If you were observing Earth from an alien planet, how would you know if life really existed here?

The renowned astronomer Carl Sagan wondered the same thing. Then he got an opportunity to find out.

In October 1989, NASA launched the Galileo spacecraft to orbit Jupiter. But because of the space shuttle Challenger disaster in 1986, the decision was made to release the probe more carefully from an orbiting shuttle rather than send it directly with a rocket booster launched from another shuttle. This meant Galileo would have to slingshot around Venus and Earth to get sufficient gravitational boosts to propel it to Jupiter.

Galileo was due to pass Earth at an altitude of only 960 kilometers on 8 December 1990. This gave Sagan the chance he was looking for. He persuaded NASA to point Galileo’s instruments at Earth just to see what it would find.

What did it find? Although the continents of Australia and Antarctica did not show any signs of civilization, Galileo did measure oxygen and methane in Earth’s atmosphere, the latter in ratios that suggested living organisms. It also identified a sharp spike in the infrared spectrum of sunlight reflecting off the planet, an indication of vegetation. The clincher was surface radio transmissions that were moderated as if engineered, causing the observing team to conclude that “A strong case can be made that the signals are generated by an intelligent form of life on Earth.” 

The complete results were published in a scientific paper titled ‘A Search for Life on Earth from the Galileo Spacecraft’ (https://www.nature.com/articles/365715a0).

So what did we learn? It was a unique way of getting us to think about ourselves. And if nothing else, it gave us an idea of how life should show itself as we survey the rest of the cosmos.

For more information, see “How Would We Know Whether There is Life on Earth? This Bold Experiment Found Out” by Alexandra Witze (https://www.nature.com/articles/d41586-023-03230-z?).