Asteroid-bound spacecraft finds signs of life—on Earth

Nearly 30 years ago, the Galileo spacecraft flew past Earth on its journey to Jupiter, prompting astronomer Carl Sagan to develop a novel experiment: to look for signs of life on Earth from space. The spacecraft found high levels of methane and oxygen, suggestions that photosynthesis was occurring on Earth’s surface. Now, astronomers have repeated the experiment, this time with an asteroid-bound spacecraft that swung around Earth in late 2017. It also found Earth to be teeming with life, but with an unsettling corollary: Atmospheric levels of carbon dioxide and methane were far higher than they were during the Galileo flyby.