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Solving NASA's great gravity mystery
Unexplained force appears to be acting against identical Pioneer probes
 NASA An artist's interpretation of NASA's Pioneer 10 probe looking back at the sun as it leaves the solar system. |
NEW YORK - It’s been years since NASA last heard from either of its two Pioneer probes hurtling 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, something exotic like dark matter, or some new facet of physics or gravity, remains in doubt.
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 perhaps something new is at work on the two spacecraft. An answer could 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 they moved farther from Earth. They found an unexpected drift in each probe’s Doppler frequency, one so small that the three-axis stabilized probes like NASA’s Voyager spacecraft—also headed out of the solar system—may have drowned it out with their in-flight activities.
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.
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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.
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