In June of 1769, an astronomer named David Rittenhouse prepared to observe a rare cosmic phenomenon, the transit of Venus. Rittenhouse had built an observatory on his farm in Pennsylvania to monitor the planet as it moved across the face of the sun, a small black dot against the glowing orb in the afternoon sky. When the moment arrived, Rittenhouse “became so over-excited that he collapsed and fainted, missing the beginning of the most important event of his scientific life,” writes the historian Andrea Wulf. “When he regained consciousness, he quickly grabbed his telescope to discover that Venus had already entered the sun, but calmed himself sufficiently to take some observations."
Astronomers across the globe entered a similar state of feverish excitement yesterday. There have been no reports of fainting—yet—but just like Rittenhouse, they are overwhelmed by a new opportunity to experience our next-door neighbor.
NASA has picked not one, but two new spacecraft missions to study the second planet from the sun. The missions—a probe that will plunge into Venus’s atmosphere and an orbiter that will remain circling overhead—are expected to leave Earth at the end of this decade. All day, the astronomy community waited for NASA Administrator Bill Nelson to make an unspecified announcement about future missions, and when he spoke the word Venus, planetary scientists, to use less-than-scientific parlance, kind of lost it. There's a spacecraft in orbit around Venus now—a Japanese spacecraft called Akatsuki—but NASA hasn’t sent a mission to the planet in more than 30 years. One mission would have been thrilling, but two feels almost surreal.
The decision was particularly satisfying for many planetary scientists, especially those who believe that NASA has overlooked Venus in favor of our other next-door neighbor, Mars. In the past three decades, NASA has sent more than a dozen robots to the red planet. Where was the love for Venus?
After all, we owe quite a bit of our understanding of the universe to this shining planet. It was Venus that helped prove the Copernican theory that the sun, not Earth, was at the center of our solar system. When Galileo saw Venus thinning into a crescent in his telescope, moving through phases just like our moon, he deduced that the planet must be reflecting sunlight in its orbit around our star. Centuries later, when people began to dispatch machines into the solar system, the first to reach another planet crash-landed on the Venusian surface in 1966. The earliest missions, dispatched by the Soviet Union, revealed an inferno of a planet, with scorching temperatures that melted the spacecraft that managed to touch down.
You have to forgive Venus for that: It wasn't always this way. Billions of years ago, the planet was balmy and as comfortable as Earth, likely with an ocean of its own, before its atmosphere ballooned with heat-trapping gases and its water vanished into space. That history, combined with similarities between Earth and Venus in size and composition, is why planetary scientists refer to the planets as twins. But it’s not hard to see why NASA has chosen to focus on another sibling. Mars has a surface that won’t melt robots, and while exploration there isn’t easy, the agency has gotten confident enough that engineers have managed to fly a first-of-its-kind helicopter without crashing. For a space agency that has long promised to send astronauts to Mars, the attention makes sense.
Astronauts may never visit the planet, but Venus is one of the most exciting targets in the search for life beyond Earth, the drive to answer that most existential of questions. Last fall, a group of scientists announced that they had discovered evidence of a rare, stinky gas called phosphine floating around in Venus’s clouds. Phosphine can’t survive for long in Venus’s acidic atmosphere, so presumably something was replenishing the supply. The scientists suggested that the mysterious something could be a chemical process no one has ever seen before or, just maybe, a form of alien life. On Earth, phosphine is produced by microorganisms—why not on this other planet too? Mars might have ancient microbes fossilized in its rock, but Venus could have lifeforms suspended in its clouds.
Since that initial discovery, other scientists have raised doubts about the research—including whether any phosphine is actually there. Debates like this have a tendency to linger, and phosphine discussion might still be going in 2028, when one of the new Venus missions is supposed to launch. (Consider that scientists spent 15 years debating the presence of methane gas on Mars before reaching some consensus, and they still disagree over whether it’s being produced by chemical processes or living organisms hiding out of view.) Although the new Venus missions are not designed to search directly for signs of phosphine—scientists had already submitted their mission concepts to NASA when the news broke last year—the probe meant to dive into the atmosphere could potentially do some sniffing around.
Venus holds many more mysteries for the science community. The atmospheric mission, known as DAVINCI+, will determine whether the planet had an ocean, as scientists suspect. The orbiting mission, called VERITAS, will map Venus’s surface in fine detail and investigate whether the planet has active volcanoes and shifting plate tectonics like Earth. “We’re gonna have a totally different view of it when we’re done,” Tom Wagner, a NASA scientist who leads the program that decided to fund these journeys, told me. “Kids who learn about this stuff in 20 years are going to have a very different perspective on Venus than we do."
The latest Venus news is understandably big news for space scientists, but the rest of us, who have little reason to contemplate the wonders of the universe in our daily lives, might be wondering: Why should we explore Venus? Why should we visit Io, a volcanic moon of Jupiter, and Triton, an icy moon of Neptune—the other targets that NASA had considered for new missions—or any worlds beyond Earth at all? Well, these places, in a cosmic sense, are all we have. We don’t have the technology to dispatch probes to planets around other stars. We will remain, certainly in this lifetime and probably for centuries to come, within the bounds of this solar system. Venus, Mars, the distant moons of the outer planets—these may be the farthest worlds that humankind and its robotic explorers will ever reach. In some ways, these are already the most familiar bodies in the solar system; in others, we are only beginning to discover them.