# Is the Flyby Anomaly still a thing?

The Flyby Anomaly is a name given to an unexplained, unexpected difference between the best theoretical calculation of a change in several different spacecraft's velocities due to flyby (Gravity Assist) maneuvers, and the best analysis of the measured changes.

While it's summarized in the linked Wikipedia article, and a long list of possible explanations is given, is there at least some movement towards a consensus on the source of the discrepancy? Or has it been (mostly) explained and the Wikipedia article needs to be updated?

Or is this still, in 2017, a credible, outstanding unexplained phenomenon in spaceflight? Are there any plans to try to detect or reproduce this anomaly with future space missions that include flybys?

Below: From The Flyby Anomaly: An Investigation into Potential Causes. Note the anomalous difference $\Delta$ is much larger than the estimated error $\sigma$ for flyby's before about 2005.

• For those wondering what you're talking about: en.wikipedia.org/wiki/Flyby_anomaly – Hobbes Nov 23 '17 at 14:14
• This anomaly was very small, for instance 2.56 mm/s difference at a speed of 13.738 km/s, that is only 0.19 parts per million or 186 parts per billion. The measured doppler shift was much less than one Hertz, only 66 mHz. – Uwe Nov 23 '17 at 16:28
• How can mm/s compared to km/s be considered significant? It seems to me that a model with that much error isn’t too shaby. Why all the fuss over so little an error? – Paul Nov 24 '17 at 6:52
• The model we're talking about here is basic physics. The error seen here means we don't have a complete understanding of gravity, which is a big deal. The error won't have operational consequences for spacecraft operation, but it might lead to a new discovery in fundamental physics. – Hobbes Nov 24 '17 at 7:46
• @Uwe remember that it's the product of $G$ times $m$ that affects the spacecrafts trajectory, the standard gravitational parameter. According to this excellent answer the uncertainty in Earth's is only about $4 \times 10^{-10}$ or 0.4 parts per billion. This is thanks in part to the LAGEOS satellites; space.stackexchange.com/q/22999/12102 Next time consider asking a new SE question. This way future readers can learn from it as well. – uhoh Nov 24 '17 at 11:32

Yes, the Flyby Anomaly is still an unsolved problem. Studies have found several candidates for the cause of the anomaly.

A recent study examined Juno's behavior on its Jupiter orbits, and found the anomaly there as well:

“Our conclusion is that an anomalous acceleration is also acting upon the Juno spacecraft in the vicinity of the perijove (in this case, the asymptotic velocity is not a useful concept because the trajectory is closed). This acceleration is almost one hundred times larger than the typical anomalous accelerations responsible for the anomaly in the case of the Earth flybys. This was already expected in connection with Anderson et al.’s initial intuition that the effect increases with the angular rotational velocity of the planet (a period of 9.8 hours for Jupiter vs the 24 hours of the Earth), the radius of the planet and probably its mass.”

Some possible origins of the anomaly have been discarded:

It is important to emphasize that this anomaly is also observed in the ranging data and cannot be attributed to a conventional or unconventional issue related entirely to the Doppler tracking. A primary evaluation of the possible conventional physical effects with could be contributing to the anomaly was carried out by Lämmerzahl et al. [31]. Ocean tides and a coupling of the spacecraft to the tesseral harmonic terms in the geopotential model have also recently been studied 2. Atmospheric friction can also be dismissed except for flybys at altitudes of 300 km or lower [4]. The same can be said of the corrections corresponding to General Relativity [28, 26], thermal effects [41] or other [14].

But the study found no conclusive cause for the anomaly:

Summarizing, we can say that in this paper: (i) We have found evidence that an anomaly could be operating also during the Juno flybys of Jupiter (ii) We have developed a theoretical model to compare with the orbital model fitted to telemetry data in order to disclose the form of the possible anomalous acceleration field acting upon the spacecraft. A significant radial component was found and this decays with the distance to the center of Jupiter as expected from an unknown physical interaction. (iii) The anomaly shows an asymmetry among the incoming and outgoing branches of the trajectory and this could be suggestive of a non-conservative interaction. The confirmation of these conclusions would require further independent analysis and we hope that our work will stimulate future research in this and other planetary flybys.

Another paper studied Juno's Earth flyby, and found that inaccuracies of the geopotential model (Earth's gravity field) used to model the flyby could account for the flyby anomaly:

The only perturbation that we examined that was found to be capable of producing something detectable in real-time and comparable to the predicted flyby velocity anomaly was truncation in the Earth’s geopotential model. Previously, we had found a 10x10 degree/order field sufficient for supporting launch operations. The uncertainties in the trajectory during that phase were large enough as to not require a higher fidelity model. However, for the EGA flyby we found that the perturbation caused by including the additional effect of the higher degree and order terms was as large as 4.5 mm/s when using a 50x50 field.

Another paper concluded that solar radiation pressure is another candidate:

Solar radiation pressure presented more interesting results: small changes in the solar radiation pressure coefficient CR could explain the anomaly. In more detail, uncertainties in the values of the reflectivity coefficients of the materials that cover the spacecraft could be held responsible for the anomalous velocity change observed. Actually, we proved that modifications of the specular and diffuse reflectivity coefficients by an amount ranging from 10−4 to 4*10−2 are sufficient to successfully eliminate the unexpected velocity shift at perigee.

• What about variations in the Earth's density? astronomy.stackexchange.com/questions/16591 – samerivertwice Aug 5 '18 at 8:56
• Those fall under "inaccuracies of the geopotential model". – Hobbes Aug 5 '18 at 10:04
• Indeed. My question proposes an enhancement to that model, whose impact could be calculated to measure whether the magnitude of the anomaly is well-fitted by this possibility. – samerivertwice Aug 5 '18 at 10:28
• Would a mascon on Jupiter cause more acceleration than a mascon of the same scale on Earth? – Magic Octopus Urn Jul 9 '19 at 14:01
• that would depend mostly on the density of the mascon relative to its surroundings. It's difficult to imagine a gas giant having mascons, though - the gas layers can only get small variations in density. – Hobbes Jul 9 '19 at 15:05