Utente:Steo85/Sandbox

Steo85 (msg) 01:21, 3 giu 2010 (CEST)

Cosa causa l'inattesa variazione di velocità durante i flyby della sonda? L'"anomalia di flyby" è un anomalo incremento di energia durante un'assistenza gravitazionale ad una sonda da parte della Terra. Quest'anomalia é stata osservata per effetto Doppler a diverse frequenze e sulle misure di ranging. Complessivamente, é causa di un aumento inspiegabile di velocità fino a 13,5 mm/s durante l'assistenza gravitazionale.

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 continuously tracked during its encounter with a planet by the Deep Space Network (DSN).

Range residuals during the Earth-flyby of NEAR

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. ScientistsTemplate:Who 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). This may suggest that the anomaly is related to Earth's rotation.

Summary of Earth-flyby spacecraft is provided in table below. It should be noted that 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

Proposed equation

Empirical equation for flyby anomaly velocity change was proposed by J.D. Anderson et al.

${\frac {dV}{V}}={\frac {2\omega _{e}R_{e}(\cos \varphi _{i}-\cos \varphi _{o})}{c}}$

where ω_e is the angular frequency of Earth, R_e is the radius of Earth, and φ_i and φ_o are the inbound and outbound equatorial angles of the spacecraft.

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.

Possible explanations

It is not known whether the flyby anomaly is related to the Pioneer anomaly. In the case of spacecraft that transit over Earth's rotational or magnetic polar regions, the flyby anomaly is an acceleration rather than a deceleration. It is possible that during an equatorial transit an unmeasured small (relative to Earth) deceleration occurs; this is very similar to the Pioneer spacecraft's deceleration relative to the Sun.

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, or the possibly related Pioneer anomaly.
• 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, e.g., frame-dragging, has been investigated as well: it turns out to be unable to account for the flyby anomaly.

References

• AIAA/AAS Astrodynamics Specialist Conf. and Exhibition, 1998, paper no. 98-4287.
• Long-range tests of the equivalence principle, in Class. Quantum Grav., vol. 18, 2001, pp. 2447–2456, DOI:10.1088/0264-9381/18/13/307.
• Is the physics within the Solar system really understood?, in Proceedings of the 359th WE-Heraeus Seminar on "Lasers, Clocks, and Drag-Free: Technologies for Future Exploration in Space and Tests of Gravity", 2006., Preprint at arΧiv:gr-qc/0604052. Associated presentation slides
• The Energy Transfer Process in Planetary Flybys, 2006, arΧiv:astro-ph/0608087.
• NASA Baffled by Unexplained Force Acting on Space Probes, at Space.com
• Anomalous Orbital-Energy Changes Observed during Spacecraft Flybys of Earth (PDF), in Phys. Rev. Lett., vol. 100, 2008, p. 091102, DOI:10.1103/PhysRevLett.100.091102.
• Wanted: Einstein Jr, at Economist.com
• K. Svozil, Microphysical analogues of flyby anomalies, 2008, arΧiv:0804.2198 [quant-ph].
• Are Flyby Anomalies and the Pioneer Effect an ASTG Phenomenon?, 2010, arΧiv:0803.1370.

External links

• 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)