It's been years since NASA last heard from either of its two Pioneer probes as they hurtle out of the solar system, but scientists are still debating the source of an odd force pushing against the outbound spacecraft.

Dubbed the Pioneer Anomaly, the unexplained force appears to be acting against NASA's identical Pioneer 10 and 11 probes, holding them back as they head away from the Sun.

Whether that force stems from the probes themselves, from something exotic such as dark matter, or from some new facet of physics or gravity remains in doubt.

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But a wealth of newly recovered data and telemetry, spanning decades of observations by both Pioneer 10 and 11, may yield the final answer to whether conventional physics — or something new — is at work on the two spacecraft.

An answer could soon arise from the new data, after about a year of analysis by an international team of researchers.

"I would like to see this story reach its finality," said Slava Turyshev, an astrophysicist with NASA's Jet Propulsion Laboratory (JPL) who has spent the last 14 years — some of it on his own time — studying the Pioneer Anomaly. "So if it's conventional physics, that's fine and we can all go about our daily business. But if it's something else, there may be another page."

He and other fellow devotees discussed the astrophysics oddity late Monday during the Seventh Annual Asimov Debate here at the American Museum of Natural History.

Turyshev's international team includes researchers from all Pioneer Anomaly camps, with some learning towards a conventional physics explanation while others trend toward the unknown fringe. Still other researchers have their own opinions.

"If I were a betting man, which I am not, I would bet a whole case of cranberry juice that the Pioneer Anomaly will have an ordinary explanation that is within known physics," said Irwin Shapiro, an astrophysicist at Harvard University unaffiliated with the Pioneer Anomaly research team, during the debate.

Shapiro said that the number of actual instances in which oddities like the Pioneer Anomaly have opened pathways to fundamentally new physics are rare, and that ongoing studies may yet yield a conventional explanation.

Perplexing push

Launched in 1972 and 1973, Pioneer 10 and 11 are both billions of miles from Earth as they zoom out of the solar system in opposite directions.

As of Feb. 6, Pioneer 10 was about 92.12 astronomical units (AU) from the Sun and headed towards the constellation Taurus.

One AU is the distance between the Earth and Sun, or about 93 million miles (150 million kilometers).

Researchers first noticed the Pioneer Anomaly as a navigation discrepancy while bouncing microwaves off each Pioneer probe as it moved farther from Earth.

They found an unexpected drift in each probe's Doppler frequency, albeit one so small that more advanced three-axis stabilized probes, such as NASA's Voyager spacecraft — also headed out of the solar system — may have drowned it out with their in-flight activities.

[The anomaly is not clearly evident with the Voyager craft, which continually fine-tune their orientations relative to Earth.]

The Doppler effect is the shortening or lengthening of waves, such as the pitch change of an ambulance as it approaches, races past, then heads away from you.

"We had a fitting model, and it had all the effects in it that would influence the spacecraft out in interstellar space, except that it didn't work," said John Anderson, a retired JPL researcher who first discovered the Pioneer Anomaly. "And all we had to do to make it work was to add a constant acceleration towards the Sun."

The discrepancy found that Pioneer 10 and 11 were each about 240,000 miles (400,000 kilometers) closer to the Sun than they should be, according to the current understanding of gravity.

Isaac Newton described gravity as a force that weakens with distance, and the Pioneer probes are speeding out of the solar system at about 30,000 miles (48,280 kilometers) per hour.

"The Pioneer spacecraft conducted the largest-ever gravitational experiment that humanity attempted to test Newton's Law, and it failed," Turyshev told SPACE.com. "If we will identify an anomaly due to conventional physics — thermal mechanism or propulsion or a combination thereof — that's a major event."

Finding a physical source will not only prove Newton right, but also allow engineers to cancel out the Pioneer Anomaly on future spacecraft to make them more stable, added Turyshev, who said that he is striving to remain unbiased about the anomaly's cause.

Researchers want to determine whether heat from the Pioneer probes' electronics or two nuclear power sources — known as radioisotope thermal generators (RTGs) — could be emitting infrared photons that then smack into the spacecraft's dish-like main antenna, causing a recoil effect that Turyshev likened to sunlight striking a solar sail.

Analysis and modeling of how the Pioneer 10 spacecraft emits heat from various sources, including its RTG, found that they account for between 55 percent and 75 percent of Pioneer Anomaly, said Gary Kinsella, a group supervisor for spacecraft thermal engineering and flight operations at JPL.

"We're really encouraged by the preliminary results and we think we're going down the right track," Kinsella said during the Monday discussion.

Edward Belbruno, a former JPL researcher and gravitational trajectory expert at Princeton University who also served on the panel but is unconnected with the anomaly research, said that another possible explanation for the Pioneer Anomaly rests in the mass of the Milky Way galaxy, which — when taken into account — yielded the exact acceleration change for Pioneer 10 as that observed.

While he found that the technique did not yield a specific direction for the acceleration, it may shed some insight into the anomaly and warrants further study, Belbruno added.

Recovered data

During their first Pioneer Anomaly analysis, researchers relied on data that spanned about 11.5 years of Pioneer 10's mission, though they only had about four years worth for Pioneer 11.

After an exhaustive search sponsored by the Planetary Society, Turyshev and his team recovered complete telemetry data sets for both Pioneer probes, as well as about 30 years of data for Pioneer 10 and a 20-year set for Pioneer 11.

Much of the data sat inert, recorded on about 400 magnetic tapes in deep storage at JPL.

Altogether, it included almost 40 gigabytes of Pioneer 10 and 11 mission data, or about the equivalent of a half hour of high-definition television (HDTV) programming from your local cable TV provider.

Transferring the data from 9-track magnetic tapes to a modern digital format, and screening it to reduce artifacts and other corrupted material, has proven time-consuming for Pioneer Anomaly researchers.

But Turyshev remains confident that once the information is ready for analysis, the anomaly shed new secrets.

He is also keeping a close watch on NASA's New Horizons probe, which may one day show signs of the anomaly as it heads out beyond Pluto's orbit after 2015, but only if the mystery force is found to be an actual effect.

"We are truly in a unique situation now with the recovery of the new data assets," Turyshev said. "Once this data set is analyzed, let's talk then."

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