© NASAArtist’s conception of Juno coming near Earth on a planned flyby Oct. 9, 2013.
"We detected the flyby anomaly during Rosetta's first Earth visit in March 2005," stated Trevor Morley, a flight dynamics specialist at the European Space Agency's European Space Operations Centre in Darmstadt, Germany.
"Frustratingly, no anomaly was seen during Rosetta's subsequent Earth flybys in 2007 and 2011. This is a real cosmic mystery that no one has yet figured out."
The phenomenon has been noticed in several spacecraft (both from ESA and NASA) since 1990. NASA's NEAR asteroid spacecraft in January 1998 had the largest change, of 13 millimeters (0.5 inches) a second. The smallest variations, with NASA's Saturn-bound Cassini in 1999 and Mercury-pointing MESSENGER in 2005, were below the threshold of measurement.
ESA won't even speculate on what's going on. "The experts are stumped," the agency says in a press release.
Those experts, however, do have some ideas on how to track that down. ESOC plans to watch Juno's flyby using a 35 meter deep-space dish in Malargüe, Argentina, as well as a 15-meter dish in Perth, Australia
"The stations will record highly precise radio-signal information that will indicate whether Juno speeds up or slows down more or less than predicted by current theories," ESA states.
What do you think is going on? Let us know in the comments!
...via the Van Allen belts, or some other electrical interaction with plasma around the Earth? Or: a supermassive, cloaked spaceship close by Earth with a large gravity well? Or said aliens 'abducting' the spacecraft hyperdimensionally, checking it out, then putting it back in place with almost no time passing? I got nothin'...