The Pioneer Anomaly Options

[ugordan]

Would would an "ideal" PA mission look like, anyway?

Probably a very long stick with an RTG at one end and a HGA at the other

[djellison]

Pioneer 10 or 11 basically

Doug

[The Messenger]

On using New Horizons to investigate the Pioneer Anomaly, here is a comment from a poster on sci.space.history --

begin quote

...In general, the systematics are predicted to be several times the Pioneer
effect, and of an unknown magnitude.  This was exactly the problem with
Cassini, which also had the RTGs mounted close in.  (They spent a month
in cruise using only the reaction wheels, which is as good as spin stabilized,
and the measured acceleration was about 3x the Pioneer effect, and well
off the pre-flight predictions, and so was useless for studying this
effect. )

  Lou Scheffer

end quote

Actually, this might be a significant and useful chunk of data, if the vector is known, and it is in either the same or opposite direction as the solar wind.

Amoung the possible explanations for the Pioneer enomally is a solar field effect that diminishes as i/r - which would be much more pronounced between 1 and 8 AU than >20. So a magnitude of three greater at closer distances is actually in-family. At distances greater than 20 AU, a 1/r attenuation reduces to a nearly constant rate, as observe by the Pioneer probes.

A higher magnitude effect in Cassini is consistent with the magnitude and attenuation of 'unmodeled forces' experienced by both Galileo and Ulysses. Since the effective 'force' of these peculiarities is in the same direction as the solar wind, it is impossible to disentangle the parameters. (Using the solar wind to model Galileo and Ulysses accelerations requires a secondary term that attenuates as 1/r. (According to Anderson & Nieto.)

[ljk4-1]

Paper: astro-ph/0601422

Date: Thu, 19 Jan 2006 10:51:50 GMT (4kb)

Title: Upper limits on density of dark matter in Solar system

Authors: I.B. Khriplovich and E.V. Pitjeva

Comments: 4 pages
\\
The analysis of the observational data for the secular perihelion precession
of Mercury, Earth, and Mars, based on the EPM2004 ephemerides, results in new
upper limits on density of dark matter in the Solar system.

\\ ( http://arXiv.org/abs/astro-ph/0601422 , 4kb)

[ljk4-1]

Paper: astro-ph/0601431

Date: Thu, 19 Jan 2006 17:08:29 GMT (79kb)

Title: Modified gravity without dark matter

Authors: R.H. Sanders

Comments: 28 pages, 10 figures, lecture given at Third Aegean Summer School,
The Invisible Universe: Dark Matter and Dark Energy
\\
On an empirical level, the most successful alternative to dark matter in
bound gravitational systems is the modified Newtonian dynamics, or MOND,
proposed by Milgrom. Here I discuss the attempts to formulate MOND as a
modification of General Relativity. I begin with a summary of the
phenomenological successes of MOND and then discuss the various covariant
theories that have been proposed as a basis for the idea. I show why these
proposals have led inevitably to a multi-field theory. I describe in some
detail TeVeS, the tensor-vector-scalar theory proposed by Bekenstein, and
discuss its successes and shortcomings. This lecture is primarily pedagogical
and directed to those with some, but not a deep, background in General
Relativity

\\ ( http://arXiv.org/abs/astro-ph/0601431 , 79kb)

[ljk4-1]

On using New Horizons to investigate the Pioneer Anomaly, here is a comment from a poster on sci.space.history --

begin quote

Unfortunately, New Horizons is a rotten design for this particular study,
even though it will be spin stabilized.  The problem is that the RTG heat
will be radiated very unevenly.  The Pioneer effect is equivalent to a few
tens of watts more being radiated antisunward than sunward.  Pioneer was
quite good for this since the RTGs are on booms, and hence are radiating
mostly into free space.  Still, one of the biggest questions is how much
IR bounces off the spacecraft, and which way it bounces.

On New Horizons, the RTG is very close to the spacecraft body, and radiating
a kW or so of IR, so hundreds of watts of IR will hit the spacecraft. 
Figuring out where this will go exactly is very hard.  The spacecraft is
very un-symmetrical in the direction of motion, so the IR reflections will
certainly favor some directions rather than others.  Also, the spacecraft
is covered with thermal blankets, making reflections even harder to predict. 

In general, the systematics are predicted to be several times the Pioneer
effect, and of an unknown magnitude.  This was exactly the problem with
Cassini, which also had the RTGs mounted close in.  (They spent a month
in cruise using only the reaction wheels, which is as good as spin stabilized,
and the measured acceleration was about 3x the Pioneer effect, and well
off the pre-flight predictions, and so was useless for studying this
effect. )

  Lou Scheffer

end quote

Why is it that the Pioneer and Voyager probes had to have their science instruments as far from the RTGs as possible, yet that does not seem to be the case with Cassini and New Horizons?

Better shielding? More advanced/protected instruments? We don't fear radiation as much as we used to?

[Borek]

Why is it that the Pioneer and Voyager probes had to have their science instruments as far from the RTGs as possible, yet that does not seem to be the case with Cassini and New Horizons?

Better shielding?  More advanced/protected instruments?  We don't fear radiation as much as we used to?

Well shielded RTG should radiate very little, shouldn't it?

[helvick]

Well shielded RTG should radiate very little, shouldn't it?

It will still radiate heat, the RTG itself isn't particularly efficient so most of the decay energy is ultimately expressed as heat which is radiated out (hence the kwatt or so referred to in the sci.space.history post) from the RTG module. A significant portion of this IR energy "hits" the main body of the spacecraft but is reflected away in arbitrary directions from it by the thermal insulation, hence the complexity of analysing the effect.

[The Messenger]

Well shielded RTG should radiate very little, shouldn't it?

The RTGs on the Pioneer probes were symetrically mounted on booms, so that the (waste) thermal energy should be symetrically distributed. One of the still plausible scenarios is that the solar wind has caused substantial aging or discoloration of the sun-facing side of the boom(s), causing the energy to be dispersed asymetrically.

This may sound like an absurdly small effect, but that it is the ballpark of the Pioneer anomalies (1x10^-9 m/s^2), and this is why it is so difficult to propose a definitive test - very, very small force.

[ljk4-1]

Paper (*cross-listing*): gr-qc/0601055

Date: Sat, 14 Jan 2006 16:11:23 GMT (36kb)

Title: What do the orbital motions of the outer planets of the Solar System
tell us about the Pioneer Anomaly?

Authors: Lorenzo Iorio

Comments: Latex2e, 12 pages, 3 tables, 4 figures

Subj-class: General Relativity and Quantum Cosmology; Space Physics
\\
In this paper we investigate the effects that an anomalous acceleration as
that experienced by the Pioneer spacecraft after they passed the 20 AU
threshold would induce on the orbital motions of the Solar System planets
placed at heliocentric distances of 20 AU or larger as Uranus, Neptune and
Pluto. It turns out that such an acceleration, with a magnitude of about 8 X
10^-10 m s^-2, would affect their orbits with secular and short-period signals
large enough to be detected with the present-day level of accuracy in orbit
determination. The absence of such anomalous signatures in the latest data
analyses rules out the possibility that in the region 20-40 AU of the Solar
System an anomalous force field inducing a constant and radial acceleration of
that size is present.

\\ ( http://arXiv.org/abs/gr-qc/0601055 , 36kb)

Paper (*cross-listing*): gr-qc/0601055

replaced with revised version Fri, 20 Jan 2006 16:04:50 GMT (37kb)

Title: What do the orbital motions of the outer planets of the Solar System
tell us about the Pioneer anomaly?

Authors: Lorenzo Iorio

Comments: Latex2e, 13 pages, 3 tables, 4 figures, 14 references. References
added. Stressed the fact that, even by assuming errors in the planetary
orbital elements 30 times larger that those published by Pitjeva, the
anomalous Pioneer effects on Uranus, Neptune, Pluto still remain well larger
and, thus, detectable if present

Subj-class: General Relativity and Quantum Cosmology; Space Physics

\\ ( http://arXiv.org/abs/gr-qc/0601055 , 37kb)

[The Messenger]

Title: What do the orbital motions of the outer planets of the Solar System
  tell us about the Pioneer anomaly?

Authors: Lorenzo Iorio

Comments: ... Stressed the fact that, even by assuming errors in the planetary
  orbital elements 30 times larger that those published by Pitjeva, the
  anomalous Pioneer effects on Uranus, Neptune, Pluto still remain well larger
  and, thus, detectable if present
...

This is an important constraint, under the tested conditions:

In particular, we will investigate the possibility that an external, unknown constant and uniform force field inducing an acceleration of (8)×10−10 m s−2 on a test particle is present in the outer regions of the Solar System within 20-40 AU.

I think this highly constrains MOND-like, Dark Matter-like, or Dark Energy-like candidates.

These results do not constrain 1) non-linear effects, 2) systemics that may cause us to incorrectly calculate the mass and/or positions of the planets, or 3) linear effects that only act upon small conductive, and/or radioactive bodies.

Although the measured Pioneer accelerations appear to be roughly linear, it is important to remember that over great distances: 1/r, 1/r^2 and 1/z^4 - these normal distant field scaling factors can be reduced to nearly linear approximations (over relatively short distances)

[Mongo]

These results do not constrain 1) non-linear effects, 2) systemics that may cause us to incorrectly calculate the mass and/or positions of the planets, or 3) linear effects that only act upon small conductive, and/or radioactive bodies.

I wonder if the magnitude of the 'Pioneer Effect' depends upon the radial velocity of the object, such that objects moving away from the Sun appear to have a force acting upon them toward the Sun, and objects moving toward the Sun appear to have a force acting upon them away from the Sun, with the magnitude of the 'force' proportional to the radial velocity of the object.

Objects in bound orbits, such as the planets, would end up having the two effects cancel out over each full orbit. The effect of such a 'force' would be to make their orbits somewhat less eccentric than they would otherwise be, but since the effect would be small (due to the low eccentricity of all the large objects with well-known orbits, resulting in low radial velocities) and would act over only half of an orbital cycle before being reversed (as the radial velocity changes from outward to inward and vice versa), the difference between the actual orbit, forced to lower eccentricity under the Pioneer Effect, and a non-Pioneer Effect orbit of sufficiently lower eccentricty to match, might well be too small to observe with current technology.

The 'Pioneer Effect' would only be easily visible in unbound trajectories such as Pioneers 10 and 11, where the effect is larger due to the larger radial velocity, and can accumulate over a much longer period of time.

Bill

[hal_9000]

From New Scientist -> http://www.newscientistspace.com/article.ns?id=dn8631

Gravity theory dispenses with dark matter

A modified theory of gravity that incorporates quantum effects can explain a trio of puzzling astronomical observations – including the wayward motion of the Pioneer spacecraft in our solar system, new studies claim.

The work appears to rule out the need to invoke dark matter or another alternative gravity theory called MOND (Modified Newtonian Dynamics). But other experts caution it has yet to pass the most crucial test – how to account for the afterglow of the big bang.

Astronomers realised in the 1970s that the gravity of visible matter alone was not enough to prevent the fast-moving stars and gas in spiral galaxies from flying out into space. They attributed the extra pull to a mysterious substance called dark matter, which is now thought to outweigh normal matter in the universe by 6 to 1.

But researchers still do not know what dark matter actually is, and some have come up with new theories of gravity to explain the galaxy observations. MOND, for example, holds that there are two forms of gravity.

Above a certain acceleration, called a0, objects move according to the conventional form of gravity, whose effects weaken as two bodies move further apart in proportion to the square of distance. But below a0, objects are controlled by another type of gravity that fades more slowly, decreasing linearly with distance.

But critics point out that MOND cannot explain the observed masses of clusters of galaxies without invoking dark matter, in the form of almost massless, known particles called neutrinos.


http://www.newscientistspace.com/article.ns?id=dn8631

[Jeff7]

From New Scientist -> http://www.newscientistspace.com/article.ns?id=dn8631

Gravity theory dispenses with dark matter

Sounds like a version of the theory that general relativity needs to be applied to gravitational models, instead of simpler Newtonian physics. That theory hclaims to be able to do away with dark matter entirely and still explain both small-scale and very large-scale systems.
I just imagine the thought of doing away with dark matter is unappealing to some people, as I'm sure some have made the study of dark matter their life's work. Another way of looking at it though is that they're just inadvertently referring to "the effects of general relativity" as "dark matter".

[ljk4-1]

Paper: astro-ph/0601581

Date: Wed, 25 Jan 2006 21:28:29 GMT (15kb)

Title: Globular Clusters as a Test for Gravity in the Weak Acceleration Regime

Authors: Riccardo Scarpa, Gianni Marconi, and Roberto Gilmozzi

Comments: Comments: To Appear in the proceeding of the "First crisis in
cosmology" conference, published in the American Institute of Physiscs'
proceedings series, Vol. 822. (includes 3 pages and 1 fig)
\\

Non-baryonic Dark Matter (DM) appears in galaxies and other cosmic structures
when and only when the acceleration of gravity, as computed considering only
baryons, goes below a well defined value a0=1.2e-8 cm/s/s. This might indicate
a breakdown of Newton's law of gravity (or inertia) below a0, an acceleration
smaller than the smallest probed in the solar system. It is therefore important
to verify whether Newton's law of gravity holds in this regime of
accelerations. In order to do this, one has to study the dynamics of objects
that do not contain significant amounts of DM and therefore should follow
Newton's prediction for whatever small accelerations. Globular clusters are
believed, even by strong supporters of DM, to contain negligible amounts of DM
and therefore are ideal for testing Newtonian dynamics in the low acceleration
limit. Here, we discuss the status of an ongoing program aimed to do this test.
Compared to other studies of globular clsuters, the novelty is that we trace
the velocity dispersion profile of globular clusters far enough from the center
to probe gravitational accelerations well below a0. In all three clusters
studied so far the velocity dispersion is found to remain constant at large
radii rather than follow the Keplerian falloff. On average, the flattening
occurs at the radius where the cluster internal acceleration of gravity is
1.8+-0.4 x 10^{-8} cm/s/s, fully consistent with MOND predictions.

\\ ( http://arXiv.org/abs/astro-ph/0601581 , 15kb)