Letter: Fly-by anomalies

From Andrew Hicks

Eric Solomon asks whether the anomalies described in Marcus Chown’s article (20 September, p 38) might be explained by approximation errors in iterative calculations (18 October, p 21).

Not only is approximation error unlikely to be the explanation for a discrepancy so consistent that it can now be predicted in advance with a degree of optimism, but the effects of such errors can also be easily detected by adjusting the iteration algorithm.

A more likely explanation is that the Earth’s core is spinning faster than assumed in mission planners’ calculations. This may be testable using some form of Doppler seismology.

The editor writes:

•The core spin has been measured using the timing of detection of pairs of earthquakes (3 September 2005, p 15).

Sydney, Australia