RETIRED NASA engineer John Anderson will be watching keenly on 13 November as the European Space Agency鈥檚 flies by Earth for the third and final time on its way to comet .
Such fly-bys give spacecraft a whip-crack of extra speed on their tour through the solar system. By using the gravitational fields of planets or moons, you can save fuel and travel much further through the solar system than would otherwise be possible. But this trick has had an unexpected effect (快猫短视频, 20 September 2008, p 38).
In December 1990, for example, NASA鈥檚 slingshotted around the Earth on its roundabout route to Jupiter. As the probe raced away from Earth, it was travelling 3.9 millimetres per second faster than it should have been, according to NASA鈥檚 calculations. The biggest such discrepancy recorded, in 1998, affected NASA鈥檚 , whose speed was boosted by an additional 13.5 millimetres per second. Rosetta has already had a boost: in 2005 it sped up by about 1.8 millimetres per second more than expected as it slingshotted around Earth.
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Nothing in known physics predicts this acceleration. Enter Anderson: he put all the data together and came up with an empirical formula that relates the probes鈥� incoming and outgoing trajectory angles and Earth鈥檚 rotational velocity to the extra acceleration experienced by the spacecraft as they swing by us. He found that the smallest anomalies arise when the incoming and outgoing trajectory are symmetrical with respect to Earth鈥檚 equator. In a bizarre twist, though, the formula also involves the speed of light. So what, ultimately, is the physics behind it?
There is no explanation from standard, accepted physics, but a variety of exotic explanations have been proposed. These incorporate dark matter, modifications to relativity, imbalances in the Earth鈥檚 gravitational field or something unknown to do with inertia or the nature of light. 鈥淥f these ideas, the least controversial is that the anomaly has something to do with dark matter bound to the Earth,鈥� says Michael Nieto of Los Alamos National Laboratory, who has been working with Anderson on the anomaly.
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