Jonathan Silverlight writes:

In message , Igor
writes
Check out this link:

http://www.newtonphysics.on.ca/Anoma...eleration.html

Very interesting! It's somehow satisfying that the explanation is
conventional, not due to some boring property of the spacecraft, and
gives new information.
Presumably the reason Cassini hasn't seen an acceleration is that it's
more than 20 x as massive.
One thing does occur to me. Paul Marmet rather fancifully suggests that
the Pioneers will gather dust as they move. It seems to me that the dust
particles will actually be moving at very high speed relative to the
spacecraft and will vaporise. More to the point, that means they will
impart their kinetic energy to the spacecraft, which scales as V^2, not

Marmet's explanation is unconvincing. It depends entirely on the
density of dust in the outer solar system, which according to Marmet:

This amount of dust in the outer region of the solar system appears
quite reasonable remembering that the daily amount of dust falling
on Earth is reported as many tons of dust grains per day.

which is a completely fallacious argument. The number of "tons" of
dust falling on the earth has nothing to do with the dust conditions
in the outer solar system, because (a) one must normalize the captured
"tons" by the cross sectional area of the earth; and (b) the
conditions are different in the outer solar system. In particular,
the dust density drops of precipitously beyond Jupiter.

It is straightforward to show that the net acceleration due to dust
is:
a_dust = -2 (A/M) n V^2 m
where A/M is the area to mass ratio of the body, n is the dust
density, V is the body velocity, and m is the mean dust mass. This
conservatively assumes elastic scattering. It is likely that the dust
will be captured, in which case a_dust will be half the value quoted
above.

Dust properties in the outer solar system have been measured, in some
cases by quantitative dust instruments on Pioneers 10 and 11
themselves (Landgraf et al 2002; Gurnett et al 1997). The there is a
continuous density distribution. According to the above equation, the
acceleration is heavily weighted to large dust particles, but these
are extremely rare. The net densities are of order 2 x 10^{-17}
cm^{-3}, with dust masses ~0.1 ug, leaving the net acceleration due to
dust to be safely less than a few times 10^{-12} cm s^{-2}, far less
than the quoted anomalous acceleration.

Craig

References

D. A. Gurnett, J. A. Ansher, W. S. Kurth, and L. J. Granroth 1997,
Geophys. Res. Lett., 24, 3125
M. Landgraf, J.-C. Liou, H. A. Zook, and E. Gr\"un 2002,
Astrophys. J., 123, 2857

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