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Pioneer 10 set out for Jupiter 20 years ago Monday, carrying a message from earthlings, just in case.

The nuclear-powered spacecraft is now 5 billion miles away - farther from Earth than any other human-made object - and still beaming back precious scientific data as it hurtles deeper and deeper into space."The great technical miracle of the whole thing is that we can receive intelligible data from 5 billion miles away using only 8 watts of radio power. That's a great human achievement," said University of Iowa physicist James Van Allen, principal investigator of one of Pioneer 10's telescopes.

"It's mind-boggling and awe-inspiring," said B.J. O'Brien, a retired engineer who managed the program for satellite maker TRW Inc.

The faint radio pulses eventually will disappear and power to the scientific instruments will cease, probably around the year 2000. But Pioneer 10's odyssey will go on and on, barring collision or capture by space beings.

No one knew what to expect when Pioneer 10 was hoisted into space by an Atlas-Centaur rocket on March 2, 1972. The 570-pound craft was designed to operate for a minimum 21 months, just long enough to fly by Jupiter and take pictures of the mysterious, giant planet.

Seven of the probe's 11 scientific instruments still work, including Van Allen's telescope. They monitor solar wind, cosmic particles and ultraviolet glow. Scientists scrutinize the data for evidence of a 10th planet and signs of the heliopause, the outer limits of the sun's influence.

For anyone or anything that might be out there, Pioneer 10 bears a plaque. It shows a naked man and woman standing side by side.

The nine planets also are depicted, as well as the spacecraft's path. A burst of radiating lines indicate the distance and direction from the sun to 14 pulsars, or pulsating stars, and the frequency of those pulsars at the time of launch.