NASA / Goddard Space Flight Center / SDO

Two weeks ago, NASA announced the selection of two new missions to explore the solar system. Psyche will fly to a metallic asteroid of the same name, and Lucy will explore “Trojan asteroids” that travel along Jupiter’s orbital path. The missions will advance our understanding of the origins of the solar system, and are by all accounts worthy missions. In choosing them, however, NASA passed on the chance to return to Venus, a planet in dire need of exploration.

A generation has now gone by since the agency set a course for the second planet from the Sun, and with this latest mission opportunity lost, the earliest an expedition there might launch (from some future selection process) would be 2027—nearly 40 years since our last visit.

For centuries, it would have been inconceivable that Venus would be in such a predicament. In the 18th century, Venus was the organizing force in international science. When humanity was finally able to stretch its arms toward the solar system, the first place it reached for was Venus. It was our first successful planetary encounter beyond Earth, and was the first planet on which humans crashed. It would later would host our first graceful landing.

Venus and Earth are practically twins. They’re alike in size, density, gravity, and physical makeup. They are both in our star’s habitable zone. Scientists have discovered no other adjacent planets in the entire galaxy that share such similarities. And yet somewhere along the way, Earth became a cosmic paradise for life as we know it, and Venus became a blistering hellscape. Beneath its sienna clouds of sulfuric acid is the greenhouse effect gone apocalyptic. At 850 degrees Fahrenheit, its surface is hotter than Mercury, though the planet itself is much farther from the Sun. A block of lead would melt on the surface of Venus the way a block of ice melts on Earth.

In recent years, the Kepler space telescope has discovered more than 3,000 planets around other stars, many of which are orbiting in habitable zones where water could be stable on a planet’s surface. These “Earth 2.0’s” are but pixels of light many light years away and are difficult to study. Conveniently, there is an Earth 2.0 next door to our own. Venus was an ocean world for much of its history. By understanding that history, how it compares to Earth, and how it lost its habitability, we might better understand potentially habitable exo-worlds.

Moreover, as scientists and lawmakers grapple with climate change, they can look to Venus. “I don't want to say that Earth can turn into Venus from global warming,” says Bob Grimm, the director of the Department of Space Studies at the Southwest Research Institute and chair of the Venus Exploration Analysis Group. “That is not going to happen. It takes a lot of carbon dioxide to make Venus's atmosphere as hellish as it is. But that lies on a continuum, on a spectrum, of how CO2 in atmospheres affect planetary climates. Earth is not going to turn into Venus, but Venus has lessons on climate evolution for Earth that we should pay attention to.”

With so much to learn on a planet so close to Earth, why, then, has NASA halted Venus exploration? For one thing, there’s not enough money. NASA receives one-half of 1 percent of the federal budget, and only a fraction of that goes to planetary science. Secondly, it’s a lot easier to explore our other neighbor, Mars, than it is to study Venus. Mars won’t melt lead. Rovers there have lifespans now measured in decades. Meanwhile, any robotic mission to the Venusian surface is bound to be short and brutal. Moreover, Mars has a romantic appeal that Venus will never possess. Humans will one day walk on Mars, and maybe settle there.

The third, and perhaps most devastating factor working against Venus exploration is NASA’s dogged focus on finding evidence of extraterrestrial life. The question of whether we are alone in the universe is too tantalizing for the agency to resist. It’s why those rovers are on Mars in the first place: to determine habitability, and with the Mars 2020 rover, to find inhabitants. The rally cry of the Mars program has long been “follow the water,” because where there is water, there might be life. The agency’s recently established ocean worlds program has similar goals, and some say, half in jest, that we must go to Europa to “follow the whales.”

Beyond planets and moons, NASA’s devotion of late to small bodies also relates directly to the question of life. Many scientists believe that water was first brought to Earth by asteroids, and that asteroids and comets delivered to us the essential building blocks of life: amino acids and precursors thereto. All of this leaves poor scorching Venus in the cold. Its surface is almost certainly lifeless, and there is at best a minuscule chance of bacteria in its clouds.          

Ellen Stofan, who two weeks ago stepped down as the chief scientist of NASA, says that the study of exoplanets brings Venus into the life-oriented NASA mission. “We’re in [a] phase of looking at these extrasolar planets and saying: Now wait a minute—Venus has something really important to tell us about why the Earth is habitable and why is Venus not.”

Early in Venus’s history, she explains, that planet had an ocean on its surface. Did life evolve in that interval? Earth has been habitable for about 3.9 billion years. Mars was habitable for maybe 500 million years. Europa might be inhabited today. “This is the question we're really struggling with now,” says Stofan. “It's not just: Are you a potentially habitable planet, but: How long did that period of habitability last? Did life take hold? Did it persist?” Venus helps answer those questions.

“If you only focus on Mars, if you only focus on Europa, you're not going to understand this. We need a broader program,” she says.

NASA cannot afford to add another multi-billion-dollar mission to its portfolio. The Venus community has therefore focused on tight Discovery-class missions in the $450 million range. The odds were in their favor in the most recent Discovery selection process. Scores of concepts were narrowed down to a final five, and two of those finalists were Venus missions. When NASA announced that it would then fund two Discovery missions, well it just seemed like Venus was set to see our return. Worst case scenario, one Venus mission would get approved, but after 28 years of nothing, that would be enough.

When NASA announced the selection of the two missions to asteroids and zero missions to Venus. Stofan, a Venus scientist who had recused herself from the process, says she was honestly surprised by the results, though describes the asteroid missions as “really great” with excellent principal investigators. She worries going forward is that NASA will soon find itself devoid of anyone with expertise in Venus missions, and the world with no one who has ever actually operated on the Venusian surface. “My huge concern is that the expertise is still there but it's going away. If we have no Venus missions it will definitely be gone. We will have lost something. We will have lost this capability and it's really unfortunate.”

Planetary exploration is a perishable skill at an institutional level. NASA is good at landing spacecraft on Mars because its engineers have been doing it for a long time. The engineering spoils of success, the lessons learned from failures, and the “We didn’t do that last time because…” conversations over coffee and in meetings are as important to exploration capabilities as saved AutoCAD drawings.                                                                                     

In the case of Venus, the chief problem facing engineers is the planet’s merciless thermal environment. When Venus scientists formulate new landing mission proposals, they reach out to the people who worked on NASA’s Pioneer-Venus, which sent probes to the surface of Venus in 1978. They speak with the Russians who landed the Venera and Vega probes. They talk, in other words, with the people who’ve actually done the job. But veterans of the Venus surface are diminishing in number.

David Grinspoon, senior scientist at the Planetary Science Institute and author of Earth in Human Hands, points also to how a lack of Venus missions affects the planetary science community as a whole. “​There’s a weird kind of feedback effect where once you have a bunch of missions somewhere, then you’ve created a community of scientists who are invested in, and interested in doing, more missions like that. You've trained a generation of grad students whose advisors were working on Mars missions who are then new scientists in the community, and they want to propose Mars missions.”

He continues, “The Venus community has shrunk because there haven’t been any missions there, so there hasn't been funding, so there haven't been as many people writing papers, training students, holding meetings—the community has sort of dissipated and withered away a bit.” Working also against a return to Venus is NASA’s emphasis on “heritage” in mission design. “That’s how you avoid risk,” says Grinspoon, who was also a co-investigator on one of the Venus proposals passed over by NASA. “You use proven technologies. The heritage for Venus [atmospheric] entry proposals is Pioneer-Venus in 1979! Clearly, if you haven’t been doing something, there are fewer experts in doing that.”

All hope is not yet lost. The European and Japanese space agencies have both mounted very successful orbital missions to Venus in 2005 and 2010, respectively. Those missions have advanced Venus atmospheric science and kept the community in new data from which to publish papers. What’s now needed, however, is a ground mission to help explain the incredibly complex interaction of the Venusian surface and atmosphere. The next shot scientists have at getting a mission concept approved is the New Frontiers medium mission class announcement of opportunity posted last month by NASA.

“​I'm really glad that other nations are picking up the slack so that somebody from Earth is paying attention to Venus,” says Grinspoon. To get the attention it needs, however, someone is going to have to build a lander. NASA has the technical expertise to do so, but the window is closing. When it shuts, we’ll be left with a lot of ideas, a stack of fading blueprints, and no certainty at all.

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