Last Wednesday was supposed to be a banner day for planetary scientists who study Venus, the closest planet to Earth. In recent decades, of more than a dozen missions proposed to explore Venus, only a handful had made it through NASA’s preliminary round of consideration. But as part of NASA’s most recent selection of planetary missions, two of the five finalists were dedicated to Venus. Two others were dedicated to asteroids, and a final one would look for near-Earth asteroids.
“With five total missions, and an expectation that two missions were going to be accepted, it seemed natural to do a Venus one and an asteroid one,” said Robert Grimm, a program director in the space science division of the Southwest Research Institute. “It’s safe to say the Venus community was very happy to see two of its missions among the finalists.”
Yet NASA did not ultimately select a Venus mission. Instead it chose to send spacecraft to both a group of asteroids in Jupiter’s orbit and a metal-rich rock in the asteroid belt. The decision left Grimm, who chairs the Venus Exploration Advisory Group but did not have a stake in either proposed mission, crestfallen. “It was both a surprise and a disappointment,” he told Ars in an interview. “And to be honest, it’s really galvanized me to change the perception of Venus.”
Perhaps the decision shouldn’t have surprised Grimm. NASA has long neglected the world often described as Earth’s wayward sister. The space agency hasn’t selected a mission to explore Venus since 1983, when it began developing what would become the Magellan probe that was tasked with making a global radar map of the cloud-shrouded world. But since Magellan burned up in the Venusian atmosphere in 1994, NASA has had no presence at Venus, even while sending probe after probe to Mars. The lack of spacecraft and new data has starved the Venus community of many of its scientists.
“Our community is passionate about Venus, but we’re getting pretty thin,” said Grimm, who mostly works on Mars data now. “Our meetings are not as well attended because we’ve got no mission. It’s getting to the point where people who were graduate students and postdocs on Magellan are going to be retiring in a decade or so. We basically have a huge generation gap with Venus, and we really need something to launch in the early- to mid-2020s so we can maintain some kind of continuity.”
Grimm can cite plenty of good reasons to explore Venus. While some may dismiss the planet as Earth’s “evil twin,” with its crushing atmosphere and searing surface temperature of 462 degrees Celsius, Grimm said it’s more apt to consider Venus a distant cousin. The two planets are closely related, sharing the same inner structure of a core, mantle, and crust. Their sizes and densities are nearly identical, and Venus also probably once had oceans along with a much more Earth-like atmosphere.
But today, the world is a hot, dead hell. Somehow the planet lost its water-based atmosphere around 2 billion to 3 billion years ago. But why? The planet’s proximity to the Sun can’t entirely explain it. And how fast did the atmosphere slough off? As Venus lost its water, the chemistry of its atmosphere changed dramatically, and its climate warmed. This also affected the planet’s geology by depriving its crust of the lubricating effects of hydrated rocks.
It seems most likely that without water, plate tectonics could not occur, so scientists believe the planet’s sparsely cratered surface today may be the result of periodic, cataclysmic volcanic resurfacing. Another mystery about Venus concerns its thick carbon dioxide atmosphere. Without a magnetosphere to protect it, what keeps the atmosphere in place? Why doesn’t the solar wind strip it away?
Answering those questions should go a long way toward helping to also understand the evolution of Earth’s atmosphere and geology. Moreover, it would give scientists another reference point when considering the limited data about exoplanet atmospheres in coming decades. Perhaps we’ll better understand whether that Earth-size world around a distant star is more like Venus or more like Earth.
One major strike
For all of its mysteries, however, Venus has one major strike against it—astrobiology. Although some modeling work has suggested that oceans existed on Venus for a couple of billion years and that it may have provided conditions somewhat favorable to life closer to the dawn of the Solar System, that is a long way from saying life might have existed there. And while it’s also fun to speculate what kind of life might exist on a hell world, in reality no such life likely exists.
“Mars I view as first among equals,” Grimm acknowledged. “People can walk on it. The planet may have had life. We need to keep exploring Mars.” And NASA has explored Mars. Since the end of the Magellan mission in 1994, the space agency has sent no fewer than 10 probes into orbit or rovers onto the surface of Mars. Generally those have been bigger-ticket items. NASA’s smaller missions, the Discovery-class probes, generally costing $500 million or less, have focused on easier targets like asteroids and comets.
Moreover, for its next flagship mission, the costliest and most ambitious spacecraft that cost in excess of $1 billion, NASA is focused on Europa. This is because the Jovian moon may harbor life in its warm oceans beneath an icy surface. In other words, NASA’s primary focus has been, and likely will remain, the search for life, both in the past and present.
“I get it, and we’re just trying to find a way to put Venus into that overall story of exploration and understanding where we came from and where we’re going,” Grimm said. He doesn’t think Venus should try to compete in the search for life. “I don’t think that past life or present, exotic astrobiology can be the driving factor for Venus exploration. It’s really the terrestrial planet evolution and the comparison of the evolutionary paths.”
Just trying to hold on
NASA’s recent decision didn’t mark the end for Venus scientists. They have more chances. In December NASA issued an “announcement of opportunity” for proposals for its next New Frontiers mission, which will provide about $1 billion in funding for a launch in 2025. This is enough money to do a serious Venus mission, and one of six concepts under consideration is a Venus lander that would perform in-situ measurements on the surface.
It’s an attractive concept for NASA, because, while the former Soviet Union landed a number of probes on Venus, the United States has never done so. Nevertheless, the Venus proposals will face significant competition from other inviting Solar System targets, including Saturn’s moon Titan (which may harbor exotic forms of life in its methane seas).
NASA has also begun discussions with Russia about follow-up missions to the Soviet Union’s Venera program. From 1970 to 1975 the Venera 7, 8, 9, and 10 probes all landed successfully on Venus. The Venera 7 mission marked the first successful landing on another planet. Venera 9 returned the first photos of the stark Venusian surface. All of the probes survived less than an hour with the hellish surface pressure.
With the Venera-D mission, Russia hopes to design a probe that might last several weeks on the surface. NASA’s administration has supported talks with Russia because of the potential for international cooperation, but Grimm said he doesn’t want to see NASA put all of its Venus exploration funds into that project alone. “Not only is Russia cash poor for something like this, but it has to come behind an entire lunar program that they’ve laid out,” he said. “So this is a 2030s kind of mission.”
So the Venus community will also continue planning its own flagship mission for consideration in the next Planetary Decadal Survey, in the early 2020s. If the scientific community deems it a priority, a major NASA mission to Venus might come, at best, before 2030. In the meantime, absent an infusion of new data that will lead to new discoveries, research papers, and more, the scientists who study the planet will likely continue to dwindle.
“You listen to the Mars program, and again I love Mars, but they say if we don’t launch every opportunity, every two years, the people are going to be heading to the doors,” Grimm said. “And the outer planets group gets up there and says, you know, it’s going to be 10 years between Cassini and the Europa fly-by, and we’re going to have a decade of darkness. But that’s nothing like what Venus is facing. We’re just trying to hold on.”
Listing image by NASA/SDO
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