A mysterious phenomenon causing small, unexpected deviations in robotic spacecraft trajectories near Earth may be a step closer to an explanation. 快猫短视频s have come up with a mathematical formula that successfully reproduces all the known occurrences of the effect, and they suspect the Earth鈥檚 rotation may somehow be to blame.
快猫短视频s first noticed something amiss in the motion of spacecraft passing near Earth when the Galileo spacecraft flew by our planet in 1990 to get a boost from the planet鈥檚 gravity. The flyby boosted Galileo鈥檚 speed by several kilometres per second, as expected, but radio tracking of the spacecraft found the boost was very slightly larger than expected, by about 4 millimetres per second.
Although the amount of this extra boost is small, it is more than 10 times larger than can be accounted for by errors in tracking by Earth-based radio dishes and other known influences, such as gas escaping from the spacecraft.
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NASA engineers reported the effect at a conference in 1998, by which time the anomaly had cropped up in an additional Galileo flyby of Earth in 1992, as well as in a 1998 Earth flyby of NASA鈥檚 Near Earth Asteroid Rendezvous (NEAR) mission.
The flyby anomaly received more attention in 2006, when former NASA scientist John Anderson, now retired, and his colleagues, posted a online describing the anomaly and raising the possibility of a link with a similar puzzle called the Pioneer anomaly. The Pioneer anomaly refers to the observation that NASA鈥檚 Pioneer 10 and 11 spacecraft are both slowing down more than expected as they fly away from the Sun in opposite directions.
Now, Anderson and colleagues James Campbell, John Ekelund, Jordan Ellis and Frank Jordan at NASA鈥檚 Jet Propulsion Laboratory (JPL) in Pasadena, US, have stumbled onto a formula that accurately reproduces all five instances where a flyby anomaly has been observed so far.
The formula involves the angle that the spacecraft鈥檚 incoming and outgoing trajectories make with respect to Earth鈥檚 equator. It accurately predicts the change in speed observed in the flybys so far.
Incoming angle
For flybys where the angle is very different for the incoming and outgoing parts of the trajectory 鈥� such as the 1998 NEAR flyby 鈥� both the formula and the observations give a large anomaly.
But for flybys where incoming and outgoing angles are similar, the anomaly predicts the anomaly to be very small, and in some cases too small to be detected. Both the 2005 flyby of NASA鈥檚 Messenger spacecraft en route to Mercury and the 2007 flyby of Europe鈥檚 Rosetta spacecraft on its way to a comet had very similar incoming and outgoing angles, and neither showed a detectable anomaly.
Team member Frank Jordan stumbled on the formula, after trying out many other possibilities that did not work. He says he was playing with different ideas for a formula when it occurred to him to use trajectory angles. 鈥淚t just worked so well, I was amazed,鈥� he told 快猫短视频. 鈥淚 immediately called everyone else [on the team] and people did independent computations鈥� showing the formula worked, he says.
Although the team has been unable to come up with an underlying physical explanation for the anomaly, Anderson thinks the formula suggests some sort of connection to Earth鈥檚 rotation.
First step
That is because the angles in the formula are defined with respect to Earth鈥檚 equator, which is perpendicular to the planet鈥檚 spin axis. 鈥淭hat suggests to me that the effect is related to the Earth鈥檚 spin direction, or perhaps to the rotation of the Earth,鈥� Anderson told 快猫短视频.
But how could Earth鈥檚 rotation influence the trajectories of spacecraft? One possibility that the team investigated comes from Einstein鈥檚 general theory of relativity, which predicts that spinning objects distort the fabric of space around them. This effect could in principle influence the trajectory of objects near a spinning body like Earth, but the team鈥檚 calculations show it would be far too weak an influence to explain the flyby anomalies.
Even though the team has not come up with a physical explanation, having the formula could be an important step towards one, because it gives physicists something to aim for, says physicist Robert Sanders of the University of Groningen in The Netherlands, who was not involved in the study.
鈥楺uite helpful鈥�
鈥淎ny theory of the effect has to aim at reproducing that formula,鈥� he told 快猫短视频. 鈥淪o, I think it鈥檚 quite helpful and it seems to be quite precise.鈥�
Sanders says the anomaly may be a sign that our current theory of gravity, Einstein鈥檚 general theory of relativity, needs to be altered. 鈥淚t suggests that we may know less about gravity than we think we do,鈥� he says. 鈥淚 think people should take it seriously.鈥�
Could the flyby anomaly be connected somehow to the Pioneer anomaly? Although there is no proof of a link, Anderson says he would be surprised if radio tracking of spacecraft had revealed two completely independent anomalies. 鈥淚 suspect they are connected somehow,鈥� he says.
Others have expressed scepticism that any new physics is involved in either anomaly. For example, some scientists suspect that uneven radiation of heat in different directions by the Pioneer spacecraft is giving them a small thrust that has not been accounted for.
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