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Long-running Pioneer spacecraft mystery may be explained

July 18, 2012
Courtesy of the Jet Pro­pul­sion Lab­o­r­a­to­ry
and World Science staff

A mys­te­ri­ous slow­ing of NASA’s Pi­o­neer 10 and 11 space­craft turns out to be due to the slight, but de­tect­a­ble ef­fect of heat push­ing back on the space­craft, ac­cord­ing to new re­search.

The so-called “Pi­o­neer Anoma­ly” has mys­ti­fied re­search­ers for years and was thought by some to point to de­fi­cien­cies in the bas­ic struc­ture of main­stream phys­ics.

An artist's view of a Pi­o­neer space­craft head­ing in­to deep space. Both Pi­o­neer 10 and 11 are on tra­jec­to­ries that will even­tu­al­ly take them out of our so­lar sys­tem. (Im­age cred­it: NA­SA )
The heat that caus­ing the slow­down em­anates from elec­tri­cal cur­rent flow­ing through in­stru­ments and pow­er sup­ply, ac­cord­ing to new re­sults pub­lished on June 12 in the jour­nal Phys­i­cal Re­view Let­ters.

“The ef­fect is some­thing like when you’re driv­ing a car and the pho­tons [par­t­i­cles of light] from your head­lights are push­ing you back­ward,” said Slava Tu­ry­shev, the pa­pe­r’s lead au­thor at NASA’s Jet Pro­pul­sion Lab­o­r­a­to­ry in Pas­a­de­na, Ca­lif. “It is very sub­tle.”

The Pi­o­neer 10 and 11 space­craft, launched in 1972 and 1973 re­spec­tive­ly, are on an out­ward trajecto­ry from our Sun. In the early 1980s, nav­i­ga­tors saw a de­celera­t­ion on the two space­craft as they ap­proached Sat­urn, which was not def­i­nitely ex­plain­a­ble by any known forc­es. They dis­missed it as the ef­fect of drib­bles of left­o­ver pro­pel­lant still in the fu­el lines af­ter con­trollers had cut off the pro­pel­lant. 

But by 1998, as the space­craft kept trav­el­ing on their jour­ney and were over 8 bil­lion miles (13 bil­lion kilo­me­ters) away from the Sun, a group of sci­en­tists led by John An­der­son of Je­t Pro­pul­sion Lab­o­r­a­to­ry real­ized the un­ex­plained slow­ing down was con­tin­u­ing—by 25 feet (7.6 meters) per day, every day. They raised the pos­si­bil­ity that this could be some new type of phys­ics that con­tra­dicted Ein­stein’s gen­er­al the­o­ry of rel­a­ti­vity.

In 2004, Tu­ry­shev de­cid­ed to start gath­er­ing records stored all over the coun­try and an­a­lyze the da­ta to see if he could fig­ure out the source of the de­celera­t­ion. In part, he and col­leagues were con­tem­plat­ing a deep space phys­ics mis­sion to in­ves­t­i­gate the anom­a­ly, and he wanted to be sure there was one be­fore ask­ing NASA for a space­craft.

He and col­leagues went search­ing for Dop­pler da­ta, a pat­tern of da­ta com­mu­ni­cated back to Earth from the space­craft, and te­lem­e­try da­ta, house­keep­ing da­ta sent back from the space­craft. At the time these two Pi­o­neers were launched, the in­forma­t­ion was be­ing stored on punch cards. But Tu­ry­shev and col­leagues were able to copy dig­i­tized files from the com­put­er of Je­t Pro­pul­sion Lab­o­r­a­to­ry nav­i­ga­tors who have helped steer the Pi­o­neer space­craft since the 1970s. They al­so found over a doz­en of boxes of mag­net­ic tapes stored un­der a stair­case and re­ceived files from the Na­t­ional Space Sci­ence Da­ta Cen­ter at NASA God­dard Space Flight Cen­ter in Green­belt, Md. They al­so worked with NASA Ames Re­search Cen­ter in Mof­fett Field, Ca­lif., to save some of their boxes of tapes. 

Tu­ry­shev col­lect­ed more than 43 gi­ga­bytes of da­ta, which may not seem like a lot now, but is quite a lot of da­ta for the 1970s. He al­so man­aged to save a vin­tage tape ma­chine that was about to be thrown out, so he could play the mag­net­ic tapes.

The ef­fort was a la­bor of love for Tu­ry­shev and oth­ers. The Plan­e­tary So­ci­e­ty sent out ap­peals to its mem­bers to help fund the da­ta reco­very ef­fort. NASA lat­er al­so pro­vid­ed fund­ing. In the pro­cess, a pro­gram­mer in Can­a­da, Vik­tor Toth, heard about the ef­fort and con­tacted Tu­ry­shev. He helped Tu­ry­shev cre­ate a pro­gram that could read the te­lem­e­try tapes and clean up the old da­ta.

They de­ter­mined that what was hap­pen­ing to Pi­o­neer was­n’t hap­pen­ing to oth­er space­craft, mostly be­cause of the way they were built. For ex­am­ple, the Voy­ag­er space­craft are less sen­si­tive to the ef­fect seen on Pi­o­neer, Tu­ry­shev said, be­cause its thrusters align it along three di­rec­tions, where­as the Pi­o­neer space­craft rely on spin­ning to stay sta­ble so its thrusters push in only one di­rec­tion.

With the new da­ta, Tu­ry­shev and col­leagues cal­cu­lat­ed the heat put out by the elec­tri­cal sub­sys­tems and the grad­u­al de­cay of plu­to­ni­um in the Pi­o­neer pow­er sources, which matched the anom­a­lous ac­celera­t­ion seen on both crafts. “The sto­ry is find­ing its con­clu­sion be­cause it turns out that stand­ard phys­ics pre­vail,” Tu­ry­shev said. “While of course it would’ve been ex­cit­ing to disco­ver a new kind of phys­ics, we did solve a mys­tery.”

* * *

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A mysterious slowing of NASA’s Pioneer 10 and 11 spacecraft turns out to be due to the slight, but detectable effect of heat pushing back on the spacecraft, according to new research. The so-called “Pioneer Anomaly” has mystified researchers for years and was thought by some to point to deficiencies in the basic structure of mainstream physics. The heat that causing the slowdown emanates from electrical current flowing through instruments and power supply, according to new results published on June 12 in the journal Physical Review Letters. “The effect is something like when you’re driving a car and the photons [particles of light] from your headlights are pushing you backward,” said Slava Turyshev, the paper’s lead author at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “It is very subtle.” The Pioneer 10 and 11 spacecraft, launched in 1972 and 1973 respectively, are on an outward trajectory from our Sun. In the early 1980s, navigators saw a deceleration on the two spacecraft as they approached Saturn, which was not definitely explainable by any known forces. They dismissed it as the effect of dribbles of leftover propellant still in the fuel lines after controllers had cut off the propellant. But by 1998, as the spacecraft kept traveling on their journey and were over 8 billion miles (13 billion kilometers) away from the Sun, a group of scientists led by John Anderson of Jet Propulsion Laboratory realized the unexplained slowing down was continuing—by 300 inches per day, every day. They raised the possibility that this could be some new type of physics that contradicted Einstein’s general theory of relativity. In 2004, Turyshev decided to start gathering records stored all over the country and analyze the data to see if he could figure out the source of the deceleration. In part, he and colleagues were contemplating a deep space physics mission to investigate the anomaly, and he wanted to be sure there was one before asking NASA for a spacecraft. He and colleagues went searching for Doppler data, a pattern of data communicated back to Earth from the spacecraft, and telemetry data, housekeeping data sent back from the spacecraft. At the time these two Pioneers were launched, the information was being stored on punch cards. But Turyshev and colleagues were able to copy digitized files from the computer of Jet Propulsion Laboratory navigators who have helped steer the Pioneer spacecraft since the 1970s. They also found over a dozen of boxes of magnetic tapes stored under a staircase and received files from the National Space Science Data Center at NASA Goddard Space Flight Center, Greenbelt, Md. They also worked with NASA Ames Research Center, Moffett Field, Calif., to save some of their boxes of tapes. Turyshev collected more than 43 gigabytes of data, which may not seem like a lot now, but is quite a lot of data for the 1970s. He also managed to save a vintage tape machine that was about to be thrown out, so he could play the magnetic tapes. The effort was a labor of love for Turyshev and others. The Planetary Society sent out appeals to its members to help fund the data recovery effort. NASA later also provided funding. In the process, a programmer in Canada, Viktor Toth, heard about the effort and contacted Turyshev. He helped Turyshev create a program that could read the telemetry tapes and clean up the old data. They determined that what was happening to Pioneer wasn’t happening to other spacecraft, mostly because of the way the spacecraft were built. For example, the Voyager spacecraft are less sensitive to the effect seen on Pioneer, he said, because its thrusters align it along three directions, whereas the Pioneer spacecraft rely on spinning to stay stable so its thrusters push in only one direction. With the new data, Turyshev and colleagues calculated the heat put out by the electrical subsystems and the gradual decay of plutonium in the Pioneer power sources, which matched the anomalous acceleration seen on both crafts. “The story is finding its conclusion because it turns out that standard physics prevail,” Turyshev said. “While of course it would’ve been exciting to discover a new kind of physics, we did solve a mystery.”