Hey cavediver,
Explaining why however is rather a large topic, as is the whole Pioneer Anomaly, if we are to treat this exhaustively. Far more sensible is to just wait for better data, ...
So I take it there is no new information on these anomalies?
Pioneer anomaly - Wikipedia
quote:
It appears to cause a constant sunward acceleration of (8.74 1.33) 10−10 m/s2 for both spacecraft. If the positions of the spacecraft are predicted one year in advance based on measured velocity and known forces (mostly gravity), they are actually found to be some 400 km closer to the sun at the end of the year. The magnitude of the Pioneer effect is numerically quite close to the product of the speed of light and the Hubble constant, but the significance of this, if any, is unknown. Gravitationally bound objects such as the solar system, or even the galaxy, do not partake of the expansion of the universe this is known both from theory[1] and by direct measurement.[2]
Data from the Galileo and Ulysses spacecraft indicate a similar effect, although for various reasons (such as their relative proximity to the Sun) firm conclusions cannot be drawn from these sources. These spacecraft are all partially or fully spin-stabilised.
The effect is much harder to measure accurately with craft that use thrusters for attitude control. These spacecraft, such as the Voyagers, acquire small and unpredictable changes in speed as a side effect of the frequent attitude control firings. This 'noise' makes it impractical to measure small accelerations such as the Pioneer effect.
The Cassini mission also had reaction wheels for attitude control, thus avoiding this particular problem, but also had radioisotope thermoelectric generators (RTGs) mounted close to the spacecraft body, radiating kilowatts of heat in hard-to-predict directions. The measured value of unmodelled acceleration for Cassini is (26.7 1.1) 10−10 m/s2, roughly three times as large as the Pioneer acceleration. Unfortunately, the measured value is the sum of the uncertain thermal effects and the possible anomaly. Therefore the Cassini measurements neither conclusively confirm nor refute the existence of the anomaly.[3]
As I recall, approximately the same acceleration would explain the rotation of galaxies without dark matter, but I can't find confirmation of that here.
Galaxy rotation curve - Wikipedia
quote:
The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis. Stars revolve around the center of galaxies at a constant speed over a large range of distances from the center of the galaxy. Thus they revolve much faster than would be expected if they were in a free Newtonian potential. The galaxy rotation problem is this discrepancy between the observed rotation speeds of matter in the disk portions of spiral galaxies and the predictions of Newtonian dynamics considering the visible mass. This discrepancy is currently thought to betray the presence of dark matter that permeates the galaxy and extends into the galaxy's halo. An alternative (less favored) explanation is a modification of the laws of gravity, MOND.[1]
There are a limited number of attempts to find alternative explanations to dark matter to explain galaxy rotation curves. One of the most discussed alternatives is MOND (Modified Newtonian Dynamics), originally proposed as a phenomenological explanation back in 1983 but which has been seen to have predictive power in the rotation curves of LSB galaxies. This posits that the physics of gravity changes at large scale but, until recently, was not a relativistic theory. However, this changed with the development of the tensor-vector-scalar gravity (TeVeS) theory[7]. A more successful alternative is the modified gravity (MOG) theory of Moffat such as scalar-tensor-vector gravity (STVG)[8]. Brownstein and Moffat (astro-ph/0506370) applied MOG to the question of galaxy rotation curves, and presented the fits to a large sample of over 100 low surface brightness (LSB), high surface brightness (HSB) and dwarf galaxies[9]. Each galaxy rotation curve was fit without dark matter using only the available photometric data (stellar matter and visible gas) and alternatively a two-parameter mass distribution model which made no assumption regarding the mass to light ratio. The results were compared to MOND and were nearly indistinguishably right out to the edge of the rotation curve data, where MOND predicts a forever flat rotation curve, but MOG predicts an eventual return to the familiar inverse-square gravitational force law. Although these alternatives are not yet considered by the astronomical community to be as convincing as the dark matter model [10], gravitational lensing studies may provide the means to separate the predictions of alternative gravity theories from the dark matter explanation. Recently, observations of the Bullet Cluster have cast considerable doubt on the ability for MOND and other dark matter alternatives to explain the spatial distribution of matter (convergence kappa-map) and ICM gas (surface density Sigma-map).
Would that be a fair summary of the current status?
I don't want to stir up old issues here, just want to keep up to date.
Enjoy.
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