It is now clear that organic chemistry has run rampant through the solar system and beyond.

Carl Sagan, Scientific American, 1997 May 21, 1999: When the Galileo spaceprobe flew by Jupiter's moon Callisto last week, the detection of life on that strange and distant world was not among the scientific objectives. After all, Callisto's heavily cratered surface is a frigid -220o F and is scarcely protected from the ravages of space by a extraordinarily thin CO2 atmosphere. Indeed most astrobiologists concur that Callisto is an unlikely abode for life.

But even if Callisto was wet and warm and teeming with life, would Galileo have noticed? The question brings to mind an earlier Galileo flyby of another curious planet -- Earth.

Above: This view of Earth's southern hemisphere centered on the South Pole was created using images from the Galileo spacecraft taken during the December 1990 flyby.

When the Galileo spaceprobe swooped by Earth in 1990, all its instruments were pointed towards us. As Galileo flew toward our planet, the Earth was centered in the windshield and then again in the rear-view mirror as Galileo continued on its journey to Jupiter.

Galileo's close encounter with Earth framed one of the most difficult questions in astrobiology:

Can a modern space instrument tell if the Earth, or any planet, is a good candidate for harboring life?

As Cornell Professor, J.R. Vallentyne, put the matter in his opinion in 1965: "Apparent inherent limitations on temperature, pressure or chemical environment for living matter are geocentric myths."

To put the 1990 flyby in perspective, the late Carl Sagan and his colleagues published a 1993 Nature article on this question. According to Sagan, the Galileo spacecraft found clear signs of life during its flight past the earth including:

1. strong absorption of light at the red end of the visible spectrum, particularly over the conti
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Contact: Linda Porter
Linda.Porter@msfc.nasa.gov
256-544-7588
NASA/Marshall Space Flight Center--Space Sciences Laboratory
21-May-1999