Thirty years ago, astronomer Carl Sagan convinced NASA to turn a passing space probe’s instruments on Earth to look for life — with results that still reverberate today.
It began the way many discoveries do — a tickling of curiosity in the back of someone’s mind. That someone was astronomer and communicator Carl Sagan. The thing doing the tickling was the trajectory of NASA’s Galileo spacecraft, which had launched in October 1989 and was the first to orbit Jupiter. The result was a paper in Nature 30 years ago this week that changed how scientists thought about looking for life on other planets.
The opportunity stemmed from a tragic mishap. Almost four years before Galileo’s launch, in January 1986, the space shuttle Challenger had exploded shortly after lift-off, taking seven lives with it. NASA cancelled its plans to dispatch Galileo on a speedy path to Jupiter using a liquid-fuelled rocket aboard another space shuttle. Instead, the probe was released more gently from an orbiting shuttle, with mission engineers slingshotting it around Venus and Earth so it could gain the gravitational boosts that would catapult it all the way to Jupiter.
On 8 December 1990, Galileo was due to skim past Earth, just 960 kilometres above the surface. The tickling became an itch that Sagan had to scratch. He talked NASA into pointing the spacecraft’s instruments at our planet. The resulting paper was titled ‘A search for life on Earth from the Galileo spacecraft’1.
The outside view
We are in a unique position of knowing that life exists on Earth. To use our own home to test whether we could discern that remotely was an extraordinary suggestion at the time, when so little was known about the environments in which life might thrive. “It’s almost like a science-fiction story wrapped up in a paper,” says David Grinspoon, senior scientist for astrobiology strategy at NASA’s headquarters in Washington DC. “Let’s imagine that we’re seeing Earth for the first time.”
It came at a time, too, when the search for life elsewhere in the Solar System was at a low ebb. US and Soviet robotic missions in the 1960s and 1970s had revealed that Venus — once thought to be a haven for exotic organisms — was hellishly hot beneath its dense clouds of carbon dioxide. Mars, crisscrossed by the ‘irrigation canals’ of astronomers’ imagination2, was a seemingly barren wasteland. In 1990, no one yet knew about the buried oceans that lay on Jupiter’s moon Europa — a discovery that Galileo would go on to make3 — or on Saturn’s moon Enceladus, both of which are now seen as potential cradles of extraterrestrial life.
Crucially, Sagan and his collaborators took a deliberately agnostic approach to the detection of life, says astrobiologist Lisa Kaltenegger, who heads the Carl Sagan Institute at Cornell University in Ithaca, New York. “Of course he wants to find life, every scientist does,” she says. “But he says, let’s take that wish and be even more cautious — because we want to find it.” The existence of life was to be, in the words of the paper, the “hypothesis of last resort” for explaining what Galileo observed.
But even through this veil of scepticism, the spacecraft delivered. High-resolution images of Australia and Antarctica obtained as Galileo flew overhead did not yield signs of civilization. Still, Galileo measured oxygen and methane in Earth’s atmosphere, the latter in ratios that suggested a disequilibrium brought about by living organisms. It spotted a steep cliff in the infrared spectrum of sunlight reflecting off the planet, a distinctive ‘red edge’ that indicates the presence of vegetation. And it picked up radio transmissions coming from the surface that were moderated as if engineered. “A strong case can be made that the signals are generated by an intelligent form of life on Earth,” Sagan’s team wrote, rather cheekily.
A powerful control
Karl Ziemelis, now chief physical sciences editor at Nature, handled the paper as a rookie editor. He says it remains one of his favourites — and one of the hardest to get in. Editorial approval for the paper was far from unanimous, because it was not obviously describing something new. But, according to Ziemelis, that was mostly beside the point. “It was an incredibly powerful control experiment for something that wasn’t really on many people’s radar at the time,” he says.
“While the answer was known, it profoundly changed our way of thinking about the answer,” says Kaltenegger. Only by stepping back and regarding Earth as a planet like any other — perhaps harbouring life, perhaps not — can researchers begin to get a true perspective on our place in the Universe and the likelihood of life elsewhere, she says.
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