August 25, 2001
A Far Out Pioneer

The signal was from Pioneer 10, an interstellar spacecraft about twice as far from the Sun as Pluto.

Controllers had been listening for the distant space probe since last August with no success, raising fears that its radio transmitter had finally run out of power after 29 years in space.  But now, says delighted Pioneer 10 project manager Larry Lasher at NASA/Ames, "Pioneer 10 lives on!"

Pioneer 10 was launched on March 2, 1972, from Cape Kennedy in Florida.  At that time it was the fastest spacecraft ever to leave Earth.

Pioneer 10 was the first spacecraft to travel through the asteroid belt, the first to visit Jupiter, and the first to use a planet's gravity to change course and reach solar-system escape velocity.  Now, as it races for interstellar space, Pioneer 10 faces its toughest challenge yet -- the march of time.

Several NASA spacecraft are searching for the boundary between interstellar space and the heliosphere. Voyager 1, at 81 AU, is currently the most distant. 

"This spacecraft is way beyond its expected life," says Larry Lasher.  "Pioneer 10 was only intended to last 21 months, but it's been going for nearly 30 years!"  The craft is powered by electricity that comes from the decay of plutonium 238.  The plutonium power should last 92 years, but the thermocouples that convert heat energy to electricity are degrading faster.  Mission controllers suspect there won't be enough electricity to power the radio transmitter much longer.

Scientists are monitoring Pioneer's faint signals as a test of communications technology for use with future interstellar missions.  "The NASA Advanced Concepts Program is funding this effort," Lasher explains.  "They're learning how to extract clear and coherent data from a noisy signal using what's known as chaos theory."

Far-away Pioneer 10 is the perfect target for such an experiment.  Its feeble signals travel nearly 11 hours to reach Earth from a distance of 7.3 billion miles -- that's 78 times farther from the Sun than our planet.

Back in December 1992, when Pioneer 10 was "only" 5.2 billion miles away, the craft made an unexpected course change.  Astronomers think it was diverted slightly by the pull of gravity from one of the Kuiper Belt Objects -- asteroid-sized bodies that circle the Sun at vast distances beyond the outermost planets.

This happened as Pioneer 10 was leaving the realm of the planets and exploring the outer limits of the heliosphere for the first time.  The heliosphere is a gigantic bubble carved out of the gases between stars by the Sun's solar wind.  No one yet knows the exact size of the heliosphere, but Pioneer 10 is probably approaching its edge.  "We could cross into interstellar space within a few years," says Lasher.  "We'll know we're there when the flux of cosmic rays being recorded suddenly increases."

Herb Lassen, one of Pioneer 10's original design engineers at TRW, discusses what the long-lived spacecraft might encounter. 
Credit: TRW.
Come to Liftoff.msfc.nasa.gov to hear the audio

Then, in the next million years, Pioneer 10 will pass ten stars at distances ranging from three to nine light years -- and it will probably still be traveling out through the Milky Way galaxy when our Sun becomes a red giant and wipes out its nearby planets about five billion years in the future.

The little space probe wasn't that long lasting in its first and only movie role, in Star Trek V: the Final Frontier.  A trigger-happy Klingon named Captain Klaa blasted Pioneer 10 to bits for target practice.  Of course, that was just a model of the spacecraft built by the moviemakers from actual design drawings of the real thing.

Plaque on the side of Pioneer 10. Each symbol is intended to tell alien intelligences about us.

As an ambassador to the galaxy, Pioneer 10's greatest and strangest adventures may still lie far, far ahead.

Author: Dr. Tony Phillips
Editors: Gil Knier and Becky Bray

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