The Flyby Mystery

In Astronomy by Brian Koberlein3 Comments

Everyone loves a good mystery. For space scientists, one of the more perplexing ones is known as the flyby anomaly.

The anomaly was first observed in the flyby of Galileo in 1990, when careful measurements of its motion found the spacecraft’s speed after the flyby was 4 mm/s faster than expected. On Galileo’s second flyby in 1992, no anomaly was observed. In 1998 NEAR gained 13 mm/s. Cassini had no anomaly in 1999. Rosetta had an anomaly in its 2005 flyby, but not in subsequent ones, and Messenger had no anomaly. There have been other anomalies in the motion of spacecraft, with the most famous one being the Pioneer anomaly. These anomalies are extremely tiny, so they can be notoriously difficult to pin down. In the case of the Pioneer anomaly it was due to a tiny push from the heat of the power source. But small anomalies can point towards revolutionary discoveries, so there’s a lot of interest in explaining them.

Delay residuals for flyby data.

Delay residuals for flyby data.

There seems to be a slight correlation to the orientation of a flyby relative to the Earth’s equator and the appearance of the anomaly. This may be an indication that the anomaly is somehow related to the rotation of the Earth, though studies of relativistic frame dragging on the probes show that effect isn’t enough to account for the anomaly. But it’s the on-again off-again nature of the anomaly that’s particularly strange. If it were due to some internal effect as with Pioneer, we would expect it to be more consistent.

Lots of solutions have been proposed for the anomaly, from modified gravity to a halo of dark matter around Earth, but a new proposal is that it’s caused by “chirps” in the radio signals. A chirp is a quickly rising or lowering of frequency from a radio signal. They are sometimes caused by lightning strikes on Earth, but can also be caused by the Doppler effect as a satellite approaches Earth. In this new work the author notes that these chirps could cause a distance-based delay in the timing signals, thus causing an anomaly. If the idea is right, then a continuous observation of a timing signal should produce no anomaly.

The paper hasn’t been peer reviewed, but it’s possible that what looks like an anomaly isn’t one after all.

Paper: V. Guruprasad. Observational evidence for travelling wave modes bearing distance proportional shifts. EPL 110 54001 doi:10.1209/0295-5075/110/54001 (2015)


  1. * Should not be the Delta-V of a flyby determined by spacecraft velocities when it is at large distances, thus not depending on tracking the velocities at perigee? The article seems to claim otherwise (at its “Explanation of the flyby anomaly” part).
    * Should not the SSN vs. DSN residuals (at “Indication of the excess delay in DSN data” part) remain greater, since being determined from DSN when the spacecraft was at a greater distance?

  2. Brian, do you know if the same sort of Pioneer ‘thermal’ cause has been exhaustively investigated for these flyby anomalies? Given the active slewing and maneuvering the spacecraft were subject to, between flybys, it may be difficult to pin this effect down.

    Also, what about the effects of micrometeorite impacts? These turned out to be significant, for HIPPARCOS, at least in terms of where it was pointing …

  3. 1. Please don’t say European Physics Journal publishes without peer-review… I submitted to it after seeing John D Anderson et al’s papers in EPL ( DOI: 10.1209/0295-5075/110/10002 ) in the April issue. 🙂
    2. The occurrence of chirp is not a conjecture – the uplink telemetry carrier is already a chirp during acceleration because its frequency is changing – it’s the Doppler rate of the carrier. That is the only cause of the change of frequencies in the carrier and in the modulated side bands envisaged in the paper – point I’m trying to clarify is that the frequency rise or fall need not be “quickly”.
    (please keep this private… thanks)

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