Observations
Gravitational assists are valuable techniques for Solar System exploration. Because the success of these flyby maneuvers depends on the geometry of the trajectory, the position and velocity of a spacecraft is continually tracked during its encounter with a planet by the Deep Space Network (DSN).
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 (observed minus computed data) 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. An investigation of this effect at the Jet Propulsion Laboratory (JPL), the Goddard Space Flight Center (GSFC) and the University of Texas has not yielded a satisfactory explanation. No anomaly was detected after the second Earth-flyby of the Galileo spacecraft in December 1992, because any possible velocity increase was masked by atmospheric drag of the lower altitude of 303 km.
On 23 January 1998 the Near Earth Asteroid Rendezvous (NEAR) spacecraft experienced an anomalous velocity increase of 13.46 mm/s after its Earth encounter. Cassini–Huygens gained ~0.11 mm/s in August 1999 and Rosetta 1.82 mm/s after its Earth-flyby in March 2005.
An analysis of the MESSENGER spacecraft (studying Mercury) did not reveal any significant unexpected velocity increase. This may be that MESSENGER both approached and departed Earth symmetrically about the equator (see data and proposed equation below). This may suggest that the anomaly is related to Earth's rotation.
Summary of Earth-flyby spacecraft is provided in table below. The Rosetta data is for its first flyby in 2005; the second flyby produced no significant anomalous increase, and the third a negligible decrease.
| Quantity | Galileo I | Galileo II | NEAR | Cassini | Rosetta-I | Messenger | Rosetta-II | Rosetta-III |
|---|---|---|---|---|---|---|---|---|
| Date | 12/8/1990 | 12/12/1992 | 01/23/1998 | 08/18/1999 | 03/04/2005 | 08/02/2005 | 11/13/2007 | 11/13/2009 |
| Speed at infinity, km/s | 8.949 | 8.877 | 6.851 | 16.01 | 3.863 | 4.056 | ||
| Speed at perigee, km/s | 13.738 | --- | 12.739 | 19.03 | 10.517 | 10.389 | 12.49 | 13.34 |
| Impact parameter, km | 11261 | 12850 | 8973 | 22680.49 | 22319 | |||
| Minimal altitude, km | 956 | 303 | 532 | 1172 | 1954 | 2336 | 5322 | 2483 |
| Spacecraft mass, kg | 2497.1 | 730.40 | 4612.1 | 2895.2 | 1085.6 | 2895 | 2895 | |
| Trajectory inclination to equator, degrees | 142.9 | 138.9 | 108.8 | 25.4 | 144.9 | 133.1 | ||
| Deflection angle, degrees | 47.46 | 51.1 | 66.92 | 19.66 | 99.396 | 94.7 | ||
| 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 | ||
| Speed increment at perigee, mm/s | 2.56±0.05 | 7.21±0.07 | −1.7±0.9 | 0.67±0.02 | 0.008±0.004 | ~0 | −0.004±0.044 | |
| Gained energy, J/kg | 35.1±0.7 | 92.2±0.9 | 7.03±0.19 |
Upcoming missions with Earth flybys include Juno and BepiColombo.
Read more about this topic: Flyby Anomaly
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