Credit: NASA
Anderson also found that the amount of the fly-by anomaly depended on the angle at which the spacecraft slid by Earth.
Messenger whipped closely around the Equator and had no measurable anomaly. NEAR and, to a lesser extent Rosetta, came in with more of a north/south angle.
"So it appears to us that the Earth fly-by anomaly is related to the spin of the Earth somehow," Anderson says. "How? We don't have an answer for that. It's another mystery."
Turyshev and Antreasian, however, doubt that it's a gravitational effect at all. Most likely, they say, the anomaly is simply an error in the computer code that is used to shift between Earth-bound and space-based coordinate systems.
"That's done quickly in the fly-by, and if you have mismodelling, you will get a velocity-dependent error and latitude-dependent error," Turyshev says.
Meanwhile, it's not likely that either anomaly will cause spacecraft engineers to crash a probe. "We navigate spacecraft really well," says Antreasian. "This kind of discrepancy doesn't affect our accuracy much."
Still, he says, "we don't know everything". He's speaking specifically of the fly-by anomaly, but the message could apply equally well to either discrepancy. "That's the thing. There's always something there to explore and try to understand in more detail."
RICHARD A. LOVETT is a science writer in Portland, Oregon, USA.
