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Mar 2008

Earth's rotation accounts for deviations in spacecraft trajectories

     London: The reason behind small and unexpected deviations in robotic spacecraft trajectories near Earth has been attributed to the rotation of our planet. According to a report in New Scientist, scientists 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's gravity. Though the flyby boosted Galileo's speed by several kilometres per second, as expected, radio tracking of the spacecraft found the boost was very slightly larger than expected, by about 4 millimeters 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. 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's Near Earth Asteroid Rendezvous (NEAR) mission. Now, NASA scientist John Anderson and colleagues James Campbell, John Ekelund, Jordan Ellis and Frank Jordan at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, US, have developed a mathematical formula that accurately reproduces all five instances where a flyby anomaly has been observed so far. The formula involves the angle that the spacecraft's incoming and outgoing trajectories make with respect to Earth's equator. It accurately predicts the change in speed observed in the flybys so far. For flybys where the angle is very different for the incoming and outgoing parts of the trajectory - such as the 1998 NEAR flyby - 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. 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's rotation. That is because the angles in the formula are defined with respect to Earth's equator, which is perpendicular to the planet's spin axis. "That suggests to me that the effect is related to the Earth's spin direction, or perhaps to the rotation of the Earth," Anderson told New Scientist.
-Mar 5, 2008