Venus might have been Earthlike in the past, and Earth might be Venuslike in the future.
Concept drawing of Venus Express aerobraking. Image: ESA–C. Carreau
Venus, Earth's radiant sister planet, has enchanted skywatchers for thousands of years. As the brightest object in the skies after the Sun and Moon, this world is often called the morning or evening star, as if its luminous glow earned it honorary stellar cred. Centuries before the advent of spaceflight, people speculated about what lay hidden under Venus's heavy cloud cover, and many imagined it as a heavenly paradise capable of supporting life.
It wasn't until May 19, 1961, exactly 55 years ago on Thursday, that the Soviet probe Venera 1 became the first spacecraft to actually visit Venus, passing 100,000 kilometers (62,000 miles) from the planet on its closest approach. That mission kicked off a golden age of Venusian exploration that revealed our nearest planetary neighbor to be a world of extremes, contradictions, and persistent mysteries.
But over the past 25 years or so, journeys to Venus have begun to taper off in favor of Mars missions, especially with regards to human exploration. At the risk of stirring up sibling rivalry between Earth's brother and sister planets, the relative abandonment of Venus in favor of Mars is a colossal missed opportunity. Venus may seem like the most unearthly environment imaginable, but it is also Earth's closest "twin," with a diameter just five percent shorter than our own planet's breadth (in contrast, Mars is dramatically smaller, measuring only half the size of Earth).
Moreover, Venus might have been more like Earth in its past, and Earth might become more like Venus in the future. To understand how one world became so hellish—and to prevent the other from following in its footsteps—we need to revive that golden age of exploration which started on this day in 1961 with Venera 1's flyby.
Nobody knows what Venera 1 saw as it sped past Venus. The probe's communications system cut out a few days after it departed from Earth, rendering it silent for the rest of its voyage. But this inaugural interplanetary mission nonetheless racked up many spaceflight milestones as the first emissary ever to visit Venus, or any other planet for that matter.
On top of that, Venera 1 was also the flagship vessel of the long-lived and productive Venera program (Venera being the Russian name for Venus). It remains the only spacecraft series to survive the punishing conditions of the Venusian surface long enough to transmit substantive reports about its environment, including this haunting color image from the Venera 13 lander, which touched down on Venus in 1982.
Needless to say, these early missions unveiled a world that could not have been more different from the lush oasis some had hoped Venus might be. The Venera series, along with select Mariner probes, revealed instead a raging inferno, with surface temperatures of around 467 Celsius (872 degrees Fahrenheit), hot enough to melt lead, and pressures equivalent to the seafloor at a depth of one kilometer.
Electrically charged droplets of sulfuric acid circulate through the suffocating clouds, while a phenomenon called super-rotation whips the upper atmosphere into a vicious global hurricane that sweeps around the planet every four Earth days, with winds of 400 kilometers (250 miles) per hour.
Even weirder, the planet's slow, retrograde rotation causes the Sun to rise in the west and set in the east, while the Venusian day, lasting 243 Earth days, is longer than the 225-day Venusian year. In short, Venus may be Earth's doppleganger in size and mass, but on many other metrics, it is a demonstrably alien and hostile environment.
Perhaps this is one reason why many space agencies have shifted their focus to Mars, especially as far as human colonization is concerned. Thousands of people have lined up to volunteer to go to Mars, but a crewed trip to Venus, in all its nightmarish glory, would no doubt be a much harder sell.
But just because Venus has devised new and inventive ways to be deadly doesn't mean it is not worthy of avid exploration—even if it must be strictly robotic for the time being.
Indeed, according to planetary scientist Alexander Rodin, docent of the Moscow Institute of Physics and Technology, Venus can shed light on everything from climate change on Earth to the discovery and characterization of exoplanets orbiting other stars.
"On Venus, we still have a huge domain of terra incognita for fundamental research," Rodin told me via email. In other words, Venus remains one of the most mysterious worlds in the solar system, despite its proximity to Earth, and is woefully underexplored considering how much useful information we stand to learn from it.
Take, for instance, the wealth of insights made by the Venus Express mission, launched in 2005 by the European Space Agency (ESA). At the time, it was the first orbiter to study the planet up close in nearly two decades.
During its nine-year lifespan, Venus Express confirmed that lightning crackles across the planet's skies, and is more prevalent there than on Earth. It found the best evidence yet of active volcanism on the Venusian surface using low resolution infrared images, and observed the mind-boggling double vortices swirling around the planet's poles.
These chaotic maelstroms of colliding atmospheric forces continue to defy firm explanation, as does the enormous hurricane that encircles the planet, sculpted by the complex effects of super-rotation.
"We still do not know whether polar vortices play any significant roles in the global dynamics," said Rodin, who was a participant on Venus Express, along with many other Mars and Venus missions. "Another unresolved issue is a role of the large-scale turbulence, [and whether] it is constructive or destructive for the global flow."
"In fact, what we know about Venus is just a very simplistic glance on this rich, fascinating world," he continued. "Even models of the atmospheric dynamics do not show convincing equilibration. Our record of monitoring is still too short to make conclusions."
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Reconstructing Venus's past is as challenging as understanding its present behavior. But the Venus Express mission was able to root out some interesting traces of its dramatic history, including tantalizing evidence that the planet may have once hosted oceans on its surface.
"The most convincing evidence [of past oceans] is extremely low abundance of water vapor in the atmosphere—about 30 parts per million," Rodin told me. "Since Venus's origins are thought to be quite similar to that of the Earth, the primordial water inventories should be similar. The almost full absence of water on Venus implies some mechanism responsible to its loss on a global scale."
This revelation, paired with the abundant evidence for past oceans on Mars, has intriguing implications for reconstructing the young solar system. Perhaps billions of years ago, Earth, Venus, and Mars were all home to oceans that may have supported life. But as our own planet's oceans flourished, nurturing ever more complex organisms, the ancient Martian ocean evaporated into space, while the speculative seas of Venus were destroyed by the devastating runaway greenhouse effect that still maintains a stranglehold over the planet.
"To date, we do not see a way how Venus may turn into more Earth-like planet again," Rodin said. "Venus has likely suffered an irreversible planetary transformation."
Scientists have long noted that Venus is the ultimate cautionary tale about the devastating effects of rapid climate change. At some point during its past, the planet reached a tipping point. Greenhouse gases began to accumulate in its atmosphere in a positive feedback loop that led to its modern incarnation as a tempestuous hellscape.
Studying this catastrophe could shed light on the worrisome climatic shifts that our own planet is experiencing, as a result of human activity. Aside from digging into the specifics of the Venusian greenhouse effect's behavior and development, the planet's conditions remain a potent reminder that permanently disrupting global climates is easier than it might seem.
"Perhaps the most important message is that the global climate system includes numerous interconnections, often nonlinear and irreversible," Rodin explained. "As on Venus, water vapor is the strongest greenhouse gas on the Earth, but its amount is controlled by weaker greenhouse species such as carbon dioxide. Similarly to a power-steering car, a relatively weak effort of a driver controls the steering machine that turns heavy tires."
"The Venus example shows that such control has limitations, and overcoming these limitations may destroy the whole climate system."
This is not to suggest that climate change on Earth is destined to transform our planet into an exact replica of Venus. But there is no doubt that anthropogenic activity is disrupting Earth's climate, resulting in rising temperatures and greenhouse gas pollution, so there are definitely worrying similarities that should be explored in future missions.
Venus can also be mined for information about finding planets orbiting distant stars. For instance, a 2015 study in Nature Communications used the transit of Venus across the Sun to spotcheck the kinds of spectral data astronomers expect to see in the atmospheres of exoplanets. In this way, Venus is shaping up to be a kind of planetary Rosetta Stone for decoding the climates and composition of alien worlds.
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Venus holds tight to its secrets, and even 55 years after Venera 1, we barely have a grip on its basic properties.
To that point, since the retirement of Venus Express in December 2014, the Japanese Akatsuki orbiter is the sole spacecraft left to study this captivating world up close. Mars, in contrast, currently has five orbiters and two rovers actively exploring it. What an attention hog.
It's heartening that NASA, ESA, and Roscosmos all have missions to Venus in the works—the Venus In Situ Explorer (VISE), the Venus Entry Probe, and Venera-D, respectively. But at this point, none of them has progressed farther than the concept phase. According to Rodin, there's no detailed timeline for the Venera-D mission, which would include an orbiter, at least one lander, and perhaps even balloons or kites.
"We will see," he said. "Definitely, this mission will involve an extensive international cooperation, and the final terms and contents of the mission would be critically dependent on it."
Rodin also added that "eventually we will need a flotilla of drones, which would continue a pioneering balloon experiment carried out in the framework of USSR's Vega mission 30 years ago."
This Vega program consisted of twin spacecraft, Vega 1 and Vega 2, which reached Venus in June 1985. In addition to ejecting surface probes to transmit data from the ground, the motherships also deployed balloon aerobots to explore the tumultuous atmosphere. They provided a lot of new insights into Venus's skies, including observations that roughly 50 kilometers (30 miles) above the surface, the planet is much more hospitable. Indeed, with balmy temperatures and habitable pressures, this region of Venus's atmosphere is the most Earthlike environment in the solar system, excepting Earth itself.
These discoveries led to fantastic terraforming and human colonization concepts for the Venusian atmosphere, such as the idea of constructing cities in the planet's clouds. Indeed, some have even hypothesized that life may already have taken root in Venus's lofty vistas. Though such claims are extremely speculative, there is some evidence to back them up, including the presence of carbonyl sulfide in Venus's atmosphere, an organic compound that is usually created by biological processes.
It's exactly these kinds of mysteries that spur scientists to propose fresh angles for exploring our sister planet, such the the dazzling idea to deploy a fleet of drones in its skies.
It was only 55 years ago that Venera 1 made the first momentous leap towards our cryptic neighbor, breaking the trail that has since been tread by spacecraft from many different countries. Hopefully, the next 55 years will yield even more efforts to pull back the veil of Venus's past, present, and future, and by extension, teach us how to prevent our own planet from sharing its fate.
Correction: An earlier version of this article mislabeled the illustration of Venus's global hurricane, mistakenly calling it the southern polar vortex. In addition, an earlier version mentioned that past tectonic activity had been detected with Venus Express, but the mission did not have the capability to construct any radar maps and the vortices were first discovered by Pioneer Venus in 1979. The story has been updated to reflect this.
Read more: Why We Should Build Cloud Cities on Venus