NASA's great gravity mystery
An unexplained force [or field] appears to be acting against identical Pioneer probes - and at least four others
An artist's interpretation of NASA's Pioneer 10 probe looking back at the sun as it leaves the solar system.
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…
“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… during the Seventh Annual Asimov Debate 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.
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 “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...
Five spacecraft experience unexpected anomalies in their motions
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U.S. researchers are now collaborating with German colleagues to search for possible anomalies in the Rosetta probe’s second flyby of the Earth on November 13.
Mysteriously, four spacecraft that flew past Earth have each displayed unexpected anomalies in their motions.
These newfound enigmas join the so-called "Pioneer anomaly" as hints that unexplained forces may appear to act on spacecraft.
A decade ago, after rigorous analyses, anomalies were seen with the identical Pioneer 10 and 11 spacecraft as they hurtled out of the solar system. Both seemed to experience a tiny but unexplained constant acceleration toward the sun.
A host of explanations have been bandied about for the Pioneer anomaly. At times these are rooted in conventional science — perhaps leaks from the spacecraft have affected their trajectories. At times these are rooted in more speculative physics — maybe the law of gravity itself needs to be modified.
Now Jet Propulsion Laboratory astronomer John Anderson and his colleagues — who originally helped uncover the Pioneer anomaly — have discovered that five spacecraft each raced either a tiny bit faster or slower than expected when they flew past Earth en route to other parts of the solar system.
‘Humble and perplexed’
The researchers looked at five deep-space probes — Galileo to Jupiter, the NEAR mission to the asteroid Eros, the Rosetta probe to a comet, Cassini to Saturn and the Messenger craft to Mercury. Each spacecraft flew past our planet to either gain or lose orbital energy in their quests to reach their eventual targets. (Galileo made two flybys.)
The scientists confirmed anomalies in five of the six flybys, involving four of the five spacecraft.
"I am feeling both humble and perplexed by this," said Anderson, who is now working on the anomaly as a retiree. "There is something very strange going on with spacecraft motions. We have no convincing explanation for either the Pioneer anomaly or the flyby anomaly."
In the one case where researchers did not confirm a noticeable anomaly, Messenger's flyby, the spacecraft approached Earth at about latitude 31 degrees north and receded from Earth at about latitude 32 degrees south. "This near-perfect symmetry about the equator seemed to result in a very small velocity change, in contrast to the five other flybys," Anderson explained. The change in velocity was so small no anomaly could be confirmed.
The five other flybys involved flights whose incoming and outgoing trajectories were asymmetrical with each other in terms of their orientation with Earth's equator.
For instance, the NEAR mission approached Earth at about latitude 20 south and receded from the planet at about latitude 72 south. The spacecraft then seemed to fly 13 millimeters (a half-inch) per second faster than expected. While this is just one-millionth of that probe's total velocity, the precision of the velocity measurements was 0.1 millimeters per second, carried out as they were using radio waves bounced off the craft. This suggests the anomaly seen is real — and in need of an explanation.
The fact this effect seems most evident with flybys most asymmetrical with respect to Earth's equator "suggests that the anomaly is related to Earth's rotation," Anderson said.
As to whether these new anomalies are linked with the Pioneer anomaly, "I would be very surprised if we have discovered two independent spacecraft anomalies," Anderson told Space.com. "I suspect they are connected, but I really do not know."
These anomalies might be effects we see with an object possessing a spacecraft's mass, between 660 and 2,200 pounds (300 to 1,000 kilograms), Anderson speculated.
"Another thing in common between the Pioneer and these flybys is what you would call an unbound orbit around a central body," Anderson said. "For instance, the Pioneers are flying out of the solar system — they're not bound to their central body, the Sun. For the other flybys, the Earth is the central body. These kinds of orbits just don't occur very often in nature — it could be when you get into an unbound orbit around a central body, something goes on that's not in our standard models."
The researchers are now collaborating with German colleagues to search for possible anomalies in the Rosetta probe's second flyby of Earth on Nov. 13.
"We should continue to monitor spacecraft during Earth flybys. We should look carefully at newly recovered Pioneer data for more evidence of the Pioneer anomaly," Anderson added. "We should think about launching a dedicated mission on an escape trajectory from the solar system, just to look for anomalies in its motion."
Montana State University physicist Ronald Hellings, who did not participate in this study, said, "There's definitely something going on. Whether that's because of new physics or some problem with the model we have is yet to be worked out, as far as I know. A lot of people are trying to look into this."
Anderson and his colleagues will detail their latest findings in an upcoming issue of the journal Physical Review Letters.
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