Time’s the Charm

Since the early days of robotic planetary exploration, NASA’s plans for a typical deep space probe mission often included constructing at least three versions of a space probe: The primary craft that would be sent to the target world, an identical or sometimes improved backup probe, and a full-scale engineering model that stays home. This third version allows the mission team members to directly test the probe and to recreate and (hopefully) resolve any technical issues the flight version may have while enroute to and at the planet.

With the sudden loss of Mariner 1, the wisdom of having built more than one version of the same probe became all too apparent. America would still have a chance to explore Venus thanks to the existing backup vessel, which became known as Mariner 2. However, with the 1962 launch window to Venus closing rapidly, the mission team would need to complete their preparations sooner than normal.

Just over one month after Mariner 1 had found itself at the bottom of the Atlantic rather than on its way to Venus, Mariner 2 found itself rising into the sky above Cape Canaveral on August 27 aboard another Atlas-Agena B launch vehicle. Several times on its way to an Earth parking orbit, the booster behaved erratically, but thankfully the situations corrected themselves before any drastic action had to be taken.

Within two hours after launch, Mariner 2 found itself in open space with its two solar panels fully extended and on its way for a four-month journey to an alien world. The solar cells were found to be generating a bit more power than expected, so on August 29 the mission team activated the craft’s science cruise experiments that would let them know what the probe was finding out there and experiencing on its way to Venus.

The interplanetary voyage for Mariner 2 across millions of miles of space was anything but calm and routine, and not just because it was something rather new for NASA JPL.

The first major technical problem for Mariner 2 arose on September 8 when the probe suddenly lost its attitude control. Three gyroscopes automatically restored stability to the vessel just three minutes later. A possible strike by a stray meteoroid was suspected but never confirmed.

Even before the official Space Age, meteoroids were a particular concern to space engineers, as no one knew to any high degree just how much cosmic debris roamed the Sol system and how much damage they could do to a spacecraft. One of the staples of early science fiction involving spaceflight often had the intrepid crew of a silvery V-2 style rocket ship being threatened by whole storms of meteoroids as they bravely explored the Universe.

Had Mariner 2 been unable to realign itself, the probe could have begun tumbling, eventually resulting in a loss of contact with Earth and a serious loss of power due to the probe being unable to keep its solar panels focused on the Sun.

The next serious threat to the health of Mariner 2 while in deep space took place right on Halloween. One of the probe’s two solar panels began to malfunction by short circuiting. Concerned that Mariner might not have enough onboard energy to examine Venus and transmit back to Earth the priceless data on that planet, the mission team turned off the science cruise instruments to conserve power.

One week later, the errant solar panel appeared to have healed itself, so the Mariner team reactivated the cruise devices. The panel’s revival was short-lived, though: By mid-November, the panel had permanently broken down, leaving Mariner 2 with just one solar panel for collecting energy, along with its short-term batteries.

Thankfully for the mission, the probe was now close enough to the Sun that it could survive on the energy from just one panel all the way to Venus. There was, of course, a down side to being much nearer to our yellow dwarf star: An increase in solar heating.

Anyone seriously putting together a satellite to operate in open space plans for the craft to deal with extremes of heat and cold among other factors in the harsh environment beyond our planet’s relatively comfortable lithosphere. In the essentially airless void of space, the outer skin of a spacecraft facing away from the Sun can be hundreds of degrees below freezing, while the sunlight side is above the boiling point of water.

Not only did Mariner 2 have to contend with these same incredible temperature variations as other spacecraft have done before and since, but the probe was also approaching 26 million miles closer to the Sun than when its journey began back in August. The scientific instruments were starting to encounter temperatures much higher than they were nominally designed for. The Mariner team became increasingly worried that their robot emissary to Venus might succumb to the equivalent of a mechanical fever before it could arrive.

Then, just two days before Mariner 2 would encounter Venus, the onboard computer failed to give what was called a “cyclic calibrate pulse” which normally took place every 16.6 hours. The team worried this error meant that when the probe came upon the planet, the computer would fail to initiate the critical final encounter sequence that would activate the science instruments to examine the planet and return all that precious information to Earth.

Venus Day

On December 14, 1962, just six hours before much of the sky would be filled with the looming presence of the planet Venus from the perspective of the Mariner probe, the mission team discovered that their fears about the vessel’s mechanical “brain” were true: The computer had not told the probe to switch to encounter mode. Mariner 2 was in danger of flying past Venus without conducting a single scientific scan of the planet.

A command was radioed from the Goldstone tracking facility in California to Mariner 2, which was 36 million miles from Earth at that moment, father than any operating spacecraft had ever been before. Traveling at the speed of light, the command was received and replied to by the probe 6.5 minutes later, which took another 6.5 minutes to let the humans conducting the mission know that all their efforts were not in vain.

Approaching the planet’s darkened hemisphere at 39,000 miles per hour, Mariner 2 flew by Venus at a distance of 21,648 miles from its surface. If the probe had possessed electronic eyes, it would have seen that cloudy world appearing 900 times larger than the full phase Moon does in Earth’s skies. For the first time in history, a space vessel had reached another planet intact and functioning well enough to return scientific information about such a world. This was a mere five years after Sputnik 1 had been sent beeping into Earth orbit.

For 35 minutes, Mariner 2’s two radiometers scanned across the planet from its night side into the daylight hemisphere. The resulting data that streamed across interplanetary space back to the eagerly waiting scientists at a mere 8.33 bits each second confirmed that the high temperatures astronomers had previously detected were coming from the very surface of the planet and not its ionosphere.

Not only was Venus a very hot world, averaging 800 degrees Fahrenheit, but that temperature remained constant across the entire globe; not just from its equator to the poles, but also both in the day and night hemispheres. Clearly the planet retained its heat through the “greenhouse effect”, allowing solar energy to penetrate its cloud layers and thick air, but not to escape back into space. No other terrestrial world approached the surface temperatures Mariner 2 found at Venus, not even Mercury.

Gone almost instantly were the humid swamps of strange plants and even stranger animals. Gone too were the seas of oil, seltzer, and especially liquid water, for not even a single drop could survive there except to become a very temporary vapor. On the planet revealed by the American robot probe, even bars made of zinc and lead would melt into puddles, and no human being exposed on the surface would last beyond mere seconds.

Image: This 1961 photo shows Dr. William H. Pickering, (center) JPL Director, presenting a Mariner spacecraft model to President John F. Kennedy. NASA Administrator James Webb is standing directly behind the Mariner model.

As for native life, anything that had ever lived on Venus or could exist there as Mariner 2 slipped by their world would be nothing like organisms from Earth. Those who envisioned the second planet from the Sun as a scorching, desolate desert had been closest to the truth.

Readings from the other science instruments aboard Mariner 2 gave further evidence that about the only aspects Venus had left in common with Earth were its size and mass. Neither a magnetic field nor a radiation belt could be found around the planet, at least to the detection limits of the probe’s magnetometer. Venus appeared to be rotating very slowly and in the direction opposite to that of our planet. If a person standing on Venus could somehow observe the Sun rising at morning, our star would appear to come up from the western horizon, crawl across the sky, and then slowly set in the east several hundred terrestrial days later.

Seven hours after its historic encounter with Venus, the mission team sent another command to the Mariner to switch the probe back into science cruise mode. Along with its invaluable and startling revelations at Venus, Mariner 2 also reported several important discoveries about the interplanetary environment to which the terrestrial worlds of the inner Sol system are immersed.

Scientists were surprised to learn that the charged particles streaming from the Sun, known as the solar plasma or solar wind, were always present and constant in deep space. Mariner 2 also revealed it had received far fewer hits from cosmic dust than expected, though at least one jarring meteoroid impact was suspected during its journey through the interplanetary realm. All of this data would do much for better preparing future deep space missions.

The Last Days of Mariner

Though Venus would soon be well behind Mariner 2 as it plunged even closer to the vicinity of the Sun, its overall mission was hardly at an end. The probe continued to send back a stream of data about existence just beyond its metal skin, none of which had ever been examined directly before.

However, as Mariner 2 made its closest approach to the Sun two days after Christmas of 1962 at 65.5 million miles, the temperatures inside and outside the probe continued to rise.

On January 3, 1963, Mariner 2 had transmitted thirty minutes of real-time telemetry data to the monitoring station in Johannesburg, South Africa. The probe appeared to be functioning as well as it often had during most of the 129 days it had spent in deep space since leaving Florida the previous August. Mariner’s home planet was almost 54 million miles distant, while Venus was reduced to a brilliant white point of light in the perpetual night sky 5.7 million miles away.

Then Mariner 2 was heard from no more.

NASA JPL attempted to find the probe via its radio signal through August of 1963, but without success. In addition to its already vaunted legacy in the history of space exploration, Mariner 2 had transmitted over eleven million measurements while traversing 223.7 million miles of deep space during its known functional lifetime, a record not to be broken until the United States and Soviet Union began sending probes to the planet Mars in the next several years. Mariner 2 now orbits the Sun inside Earth’s solar circuit once every 345.9 days – silent, inoperative, and presumably largely intact.

Image: Mariner 2 celebrated at the Tournament of Roses Parade.

Oran W. Nicks, a man most personally invested in the early Mariner missions, eulogized the space probe in his work, The Far Travelers, thusly:

“Thus ended the saga of Mariner 2 – A robot, designed and directed by men, given a mission to extend the search for knowledge beyond the limited reach of Homo sapiens. Though it accomplished a voyage that was clearly “superhuman”, Mariner was a simple exploring machine, with only a very modest capability to perform on its own. A total of 11 real-time commands and a spare were possible, along with a stored set of 3 onboard commands which could be modified. Other functions, such as updating antenna position and adjusting thermal control louvers, were provided, but in every sense it was a simple robot with the capability for only a small amount of human interaction.

“From the meager information returned by telemetry, we know that Mariner 2 endured significant stress, but how many meteorite impacts it received and why it developed an ultimately fatal fever will forever remain a mystery. Perhaps in its passage from Earth to Venus and its transfer from orbit to orbit, it had other experiences which we will better understand when man repeats the voyage in person, with his own sensors and the additional capabilities that will exist at the time.”

The Legacy of Mariner 2

It is not an exaggeration to state that what Mariner 2 accomplished at Venus on December 14, 1962 changed the course of the American and Soviet space programs. When the intrepid probe reported back to humanity that what lay beneath the clouds of our neighboring world was anything but neighborly to all known forms of terrestrial life, the United States turned its planetary focus towards Earth’s other celestial neighbor, Mars.

Though almost no professional scientist of the early Space Age still looked upon the Red Planet as a haven for an alien civilization, many members of the relevant sciences did seriously think that Mars supported some form of simple plant life or perhaps even more complex creatures, albeit not highly intelligent. In addition, Mars looked much easier to land and maintain machines and astronauts upon compared to Venus. Now while NASA did not intend to stop exploring Venus altogether, being closer to Earth than any other planet in the Sol system was no longer good enough to devote a substantial amount of the space agency’s time, resources, and funds to that shrouded inferno. It was Mars that appeared to offer the best planetary return on their investment in terms of finding extraterrestrial life and founding permanent colonies.

Soviet space scientists had a decidedly different reaction to the findings of Mariner 2. Already invested in the concept of a Venus with a surface covered in some form of liquid, plus the indignity of being scientifically and politically trumped at once by their chief Cold War rival, the Soviet space agency was determined to learn the true nature of Venus for themselves. After all, Mariner 2 actually opened more key questions about the planet than it had answered, among them being: Exactly how dense was the atmosphere of Venus? What were the main constituents of the air? What did the surface of Venus really look like? In addition, the Soviets were far ahead of the United States when it came to developing and launching planetary landers, even if none had yet made it to the second planet from the Sun.

The years 1964 and 1965 saw America fling its next two Mariner probes away from the Sun towards the Red Planet. In the same time period, the Soviets hurled three more flyby probes and three more landing vehicles at Venus. While none of them achieved their intended scientific goals, suffering either from launch failures or overheating in space enroute, the Soviets were at least able to glean another planetary first in the Space Race from these efforts, specifically the first craft to land on another planet with Venera 3. Although the probe had stopped transmitting to Earth by the time it reached Venus in March of 1966, Venera 3 may have been functioning when it impacted on the night side of that world. Despite this technical achievement, the scientific questions about Venus left by Mariner 2 were still wide open.

Mariner 5: The Sequel

By the late 1960s, NASA was prepared to solve some of those critical questions about the second world from the Sun left open by the second Mariner. Taking leftover parts from the two 1964 Mariners they built for exploring Mars, JPL put together a single deep space probe they named Mariner 5.

Modified to survive in the much warmer vicinity of Venus, the fifth member of the Mariner probe series was lofted upwards on the night of June 14, 1967 from the same launch complex with the same kind of rocket configuration that its trailblazing predecessor had used almost five years earlier.

Meanwhile, just two days earlier on the other side of the globe, the Soviet Union had launched their latest effort to reach not just Venus but the very surface of the planet itself.

Named Venera 4 – once the probe had successfully escaped its Earth parking orbit – the nearly spherical landing capsule it carried strongly reflected the Soviet view of a liquid-covered planet: Not only could the one yard-wide capsule float, the probe’s makers also included with it a protective lock composed of sugar. Upon being dunked in the alien sea, the sugar lock would dissolve and release the probe’s transmitter antennae, which would send home the incredible news that Earth was no longer the only known world able to keep water in a liquid state on its surface. That was the expectation, at least.

As Venera 4 descended towards whatever fate awaited it on that distant realm dangling beneath a large parachute, the vessel’s scientific instruments would perform the first direct readings of Venus’ dense atmosphere. While some of the elements and molecules making up the planet’s air were already know to astronomers, exactly how much of each constituent was present and if any other kinds were waiting to be discovered remained a mystery as Venera 4 plunged through interplanetary space to Venus, with the American Mariner 5 probe following close behind.

Having been launched first, Venera 4 arrived at Venus one day ahead of its American counterpart on October 18, 1967. The entire probe sailed right into the planet’s night side. The main bus carrying the lander capsule conducted several scientific measurements of its own before its demise as an undoubtedly brilliant meteor far above the cloud deck.

In agreement with Mariner 2, the Soviet probe bus could find no evidence for either a magnetic field or radiation belts around Venus. What Venera 4 did discover was a weak corona of hydrogen particles sixteen thousand miles above the planet.

The automated capsule, operating on batteries with a 100-minute lifetime and drifting slowly downwards beneath a successive series of braking parachutes, began letting the scientists back on Earth know exactly what the Venusian air is made of: To their surprise, at least 93 percent of the planet’s atmosphere was composed of carbon dioxide, with nitrogen following at around seven percent and just traces of oxygen and water vapor. Many assumed nitrogen would have been the dominating element of that alien air as it is with Earth, but the reverse was true.

Venera 4 ceased transmitting 93 minutes into its mission. The last atmospheric pressure reading the probe reported was about 18.5 times what is experienced at sea level on Earth’s surface, not far from what the capsule was designed to tolerate. Since about 25 Earth atmospheres are what most contemporary scientists estimated about the density of Venus’s air, it was logical (and politically expedient) for the Soviets to conclude that Venera 4 had reached the surface of the planet while still transmitting data – yet another space first.

The next day, October 19, Venus received the other mechanical emissary of 1967 from the inhabitants of the third world from the Sun. Although Mariner 5 carried neither an imaging system nor its own lander capsule (the latter was considered but never implemented, as the probe was under tight budget and time constraints), the vessel would use its radio signal to greatly refine the then current information about the atmosphere of Venus. This revised data would also let the scientists on both sides learn if Venera 4 did indeed survive intact and functioning to the planet’s surface – or not.

Flying past the veiled planet much closer than Mariner 2 did at a distance of just 2,480 miles, Mariner 5 spent 26 minutes behind Venus as seen from Earth. As the robotic explorer was being eclipsed by the bulk of the planet, Mariner’s radio signals cut through Venus’ atmosphere on their way to Earth, revealing new details on that planet’s mantle of air.

The atmospheric pressure of Venus at its very bottom was much higher than the majority of scientists expected, ranging from 75 to 100 Earth atmospheres. There appeared to be two different cloud layers over Venus, one approximately 37 miles above the surface and the other at 31 miles in altitude. The global ground temperature was even higher than recorded by Mariner 2, approaching up to 981 degrees Fahrenheit. Tracking Mariner 5 also helped to refine the planet’s mass at 81.5 percent that of Earth’s. This meant that an object weighing one hundred pounds on Earth would weigh just nine pounds less on Venus.

Over the next two years, American and Soviet scientists met formally several times to discuss the data results from their indirectly joint space mission to Venus. The Soviets initially argued that Venera 4 could have landed on a very high mountain or plateau to account for their readings, but the Mariner evidence and later Earth-based radar observations showed Venus’ crust to be relatively flat overall. The conclusion was that Venera 4 had been crushed into radio silence by the increasing air pressure just sixteen miles above surface. Eventually the capsule would have landed on the face of Venus in any event, just not in working order.

The Soviets would eventually succeed in reaching Venus with a functioning lander and refine the planet’s air pressure and temperature even further. Venera 7 touched down on December 15, 1970 in the southwestern section of what would one day be called Tinatin Planitia. There the instrumented capsule spent the 23 remaining minutes of its working life letting scientists know that the temperature of its landing site averaged 887 degrees Fahrenheit, while about ninety Earth atmospheres of pressure weighed down upon its hull (the Soviets had designed Venera 7 to withstand double that amount, just to be safe). One would have to dive over three thousand feet beneath the oceans to find an equivalent natural pressure on Earth. Any lingering thoughts of a cool and watery Venus brimming with life were finally crushed with Venera 7, the first probe to transmit from another planet.

Venus Exploration Heats Up

The robotic exploration of Venus reached its peak in the 1970s and 1980s, thanks largely to Soviet efforts but also with several notable American missions in those same decades.

Between 1972 and 1985, the Soviet space program succeeded in landing eight more probes on our celestial neighbor. These technologically superior Veneras, while lasting no more than a few hours in the very harsh conditions of the planet, returned the first (and so far only) optical images of Venus’ rocky surface and the first data on the composition of the crust.

These Veneras revealed that much of Venus is covered in various types of basalt, a mineral produced by volcanic lava. The surface readings and images, along with other data returned by that generation of Soviet probes, gave a pretty clear indication that most of the face of the second planet has been long shaped by intense volcanic activity, past and very likely present. The images also showed that daytime on Venus is not as gloomily dim as thought and that atmospheric refraction turns the sky bright orange in color, compared to Earth’s blue skies and the salmon pink ones of Mars.

Image: Color images of Venus, first obtained by Venera 13 and 14.

The last two space probes in the Venera series remained in orbit about Venus upon their arrival in 1983 and conducted the most sophisticated radar mapping of the planet until the American Magellan probe arrived there seven years later. The Soviets also succeeded in placing two instrumented balloons into the atmosphere of Venus with the twin Vega missions in 1985. These balloons spent two days traveling over five thousand miles among the thick and turbulent cloud layers.

While not quite as ambitious as the Soviets, the United States did send several more space probes to Venus, among them the last of the robot vessels officially part of the Mariner series.

Although its primary goal was to become the first probe to flyby the planet Mercury, Mariner 10 utilized Venus for a historic first gravity assist to reach the world closest to the Sun. Launched from Cape Canaveral in Florida in November of 1973, the probe flew by Venus on February 5, 1974. Equipped with an imaging system, Mariner 10 proved that scientists could see physical features in the planet’s clouds, especially in the ultraviolet range of the spectrum. The visiting probe revealed that hurricane-level winds whipped the Venusian clouds around the planet in just four Earth days, much faster than the global rotation rate of over 243 days, which is longer than it takes for Venus to go once around the Sun!

Four years later, the United States lofted its own armada of Venus explorers towards the planet as the last official members of the Pioneer family of deep space probes. While one spacecraft would circle the planet and return the first radar map of its surface made from orbit, the other member of the fleet would drop four small probes onto the planet itself. They would directly examine the atmosphere from four different regions over Venus until impact with the ground.

Along with determining that Venus has three distinct cloud layers instead of two as indicated by Mariner 5 eleven years earlier, the Pioneer Venus Multiprobes greatly added to the planet’s reputation as a hellish place when it was realized they were plunging through an atmosphere saturated with droplets of sulfuric acid!

Although none of these drop probes were designed or expected to survive their encounters with the surface, one called Day did manage to keep functioning after its rough landing. The vessel transmitted for 67 minutes before being overcome by the intense heat of its surroundings. The Pioneer Venus Multiprobes were the first American craft to land on the veiled world and remain the only ones to do so to this day.

A Mariner Called Magellan

The last American robotic probe mission dedicated to the exploration of Venus was a spacecraft named Magellan. Other later deep space machines built and launched by NASA such as Galileo, Cassini, and MESSENGER would fly by and aim their scientific instruments at the second planet from the Sun in the 1990s and 2000s, but their ultimate goals lay elsewhere in the Sol system. For them, Venus was primarily a gravity boost to save on fuel and costs. Magellan’s focus was Venus, namely to radar map its entire surface in unprecedented detail.

Although Magellan was never officially listed as a member of the Mariner family of space probes, the automated craft did more than just emulate the spirit of the Mariners that went to Venus: Magellan was actually put together from the spare parts of previous space probes, including Mariners and their kin.

The main body of Magellan came from a spare bus of the Voyager probes, which famously explored the outer Sol system from 1979 to 1989 and are expected to keep returning data from the fringes of our planetary neighborhood until 2025. The twin Voyager craft were originally going to be known as the eleventh and twelfth Mariners until NASA officials wanted them to have what they considered to be a more dynamic nomenclature. Despite the change of labels, if one looks at the structural design of Voyager 1 and 2, it is not hard to see their Mariner family heritage going all the way back to the first two Mariner probes to Venus.

Other features that Magellan literally shared with the Mariner probes were the large white high-gain antenna that also came from the Voyager mission and a medium-gain antenna that was originally a spare for the Mariner 9 probe that became the first spacecraft to orbit Mars back in 1971.

Whether anyone considered Magellan to be an actual part of the Mariner family or not, the craft did honor to that historic legacy by revealing the face of Venus as never before from 1990 until its demise in the planet’s atmosphere in 1994.

Many thousands of volcanoes of wondrous variety cover the surface of that planet, an unknown number of which may be active at present. Lava channels thousands of miles long snake across the landscape. By contrast, no impact craters smaller than a few miles wide exist due to the very thick atmosphere preventing all but the largest planetoids and comets from getting through to the ground.

Image: Color information from the Soviet Venera landers and radar data from the Magellan spacecraft were used to construct this striking perspective view of the Venusian landscape. (In this computer generated image, the vertical scale has been exagerated.) In the foreground is the edge of a rift valley created by faulting in the crust of Venus. The valley runs all the way to the base of Gula Mons, a 2 mile high volcano seen here on the right, some 450 miles in the distance. On the left is another volcano, Sif Mons. Using radar to pierce the dense clouds continuously shrouding the Face of Venus, Magellan was able to explore over 98% of the Venusian surface, revealing a a diverse and tantalizing topography. Credit: The Magellan Project, JPL, and NASA.

The Magellan radar data also seems to indicate that the face of Venus is about 800 million years old – rather young in geological terms for a planet created about 4.6 billion years ago. While there seems to be no evidence for plate tectonics, the planet’s interior may erupt in massive lava flows onto the surface, completely reshaping it over and over. Scientists also wonder if Venus has a rotating iron core, a lack of which would explain the very weak magnetic field that was later discovered by probes with much more sensitive detectors than the early Mariners.

Venus Now and in the Future

Other nations have since gotten into exploring Venus with their own robotic probes. The European Space Agency (ESA) built their first Venus craft called Venus Express, which they based on the Mars Express probe design, equipped with seven scientific instruments and launched on a Russian rocket in November of 2005 to go into polar orbit around the planet the following April. The primary goal of Venus Express is to monitor the atmosphere for a long time period, at least through the end of 2014. Scientists also hope to use what they learn about the climate of Venus to better explain Earth’s atmosphere and climate and ensure that what happened to our neighbor does not happen to our world.

One interesting finding from Venus Express is that the second planet may have slowed down its rotation rate by over one minute since Magellan measured this activity in the early 1990s. Scientists think if this is a real phenomenon, then the high winds and other harsh conditions of Venus turgid atmosphere may make an important contribution to this relatively pronounced rotational slowing.

Japan is another nation with a long and vibrant space program which has attempted to send their first probe to Venus, which they call Akatsuki, the Japanese word for “dawn”.

Launched from Japan in May of 2010, Akatsuki was supposed to go into orbit about Venus that December, where among other things the probe would attempt to learn if the planet has lightning storms and if any of its many volcanoes are active. Unfortunately, something went wrong with the critical braking burn when the probe arrived at Venus and Akatsuki and its mission team found themselves adrift in solar orbit. The hope is that when Akatsuki comes into the vicinity of Venus in 2015, it can be placed into orbit around the shrouded world using a different set of rockets on the probe.

Image: Launch of the Akatsuki Venus Probe. Credit: JAXA.

Not all was lost with Akatsuki. Riding along with Japan’s first Venus orbiter was a satellite named IKAROS, which stands for Interplanetary Kite-craft Accelerated by Radiation Of the Sun. When IKAROS arrived at Venus just one day after Akatsuki’s less than triumphant attempt to orbit the planet on December 8, 2010, the sail craft successfully flew 80,000 miles past Venus and demonstrated the feasibility of solar sailing in interplanetary space.

Talk of future plans for Venus include balloon probes and landers with methods to extend their life on the planet into months instead of just days. Other talk includes nuclear-powered rovers, gondolas, and aircraft on and just above Venus. Even a surface sample return mission has been considered. When and if these and other missions come to pass in the next few years and decades is up to those in charge of humanity’s future in space.

Scientists have even shown some confidence in the last few years that life might exist on the hellish world of Venus after all. Certain layers in the Venusian atmosphere are rather mild in comparison to other layers of air above that planet. It is considered possible that colonies of microbes may live up there in the more biologically friendly zones, floating, eating, and reproducing on the winds. Others think that just below the roasting and crushing realm that is the Venusian surface, the temperature remains milder and fairly constant, a better place for native organisms. Perhaps certain space agencies will be intrigued and encouraged enough to test these theories by sending missions to Venus equipped to answer the questions about life on or maybe under that world.

As for future humans living on Venus, the planet would have to undergo some pretty sophisticated terraforming to make conditions comfortable enough for people to live on Venus without protective gear. One possible method for making Venus more like Earth is to strategically slam comets into the planet. They may be able to provide the water and other elements necessary to cool Venus down and thin out its acidic and roasting atmosphere. Properly directed comet impacts may also speed up Venus’ rotation rate, which at the moment is painfully slow. Genetic engineering and other future technologies may provide life forms that will hasten Venus’ transformation into a place terrestrial organisms could call home one day.

For those who want to see an actual rendition of the probe that started it all for deep space exploration, the Smithsonian National Air and Space Museum in Washington, D.C. has a full-scale model of Mariner 2 hanging from its ceiling, as shown in the image below:

The spacecraft on display was constructed from test components by engineers from NASA’s Jet Propulsion Laboratory. Until the day that someone goes out into interplanetary space and recovers the original Mariner 2, this replica made of actual test components will have to suffice for now.

No matter how far we eventually go into space, or what forms our space vessels may take as they evolve to handle the new and strange celestial environments they will encounter, they will all owe their existence and success to a rather primitive and ungainly probe named Mariner 2 that was first to succeed in exploring another world fifty years ago. And humanity will continue to be grateful for the vast amounts of knowledge that Mariner 2 and all its mechanical descendants have delivered to us about the amazing Universe we live in.