Flyby anomaly
The flyby anomaly is a discrepancy between current scientific models and the actual increase in speed observed during a planetary flyby by a spacecraft. In multiple cases, spacecraft have been observed to gain greater speed than scientists had predicted, but thus far no convincing explanation has been found. This anomaly has been observed as shifts in the S-band and X-band Doppler and ranging telemetry. The largest discrepancy noticed during a flyby has been 13 mm/s.
Observations
are valuable techniques for Solar System exploration. Because the success of such flyby maneuvers depends on the exact geometry of the trajectory, the position and velocity of a spacecraft during its encounter with a planet is continually tracked with great precision by the Deep Space Network.The flyby anomaly was first noticed during a careful inspection of DSN Doppler data shortly after the Earth flyby of the Galileo spacecraft on 8 December 1990. While the Doppler residuals were expected to remain flat, the analysis revealed an unexpected 66 mHz shift, which corresponds to a velocity increase of 3.92 mm/s at perigee. Investigations of this effect at the Jet Propulsion Laboratory, the Goddard Space Flight Center and the University of Texas have not yielded a satisfactory explanation.
No such anomaly was detected after the second Earth flyby of the Galileo spacecraft in December 1992, where the measured velocity decrease matched that expected from atmospheric drag at the lower altitude of 303 km. However, the drag estimates had large error bars, and so an anomalous acceleration could not be ruled out.
On 23 January 1998 the Near Earth Asteroid Rendezvous spacecraft experienced an anomalous velocity increase of 13.46 mm/s after its Earth encounter. Cassini–Huygens gained around 0.11 mm/s in August 1999, and Rosetta gained 1.82 mm/s after its Earth flyby in March 2005.
An analysis of the MESSENGER spacecraft did not reveal any significant unexpected velocity increase. This may be because MESSENGER both approached and departed Earth symmetrically about the equator. This suggests that the anomaly may be related to Earth's rotation.
In November 2009, ESA's Rosetta spacecraft was tracked closely during flyby in order to precisely measure its velocity, in an effort to gather further data about the anomaly, but no significant anomaly was found.
The 2013 flyby of Juno on the way to Jupiter yielded no anomalous acceleration.
In 2018, a careful analysis of the trajectory of the presumed interstellar asteroid ʻOumuamua revealed a small excess velocity as it receded from the Sun. Initial speculation suggested that the anomaly was due to outgassing, though none had been detected.
Summary of some Earth-flyby spacecraft is provided in table below.
| Galileo I | Galileo II | NEAR | Cassini | Rosetta-I | MESSENGER | Rosetta-II | Rosetta-III | Juno | Hayabusa 2 | OSIRIS-REx | BepiColumbo | |
| Date | 1990-12-08 | 1992-12-08 | 1998-01-23 | 1999-08-18 | 2005-03-04 | 2005-08-02 | 2007-11-13 | 2009-11-13 | 2013-10-09 | 2015-12-03 | 2017-09-22 | 2020-04-10 |
| Speed at infinity, km/s | 8.949 | 8.877 | 6.851 | 16.01 | 3.863 | 4.056 | 4.7 | |||||
| Speed at perigee, km/s | 13.738 | 8.877 | 12.739 | 19.03 | 10.517 | 10.389 | 12.49 | 13.34 | 14.93 | 10.3 | 8.5 | |
| Impact parameter, km | 11261 | 12850 | 8973 | 22680.49 | 22319 | 19064 | ||||||
| Minimal altitude, km | 956 | 303 | 532 | 1172 | 1954 | 2336 | 5322 | 2483 | 561 | 3090 | 17237 | 12677 |
| Spacecraft mass, kg | 2497.1 | 2223.0 | 730.40 | 4612.1 | 2895.2 | 1085.6 | 2895 | 2895 | ~2720 | 590 | 4000 | |
| Trajectory inclination to equator, degrees | 142.9 | 138.9 | 108.0 | 25.4 | 144.9 | 133.1 | ||||||
| Deflection angle, degrees | 47.46 | 51.1 | 66.92 | 19.66 | 99.396 | 94.7 | 80 | |||||
| Speed increment at infinity, mm/s | 3.92±0.08 | −4.60±1.00 | 13.46±0.13 | −2±1 | 1.82±0.05 | 0.02±0.01 | ~0 | ~0 | 0±0.8 | ? | ? | ? |
| Speed increment at perigee, mm/s | 2.560±0.050 | -9.200±0.600 | 7.210±0.0700 | −1.700±0.9000 | 0.670±0.0200 | 0.008±0.004 | ~0.000±0.000 | −0.004±0.044 | ? | ? | ? | |
| Gained energy, J/kg | 35.1±0.7 | 92.2±0.9 | 7.03±0.19 | ? | ? | ? |
Anderson's empirical relation
An empirical equation for the anomalous flyby velocity change was proposed in 2008 by J. D. Anderson et al.:where ωE is the angular frequency of the Earth, RE is the Earth radius, and φi and φo are the inbound and outbound equatorial angles of the spacecraft. This formula was derived later by Jean Paul Mbelek from special relativity, leading to one of the possible explanations of the effect. This does not, however, consider the SSN residuals – see "Possible explanations" below.
Possible explanations
There have been a number of proposed explanations of the flyby anomaly, including:- It has been postulated that the Flyby Anomaly is a consequence of the assumption that the speed of light is isotropic in all frames, and invariant in the method used to measure the velocity of the space probes by means of the Doppler Effect. The inconsistent anomalous values measured: positive, null or negative are simply explained relaxing this assumption. During flyby maneuvers the velocity components of the probe in the direction of the observer Vo are derived from the relative displacement df of the radiofrequency f transmitted by the probe, multiplied by the local speed of the light c' by the Doppler effect: Vo = c'. According to the Céspedes-Curé hypothesis, the movement through variable gravitational energy density fields produces slight variations of the refractive index n' of space and therefore of the speed of light c' which leads to unaccounted corrections of the Doppler data that are based on an invariant c. This leads to incorrect estimates of the speed or energy change in the flyby maneuver on the Earth’s frame of reference.
- Unaccounted transverse Doppler effect—i.e. the redshift of light source with zero radial and non-zero tangential velocity. However, this cannot explain the similar anomaly in the ranging data.
- A dark-matter halo around Earth.
- A modification of inertia resulting from a Hubble-scale Casimir effect, related to the Unruh effect.
- The impact of general relativity, in its weak-field and linearized form yielding gravitoelectric and gravitomagnetic phenomena like frame-dragging, has been investigated as well: it turns out to be unable to account for the flyby anomaly.
- The classical time-retarded gravity explanation proposed by Joseph C. Hafele.
- Range-proportional excess delay of the telemetry signal revealed by the United States Space Surveillance Network range data in the NEAR flyby. This delay, accounting for the anomaly in both Doppler and range data, as well as the trailing Doppler oscillations, to within 10–20%, points to chirp modes in the reception due to the Doppler rate, predicting a positive anomaly only when the tracking by DSN is interrupted around perigee, and zero or negative anomaly if tracked continuously. No anomaly should occur in Doppler tracked by non-DSN stations.
- The action of a topological torsion current predicting flyby anomalies in retrograde direction, but null-effect when spacecrafts approach the planet in posigrade direction with respect to the planetary sense of rotation.
- The analysis of the Juno flyby looked at analysis errors that could potentially mimic the flyby anomaly. They found that a high-precision gravity field of at least 50x50 coefficients was needed for accurate flyby predictions. Use of a lower-precision gravity field, would yield a 4.5 mm/s velocity error.
Future research
Literature
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