Pioneer 10: Farewell to an Old Friend

by Peter Backus - Observing Programs Manager

We first met at Venus, in the desert. Barely a teen, our new friend was already a well-seasoned traveler when we received its hailing signal in March of 1985. NASA SETI team members and the local staff crowded around the monitor of a Sun 100 workstation to see a signal from beyond our solar system, a white diagonal line boldly marching down the computer screen. This cheery greeting was from the Pioneer 10 spacecraft. We were using a research and development antenna, the "Venus" station in the Mojave Desert, part of NASAs Deep Space Network when we saw the first real demonstration of our prototype signal processors capability. Since that moment, Pioneer 10 (often referred to affectionately as P10) has been an unofficial member of our SETI team.

On March 2, 1972, Pioneer 10 embarked on a two-year mission to Jupiter, and possibly Saturn, if all went well. With such an ambitious plan, Pioneer was well named. Although the US and USSR had sent probes to Mars, Pioneer 10 and its mission staff at NASAs Ames Research Center would be the first to "boldly go," through the asteroid belt then on to the realm of the giant planets. P10 accomplished those goals with spectacular success and more as it ventured further, probing and measuring the outer reaches of our planetary system. It passed the orbit of Neptune (then the outermost planet) in 1983, and began looking for the "heliopause," the outer boundary of our solar system. All the while P10 transmitted data, "writing home" even as it approached the threshold of interstellar space

Sometimes serendipity happens. Though not a part of Pioneer 10s mission planning, had the NASA SETI Program been asked to design a test signal to be broadcast from deep space, it would have looked very much like a transmission from Pioneer 10. The spacecrafts signal consisted of a "carrier" and "sidebands." The carrier is on continuously and concentrated in frequency, an ideal test signal for our continuous wave detectors. The sidebands contain data and so are more complex, providing a good test for our pulsed signal detectors.

So, when it came time to determine how well NASAs prototype SETI detector system worked, we looked for Pioneer 10. And we found it. At Venus in the desert.

Since that time weve observed other spacecraft using other telescopes and two generations of detector systems, but weve always favored the signal from Pioneer 10. Whenever we deployed to a new telescope, or tested a new detection algorithm, Pioneer 10 was always the touchstone.

When Project Phoenix began using two widely separated antennas to discriminate against terrestrial signals, daily observations of Pioneer 10 showed that we could indeed detect an extraterrestrial signal and differentiate it from terrestrial interference. Pioneer 10 has been a part of Project Phoenix since 1995. It will seem strange and sad during our next observing session to know that one team member will not be there.

Thirty-one years into its five-year mission, Pioneer 10 broadcasts no more. It has been gradually failing in recent years. One by one its experiments were turned off. Then a key component in its transmitter failed. It could only transmit when it received a strong reference signal from the Earth. Finally, the power levels dropped so low that the final experiment was shut down. With nothing to report, theres no point in transmitting the reference signal. Pioneer 10 will drift silently into interstellar space, toward the constellation of Taurus the Bull.

Perhaps, someday, Pioneer 10 will speak again, but not to Earth. Attached to the side of the spacecraft is a plaque showing a human man and woman, and the location of the Earth, a message from our planet to whatever space-faring civilization might find the spacecraft. No longer the distant correspondent, P10 has become Earths silent courier to distant worlds in its final phase of life.

Farewell old friend.