During Galileo’s December 1990 flyby of Earth, astronomers trained its instruments and cameras on our own planet. Onboard sensors were able to measure the signatures of life: an atmosphere rich in the two highly reactive gases, oxygen and methane. Green light from plants reflected off the surface that covered most of the land surface. Finally, its radar detectors noticed a great deal of radio wave emission emitting within narrow bands of the electromagnetic spectrum—signals too orderly and well organized to come from lightning, aurorae, or other natural energy bursts—that indicated communication between intelligent beings.

In 2008, the EPOXI spacecraft (the continuation of the Deep Impact mission) took data of Earth in a similar way but at a greater distance, using its more advanced scientific instruments to examine Earth as if it were an exoplanet viewed from afar. Future space probes will use these readings from Galileo and EPOXI as a baseline to refer to when searching for extraterrestrial life.