On Dec. 8, a spacecraft traveling the solar system swung within 600 miles of an unusual planet. From the fleeting photographic glimpses and remote-sensing data the craft transmitted, scientists began to compose an image of what that strange new world must be like.
From its strong gravitational pull, they inferred that the planet must be composed mainly of rock and iron. It is clasped by intense belts of trapped radiation. Bursts of lightning flare across its face.
Dazzling auroras veil its southern pole. The planet's surface is mostly water, but the spacecraft's cameras caught sight of a continent of rock and ice at its south pole.
Its atmosphere consists mostly of nitrogen. The spacecraft's sensors also detected significant quantities of oxygen, nitrous oxide and methane, all byproducts of biological processes. The planet apparently abounds in life.
But intelligent life? "We don't see anything in the pictures that would indicate intelligent life," said Torrence V. Johnson, the chief project scientist at the Jet Propulsion Laboratory in Pasadena, Calif.
The planet is, of course, Earth.
When the Galileo spacecraft passed by Earth this month on its roundabout voyage to Jupiter, the scientists decided to exercise the analytical skills of first-look reconnaissance they have practiced at Mars, Saturn and most of the other planets.
Why not see how reliable these techniques of observation, analysis and inference are by testing them on the planet they know best?
The experience has left the planetary scientists bemused. In failing to reveal evidence for intelligent life on Earth, the spacecraft left them wondering if they were not sometimes like the blind men who one by one try to describe an elephant.
"This is an excellent example of why, if you have been to a planet once, you want to go back again for another, longer look," Dr. Johnson said.
Scientists were reminded in particular of how in the 1960s they were misled by the first three spacecraft visits to Mars. As it happened, all three Mariners got a distant view only of the planet's southern hemisphere, a bleak, cratered landscape, not unlike the moon.
Not until Mariner 9 orbited Mars in 1971 and mapped the entire planet did scientists learn of its towering volcanic peaks, yawning canyons and many dried channels telling of water that once flowed across Mars in abundance.
Chance likewise accounted for Galileo's incomplete picture of Earth. The spacecraft happened to come closest to Earth over the Atlantic Ocean and obtained its best images in data over the South Pacific, Central Australia and Antarctica, some of the most unprepossessing regions on Earth.
If Galileo had explored the Northern Hemisphere, Dr. Johnson said, it would more likely have made out clues of intelligent life, such as the Great Wall of China or geometric patterns cut through Canadian forests for power lines.
The spacecraft's cameras were incapable of detecting features any smaller than a half-mile across. But they might have resolved the lights of cities, if the craft had flown over them at night.
In one photograph transmitted by Galileo, analysts think they can pick out a human-built structure that stands out against the white background of the South Pole.
It may be part of the scientific research station there. But would scientists from an alien world have been able to draw such a conclusion?
"That is possibly the only direct evidence of the people who occupy this planet," said Michael J. B. Belton of the National Optical Astronomy Observatories, who is the leader of the photo-interpretation team.
The search for signs of extraterrestrial life far beyond the solar system is conducted with radio antennas set to receive any possible unnatural radio signals.
Galileo's antennas were not tuned especially to listen for similar signals from Earth, such as television shows that might be bouncing through the upper atmosphere. It did pick up some odd radio signals, probably sounds from over-the-horizon radar and submarine communications.
Galileo's infrared mapping instruments provided the unambiguous evidence for some forms of life. From the presence of oxygen, methane and nitrous oxide in the atmosphere, said Robert W. Carlson of the Jet Propulsion Laboratory, "I can tell you that life abounds on the Earth."
Galileo also provided pictures of the Great Barrier Reef of Australia. Alien scientists could be excused for not recognizing this as a spectacular manifestation of coral life.
Earth's moon would have intrigued and puzzled alien scientists examining the Galileo data.
By studying its gravity and inferring its density, Dr. Johnson said, the scientists would have concluded that the moon and Earth have different compositions and wondered how this could be. Even Earth scientists, after all these years and the analysis of Apollo results, are not sure how Earth came to have its large companion.
Galileo's excursion by Earth, after first swinging by Venus, was only a way station in its journey to Jupiter. Its primary purpose was to use Earth's gravity to gain velocity and change direction.
A second pass by Earth, in December 1992, should give the spacecraft its final boost into the outer solar system for a rendezvous with Jupiter in 1995.
There, Galileo is to launch a probe into Jupiter's atmosphere and then orbit the giant planet and survey its four major satellites: Io, Europa, Ganymede and Callisto.