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Frame dragging on flybys

The Quick Study on Earth flyby anomalies (PHYSICS TODAY, October 2009, page 76) teases the reader who is unfamiliar with the subject. We’re told of a microscopic, nonconserving change in the speed of a satellite as it flies by Earth. The slight change in kinetic energy may be increasing or decreasing, as if Earth’s rotation were being weakly added to the velocity of the satellite. Up to an altitude of 2000 km, an empirical fit of the data depends on a constant of proportionality equal to twice the product of Earth’s radius and angular velocity divided by the speed of light. What jumps immediately to mind is frame drag—the idea, according to general relativity, that spacetime in the vicinity of a rotating mass will be dragged around as the mass spins. Yet this point is absent from several proposed and seemingly far-fetched explanations. Even if frame drag fails quantitatively or in some omitted detail, it seems intuitive to the uninitiated and should have been addressed, given PHYSICS TODAY’s diverse readership.

Nieto and Anderson reply: The magnitude of the frame-dragging effect would be impossible to detect for satellites that fly by Earth. But Jupiter is another story. Indeed, NASA’s Juno mission, scheduled for launch in 2011, will place a polar orbiter about Jupiter in 2016. The orbiter will approach Jupiter at altitudes ranging from about 4000 to 6000 km every 11 days over about 31 orbital revolutions. The very real possibility that frame dragging will have a measurable effect should be addressed by the Juno gravity team, of which one of us (Anderson) is the team leader.