WHEN the space shuttle Challenger blew up 73 seconds into its flight on a January morning in 1986, the consequences rippled through the space industry. One lesser known casualty was the Galileo mission to Jupiter, a $1 billion NASA spacecraft designed to orbit the giant planet, study its many moons and drop a probe into its atmosphere.
Galileo had been due to begin this journey sitting on the tip of a Centaur rocket stage, which would power it to Jupiter after it was hefted into space inside a space shuttle鈥檚 cargo bay. But in the wake of the disaster, NASA decided that launching an unlit Centaur rocket using the shuttle鈥檚 booster was just too risky.
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No other set-up was powerful enough to lift the Galileo spacecraft into orbit attached to this rocket stage, so the Centaur was ditched, leaving the mission team to find another way to get to Jupiter. The solution was gravitational slingshots that would send Galileo around Venus and twice past Earth to build up enough speed to hurl it at Jupiter. This workaround set the stage for one of the most inspired experiments in space science.
Galileo blasted off on its circuitous journey aboard the shuttle Atlantis on 18 October 1989. Only then did one of the project scientists, the astronomer Carl Sagan, come up with an extraordinary idea: to use an Earth flyby as an opportunity to point Galileo鈥檚 instruments at our planet, to see if they could discern signs of intelligent life solely from the data sent back. At the time, NASA spacecraft had flown by upwards of 60 planets and moons, and none had spotted any hint of life. 鈥淚f we find signs of life on Earth, it means the negative results we find elsewhere really are significant,鈥� Sagan said in a television broadcast. NASA liked his plan.
As it happened, Galileo approached Earth first-time round from its night-time side, flying past on 8 December 1990. It got to just 960 kilometres above the dark Caribbean Sea 鈥� 40 times closer than a geostationary satellite. Its imaging system and three spectrometers were trained on Earth in the visible, ultraviolet, infrared and radio regions of the spectrum. The data would be treated as if collected from an alien planet, and the presence of living organisms would be 鈥渢he hypothesis of last resort鈥� for the life-hunting detectives. At such close range, how difficult could it be to spot telltale signs of life?
鈥淭hey struggled hard to find any proof at all to start with,鈥� says Don Gurnett. He was a senior member of the team who ran the spacecraft鈥檚 plasma wave spectrometer, which detected radio waves.
With its 1980s technology, Galileo鈥檚 photographic resolution was just 1 kilometre per pixel at best when it reached Earth鈥檚 day-lit side. That meant only artificial, geometric structures with a scale greater than that would show up 鈥� cities, swathes of agricultural fields and so on. Unfortunately, its highest-resolution images were of Australia and Antarctica, both of which are sparsely inhabited (see photos above). Australia鈥檚 coastal agriculture offered a hint of something, but wasn鈥檛 judged 鈥渟ufficiently distinctive to be鈥� indicative of intelligent life鈥�. Ouch.
The life detectives were off to a poor start with this, their only eyewitness, but the good news was that Earth clearly had water galore, in all its forms. Liquid water is probably necessary for the existence of life, but not sufficient.
There were further lines of enquiry. Galileo also spotted that Earth鈥檚 atmosphere contains methane, for example. Methane is not an unambiguous sign of life, but sunlight breaks it down, so any left over from the formation of the solar system should have long ago disappeared 鈥� something on the surface must be producing it. We know that much of Earth鈥檚 methane comes from bacterial respiration, rice farming and, lest we forget, flatulent cows. But methane is also generated by volcanic activity, so it could have been a red herring.
The high level of oxygen in the atmosphere was tantalising too. This very reactive gas ought to form more stable compounds over time, so something on the surface must have been pumping it out.
Earth鈥檚 land masses provided another important clue. Much of the planet鈥檚 surface was covered with a green pigment that strongly absorbed light in the red part of the spectrum. Crucially, the team reported, this pigment corresponded to 鈥渘o plausible mineral鈥�. The pigment 鈥� chlorophyll 鈥� puts a cliff-like dip in the spectrum of light Earth reflects. Astrobiologists now call this the 鈥渞ed edge 鈥� and think its presence is uniquely indicative of the light-harvesting molecules involved in photosynthesis.
The red edge, combined with the other leads, pointed strongly towards life on Earth 鈥� but not necessarily the intelligent kind. 鈥淢ost of the evidence uncovered by Galileo would have been discovered by a similar flyby spacecraft as long ago as about 2 billion years,鈥� the team noted.
鈥淰isual clues weren鈥檛 deemed distinctive enough to indicate intelligent life in Australia. Ouch鈥�
One last clue blew the case wide open. Gurnett鈥檚 plasma wave spectrometer picked up narrow-band radio transmissions coming from the surface (though Galileo couldn鈥檛 鈥渢une in鈥� to them). 鈥淵ou just don鈥檛 see natural radio signals looking like that,鈥� says Gurnett. 鈥淲e were picking up taxi communications from South America.鈥�
Busted! This was the smoking gun. 鈥淥f all Galileo science measurements, these signals provide the only indication of intelligent, technological life on Earth,鈥� the team wrote in their paper , Nature鈥榮 cover story in 1993.
Surprisingly, the paper caused little excitement at the time, says Gurnett, especially compared with another spacecraft he worked on. 鈥淚 gave a few radio interviews, but it didn鈥檛 generate the media interest that Voyager 1 did when it left the solar system.鈥�
However, 25 years on, that view has changed. 鈥淚 thought of it as a novelty at the time, but now as a seminal paper,鈥� says Jim Green, head of planetary science at NASA HQ in Washington DC. 鈥淐arl Sagan was ahead of his time, probably by a decade or more.鈥�
Sagan died in 1996, aged 62, at the dawn of a new era. Just a year before, the first exoplanet orbiting a sun-like star was discovered. Today, exoplanets are being found everywhere 鈥� almost 4000 have been confirmed so far 鈥� and the Galileo flyby experiment is inspiring a new generation. NASA is focusing on the search for life elsewhere in the universe. Its Transiting Exoplanet Survey Satellite (TESS) spacecraft, scheduled for a 16 April launch when New 杏吧原创 went to press, will look for exoplanets orbiting the brightest and nearest stars. Then the James Webb Space Telescope, expected to launch in 2020, will bring its unparalleled resolving power to bear on TESS鈥檚 most promising finds. It should deliver high-quality spectra of the atmospheres of exoplanets in our galactic neighbourhood.
Will starlight passing through the atmospheres of these distant worlds betray signature 鈥渆dges鈥�? Or will it reveal other features that can鈥檛 easily be explained away without invoking life, just as Galileo鈥檚 flyby revealed such signs on Sagan鈥檚 鈥減ale blue dot鈥�? We鈥檒l soon find out.
This article appeared in print under the headline 鈥淚s there intelligent life on Earth?鈥�
