This is a captivating mystery. In 1990 and 1992 when the Jupiter probe Galileo used the Earth for gravitational assists (or “slingshots”), ground-based observers noticed a small (unexpected) boost in velocity as the spacecraft approached Earth. A boost in a few millimetres per second had also been observed in the slingshot of NASA’s NEAR probe two years previously. The same was seen in the flybys of Cassini (in 1999), MESSENGER and Rosetta (in 2005). Many explanations have been put forward – including my favourite that it could be dark matter in Earth orbit kicking our robotic explorers around – but flyby anomalies may have a more mundane explanation.
In keeping with Occam’s Razor (i.e. the simplest explanation is usually the right one), a short paper has been published suggesting that flyby anomalies can be accounted for by using conventional physics…
In my first month of writing for the Universe Today, I was engrossed with the mystery of flyby anomalies. At the time, I erroneously thought flyby anomalies and the “Pioneer effect” were one of the same thing, but they are totally different, operating over different scales. Flyby anomalies are the very fast but small boosts in spacecraft velocity, whereas the Pioneer effect is a long-term deviation observed in spacecraft trajectories (i.e. the 1970’s Pioneer and Voyager missions).
By “small boosts” in flyby velocities, we are talking about a few millimetres per second; the largest being a 13.5mm/s boost for the NEAR mission to the asteroid Eros as the craft flew past the Earth in 1998. The Pioneer effect is a deviation of hundreds of thousands of kilometres after a couple of decades of space flight (now this is a true Solar System mystery in my opinion). Obviously two very different creatures.
Although the Pioneer effect will probably remain a mystery for some time to come, flyby anomalies may not remain in the “mysteries” folder for much longer. Although a few exotic (dark matter) and complex (Unruh radiation) explanations have been investigated, it might just be an overlooked artefact of the Doppler effect and the spin of the Earth.
To measure flyby anomalies, communication signals between the ground-based antennae of the Deep Space Network (DSN) and the passing spacecraft are analysed. The photons transmitted are subject to a shift in frequency due to the relative motion of the craft and spin of the Earth. Basically, the explanation put forward by Jean Paul Mbelek at the Service d’Astrophysique, France, is that flyby anomalies are actually a consequence of an overlooked effect of classical Special Relativity (SR). Considering the transverse Doppler effect of the Earth’s spin and the incoming velocity of the craft, by using well known SR equations, the flyby anomaly for orbiting spacecraft gets ironed out.
It’s a bit of an anticlimax, I was hoping dark matter might have been to blame…
So could this be “case closed” for flyby anomalies? Have a read of the paper yourself and decide…
[Update: Nancy over at the Universe Today has also been following this story…]
Sources: arXiv Blog, arXiv
astroengine.com 
That is so cool.
It simply amazes me that SR is still providing simple solutions for “unexplained” effects over 100 years after it was first proposed.
What really strikes me about that paper is it’s brevity. That was a characteristic of Einstein’s first paper and also pretty typical of SR explanations for phenomena.
The dark matter explanation in this case is exceedingly contorted (in my opinion) and makes all sorts of exotic assumptions — exotic even by dark matter standards. That says a lot, given the fact that dark matter may not even exist!
Invoking the dark matter explanation in this case kind of reminds me of the contorted explanations (eg, involving sunspots and cosmic rays) that some people invoke to explain the warming of the earth’s surface in recent decades.
Simple explanations (eg, the greenhouse effect) often are the correct ones.
I have a feeling (and it is just a hunch) that the physics community as a whole may have gone too far afield of such simple explanations. The whole string theory “landscape” strikes me as one such case.
As Mr O’Neil says the simple answer is often an anticlimax, and as humans it is in our nature to be looking for something new and exciting. Similarly ‘intelligent’ people are often offended by simple solutions as they can be a waste of an opportunity to sound clever!
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funny
I did not agree with Mbelek paper. DSN station are not static with respect to the GCRS during a telemetry session. First of all, read the book of Moyer (more than 300 pages of relativistic modelling…) and you will be sure that the Earth is rotating in the model. Henceforth, this transverse doppler shift (and also range) is already included in the JPL ODP software.
I did not agree with Mbelek paper. DSN station are not static with respect to the GCRS during a telemetry session. First of all, read the book of Moyer (more than 300 pages of relativistic modelling…) and you will be sure that the Earth is rotating in the model. Henceforth, this transverse doppler shift (and also range) is already included in the JPL ODP software.