The Flyby Mystery

4 October 2015

Artist's concept of Cassini's Sept. 24, 2010, flyby of Saturn's moon Titan. NASA/JPL-Caltech
Artist’s concept of Cassini’s Sept. 24, 2010, flyby of Saturn’s moon Titan.

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. V. Guruprasad
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.1 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.

  1. Guruprasad, V. “Observational evidence for travelling wave modes bearing distance proportional shifts.” Europhysics Letters 110.5 (2015): 54001. ↩︎