ESA spacecraft may help unravel cosmic mystery

ESA spacecraft may help unravel cosmic mystery - 12 November 2009 - When Europe's comet chaser Rosetta swings by Earth tomorrow for a critical gravity assist, tracking data will be collected to precisely measure the satellite's change in orbital energy. The results could help unravel a cosmic mystery that has stumped scientists for two decades. Since 1990, scientists and mission controllers at ESA and NASA have noticed that their spacecraft sometimes experience a strange variation in the amount of orbital energy they exchange with Earth during planetary swingbys. The unexplained variation is noticed as a tiny difference in speed gained or lost during the swingby when comparing that predicted by fundamental physics and that actually measured after the event. Tiny unexplained speed variations - The unexplained speed variations are extremely small: NASA's Galileo satellite experienced an increase of just 3.9 mm/s above the expected value when it swung past Earth in December 1990. The largest unexpected variation - a boost of 13.0 mm/s - was observed with NASA's NEAR spacecraft at its Earth swingby in January 1998. On the other hand, variations seen at the swingbys of NASA's Cassini in 1999 and Messenger in 2005 were so small that they lay within the bounds of uncertainty.