|This article is outdated. (March 2010)|
|What causes the unexpected change in acceleration for flybys of spacecraft?|
The flyby anomaly is an unexpected energy increase during Earth-flybys of spacecraft. This anomaly has been observed as shifts in the S-Band and X-Band Doppler and ranging telemetry. Taken together it causes a significant unaccounted velocity increase of over 13 mm/s during flybys.
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 because MESSENGER both approached and departed Earth symmetrically about the equator (see data and proposed equation below). This suggests that the anomaly may be related to Earth's rotation.
Summary of Earth-flyby spacecraft is provided in table below. The Rosetta data are 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||Juno|
|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|
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.
Upcoming missions with Earth flybys include BepiColombo.
An empirical equation for the anomalous flyby velocity change was proposed by J.D. Anderson et al.:
Possible explanations of the flyby anomaly include
- 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 the Earth;
- A Modification of Inertia resulting from a Hubble-scale Casimir effect (MIHsC);
- 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 anomaly may be due to the rotation of the Earth. This rotation induces an azimuthally symmetric gravitational field.
- "ESA's Rosetta spacecraft may help unravel cosmic mystery". European Space Agency. November 12, 2009. Retrieved 13 March 2010.
- Anderson, John D.; James K. Campbell, Michael Martin Nieto (July 2007), "The energy transfer process in planetary flybys", New Astronomy 12 (5): 383–397, arXiv:astro-ph/0608087, Bibcode:2007NewA...12..383A, doi:10.1016/j.newast.2006.11.004
- "Mystery remains: Rosetta fails to observe swingby anomaly". ESA.
- "Probing the Flyby Anomaly with the future STE-QUEST mission".
- Anderson, et al. (7 March 2008), "Anomalous Orbital-Energy Changes Observed during Spacecraft Flybys of Earth", Phys. Rev. Lett..
- J. P. Mbelek (2008). "Special relativity may account for the spacecraft flyby anomalies". arXiv:0809.1888 [gr-qc]. Bibcode 2008arXiv0809.1888M
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- J.D. Anderson; J.K. Campbell; M.M. Nieto (2006), "The Energy Transfer Process in Planetary Flybys", New Astronomy 12 (5): 383, arXiv:astro-ph/0608087, Bibcode:2007NewA...12..383A, doi:10.1016/j.newast.2006.11.004
- NASA Baffled by Unexplained Force Acting on Space Probes, at Space.com
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- Wanted: Einstein Jr, at Economist.com
- K. Svozil (2008). "Microphysical analogues of flyby anomalies". arXiv:0804.2198 [quant-ph]. Bibcode 2008arXiv0804.2198S
- Claus Lämmerzahl, University of Bremen:The Pioneer Anomaly or Do We Really Understand the Physics With the Solar System? (PDF file; 6.25 MB, 139 pages
- Andreas Aste, University of Basel:Spacecraft Anomalies: An Update(PDF file; 9.8 MB, talk/slides)