The lack of any flyby anomaly effect when the Rosetta spacecraft passed Earth in November 2009 is what, an anomaly? No. Anomalies arise when there is a mismatch between a predicted and an observed value. When it happens our first thought shouldn’t be that OMG there’s something wrong with physics! We should probably start by reviewing whether we really got the math right.
The flyby anomaly story starts with the Galileo spacecraft‘s flyby of Earth in December 1990 – where it was measured to have gained a speed increase (at least, an increase over the predicted value) of 2.5 millimeters per second at perigee. In its second pass in December 1992, the predicted value was the same as the observed value, although it has been suggested that atmospheric drag effects confound any analysis of this particular flyby.
The next, and biggest anomaly so far detected, was the NEAR spacecraft‘s flyby in 1998 (a whopping 7.2 millimeters per second at perigee increase over the predicted value). After that you have Rosetta showing an anomaly on its first flyby in 2005. Then a quantitative formula which aimed to model the various flybys to date was developed by Anderson et al in 2007 – predicting a small but detectable speed increase would be found in Rosetta’s second fly-by of 13 November 2007. However (or should I say anomalously), no such increase was detected in this, or in Rosetta’s third (2009), pass.
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So, on balance, our spacecraft (and often the same spacecraft) are more likely to behave as predicted than to behave anomalously. This reduces (though not negates) the likelihood of the anomaly being anything of substance. One might sagely state that the intermittent absence of an anomaly is not in itself anomalous.
More recently, Mbelek in 2009 has proposed that the anomalous flyby data (including Anderson et al’s formula) can be explained by a more rigorous application of special relativity principles, concluding that ‘spacecraft flybys of heavenly bodies may be viewed as a new test of SR which has proven to be successful near the Earth’. If such recalculated predicted values match observed values in future flybys, that would seem to be that.
Then there’s the Pioneer anomaly. This has no obvious connection with the flyby anomaly, apart from a common use of the word anomaly, which gives us another epistemological maxim – two unrelated anomalies do not one bigger anomaly make.
Between around 20 and 70 AU out from Earth, Pioneer 10 and 11 both showed tiny but unexpected decelerations of around 0.8 nanometers per second2 – although again we are just talking about an observed value that differed from a predicted value.
Some key variables not considered in calculating the original predicted value are radiation pressure from sunlight-heated surfaces, as well as internal radiation generated from the spacecrafts’ own (RTG) power source. A Planetary Society update of an ongoing review of the Pioneer data indicated that revised predicted values now show less discrepancy from the observed values. Again, this doesn’t yet negate the anomaly – but given the trend for more scrutiny equals less discrepancy, it’s fair to say that this anomaly is also becoming less substantial.
Don’t get me wrong, this is all very useful science, teaching us more about how our spacecraft operate out there in the field. I am just suggesting that when faced with a data anomaly perhaps our first reaction should be Doh! rather than OMG!