A new study by researchers from the Jet Propulsion Laboratory shows that spacecraft that swing by the Earth are subject to a small but unexplained increase in their velocity. The Flyby Anomaly is in some ways similar to that other cosmic enigma – the Pioneer Anomaly, which seems to be affecting Pioneer 10 and 11 as they speed out of the solar system. (Image Credit: Gravity Probe B Image Archive)
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A new study by researchers from the Jet Propulsion Laboratory shows that spacecraft that swing by the Earth are subject to a small but unexplained increase in their velocity. Is an unknown physical force at work, or something far more mundane? As in the case of the Pioneer anomaly, all investigators know so far is that a great deal more research is needed. But they have already given this new mysterious phenomenon a name: "the flyby anomaly."
The first indication that something unusual was going on came on December 8, 1990, as the spacecraft Galileo swung by Earth on its meandering road to Jupiter. As the spacecraft flew by, engineers at the Jet Propulsion Laboratory (JPL) in Pasadena carefully tracked its Doppler signal to determine its speed and trajectory. But something didn't fit: as the spacecraft was moving away from Earth, it was traveling ever so slightly faster than calculations said it should be. The deviation was small, only about one millionth the velocity of the spacecraft, but it was clearly detectable. It was as if a mysterious force had given Galileo a miniscule push as it was swinging by its home planet.
What was going on? Did the JPL engineers misinterpret the data they received from the spacecraft? Was Galileo affected by a mysterious cosmic force unknown to science? Or was it something else entirely? As of now no one knows the answer.
JPL engineers have pondered the anomaly from the beginning and what they have learned is the result of 18 years of work and data analysis. Skeptical by nature and training, the space engineers initially doubted whether this apparent inconsistency in the Galileo data was real. It is far more likely, they reasoned, that the anomaly was an artifact of their own tracking instruments rather than a shift in the spacecraft's actual velocity. Over the next several years, John D. Anderson, along with James K. Campbell and James F. Jordan of JPL looked hard at the tracking procedures and equipment, searching for a possible cause that could explain away the speed discrepancy. They found nothing: the Galileo flyby anomaly remained stubbornly apparent in the tracking data.
Two years after its first visit, Galileo flew by Earth one more time for a gravity assist on its way to Jupiter, and Anderson and his colleagues were eager to see whether the mysterious effect repeated itself. This time, however, Galileo passed much closer to Earth, a mere 300 kilometers above the Earth's surface. In such a close flyby, the astrophysicists found the significant effects of atmospheric drag on the spacecraft drown out any possibility of detecting the miniscule velocity shift of the flyby anomaly.
But in the following years four other spacecraft swung by Earth on their way to their celestial destinations, giving engineers a chance to see if the so-called "flyby effect" reappeared. First came NEAR, which visited Earth in January 1998, and then Cassini in August of 1999. Rosetta, the European asteroid chaser visited in March 2005, followed by MESSENGER in August of the same year.
Surveying the information Anderson and his colleagues found that the Cassini data was useless, because the spacecraft was using its navigational thrusters at the time of its closest approach to Earth, and the MESSENGER data showed no unexpected change in velocity during the flyby. But the Rosetta data did indicate an anomaly similar to the one detected 15 years earlier in Galileo, and the spacecraft's European controllers confirmed that they too were seeing the effect. The data from NEAR was most striking of all, providing the engineers with the clearest example of the anomaly. In addition to the Doppler measurements, the spacecraft's velocity change was conformed by independent "ranging" data, which measure the time it takes for a signal from Earth to be transmitted back from the spacecraft.
All this led Anderson and his colleagues to conclude that the flyby anomaly was not a fluke related to the unique conditions of the Galileo spacecraft and its trajectory, but a consistent effect influencing the speed of spacecraft flying by our planet. Suggestions that it was caused by General Relativity's "frame dragging" (known as the "Lense-Thirring effect") led nowhere, when Anderson's calculations showed that the actual velocity change was too large to be explained by this phenomenon. But if the flyby anomaly was real, as data suggested, and if General Relativity had nothing to do with it, then what? What is the cause of the flyby anomaly?
Nobody knew, and it was at this point that Anderson and his colleagues decided to go public with an article. Talk about the flyby anomaly, Anderson recounted, "had been floating around JPL for years, and no one was able to explain it." Now, with the accumulation of data from several different spacecraft, Anderson said, the anomaly could no longer be dismissed. "It was time," he said, "to tell people that there was a problem with earth flybys." If the engineers at JPL couldn’t' explain the effect, perhaps the broader scientific community could come up with an explanation.
With this goal in mind, Anderson, Campbell, and Jordan, along with John E. Ekelund and Jordan Ellis, spent 18 months closely analyzing the data from all Earth flyby's. By the time they were done they had come up with a formula that accurately predicted the size of the anomaly based on the spacecraft's flight path. The extent to which the velocity of a spacecraft deviates from its expected value during a flyby, they found, depends of the difference in latitude (or "declination") between the spacecraft's incoming and outgoing trajectories. The greater the difference in latitude, the greater the anomalous velocity shift after the flyby.
The spacecraft NEAR, for example, approached Earth from a near-equatorial latitude, but left close to a polar latitude. According to the formula, this large difference between the two should result in a substantial flyby anomaly, and this was indeed the case. The NEAR flyby became most clear-cut case-study for the mysterious effect. MESSENGER, in contrast, approached and departed along nearly the same latitude, which according to the formula should result in a miniscule effect. And indeed, no flyby effect was detected in the MESSENGER data.
But as Anderson points out, coming up with a mathematical formula that can predict an effect is very different from having a physical explanation for it. Could it be some as yet undiscovered physical force, or something known as "dark energy" at work? Such revolutions in physics, Anderson mused, don't happen very often, but in the absence of a better explanation such radical hypotheses cannot be ruled out. "The formula doesn't suggest anything to us" he readily admitted, but perhaps some physicists will be able to come up with an explanation.
As more food for thought, Anderson points out that the flyby anomaly is in some ways similar to that other cosmic enigma – the Pioneer anomaly, which seems to be affecting Pioneer 10 and 11 as they speed out of the solar system. There is, he said an important difference between the two, because the flyby anomaly affects spacecraft's velocity, whereas in the Pioneers' case it is their acceleration that is being affected, slowing them down on their escape path from the solar system. But there is also an important similarity: "The Pioneers," Anderson said, "like all spacecraft swinging by Earth, are on a hyperbolic trajectory." This is a very unusual path, seeing that the vast majority of spacecraft are on parabolic or ellipsoid trajectories. Is there something then about a hyperbolic path that produces such anomalies? As of now, that too is a mystery…
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