Flyby anomaly

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Unsolved problems in physics: What causes the unexpected change in acceleration for flybys of satellites?

The flyby anomaly is an unexpected energy increase during Earth flybys of satellites. This has been observed as an anomalous shift in the S-Band and X-Band Doppler data which translates to a significant velocity increase of a few mm/s at perigee. So far, no conventional mechanism has been found that can explain this effect.

[edit] Observations

Gravitational assists, i.e. the technique of changing a spacecraft's heliocentric orbital velocity by means of an exchange of potential and kinetic energy between a planet and a spacecraft, provide an indispensable tool for solar system exploration. Since the success of such a flyby maneuver depends crucially on the geometry of the trajectory, the position and the velocity of a spacecraft is continuously tracked during its encounter with a planet by the Deep Space Network (DSN).

The flyby anomaly was first noticed shortly after the Earth flyby of the Galileo spacecraft on 8 December 1990 by a careful inspection of the DSN Doppler data. While it was expected that the Doppler residuals (observed minus computed data) should 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 could not give a satisfying explanation. However, no anomaly was detected after the second Earth flyby of the Galileo spacecraft in December 1992 since any possible velocity increase was masked by atmospheric drag due to the lower altitude of 303 km.

On January 23, 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.

Interestingly, an analysis of the MESSENGER spacecraft (studying Mercury) did not reveal any significant unexpected velocity increase. Scientists think the reason for this may be that MESSENGER both approached and departed earth symmetrically about the equator (approached: latitude 31 degrees north; departed: latitude 32 degrees south), suggesting that this anomaly is somehow related to Earth's rotation.

It is not known whether this phenomenon is related to the Pioneer anomaly, a similar deviation associated with the Pioneer 10 and Pioneer 11 spacecraft. Electromagnetic forces due to an electric charge and the simple ferromagnetic properties of the Pioneer spacecrafts are a possible suspect. It is known that the flyby anomaly is an acceleration rather than a deceleration in the case of spacecrafts which transit over polar Earth regions (rotational AND magnetic) during their voyage. It is possible that in the case of an equatorial transit an unmeasured small deceleration relative to Earth occurs that is very similar to the Pioneer spacecraft's deceleration relative to the Sun.

Two-way X-Band Doppler residuals during Earth flyby of NEAR
Two-way X-Band Doppler residuals during Earth flyby of NEAR
Range residuals during Earth flyby of NEAR
Range residuals during Earth flyby of NEAR


[edit] References

  • C. Lämmerzahl, O. Preuss, H. Dittus, Is the physics within the Solar system really understood?, Proceedings of the 359th WE-Heraeus Seminar on "Lasers, Clocks, and Drag-Free: Technologies for Future Exploration in Space and Tests of Gravity" , Preprint: [1], 2006
  • J.D. Anderson, J.G. Williams, Long-range tests of the equivalence principle, Class. Quantum Grav., 18, 2447-2456, 2001
  • P.G. Antreasian, J.R. Guinn, Investigations into the unexpected delta-v increase during the Earth Gravity Assist of GALILEO and NEAR, paper no. 98-4287 presented at the AIAA/AAS Astrodynamics Specialist Conf. and Exhibition, Boston, 1998
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