The Pioneer Anomaly Options

[remcook]

http://www.planetary.org/news/2005/pioneer_anomaly_faq.html

The planetary society may be checking it out...

The Planetary Society has committed to raise the funds to preserve the priceless Pioneer data from destruction.


After years of analysis, but without a final conclusion, NASA, astonishingly, gave up trying to solve the "Pioneer Anomaly" and provided no funds to analyze the data. The Pioneer data exists on a few hundred ancient 7- and 9-track magnetic tapes, which can only be read on "antique" outdated computers. The agency is going to scrap, literally demolish, the only computers able to access and process that data in the next few months!

[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)

[ljk4-1]

From The Planetary Society update:

Our strategy for the upcoming analysis of the newly available data can be summed up as follows: First, studying the early mission data may help us unambiguously determine whether the acceleration points towards the Earth, the Sun, or some other direction. Second, we hope to find out how the anomaly begins or if it was present throughout the mission. Third, we hope to be able to determine how this anomalous acceleration changed over time. Fourth, we will compare data from the two Pioneers to see if we can discern any notable differences in their behavior. Lastly, we will use the MDRs to develop a better engineering model of the spacecraft, making use, for instance, of finite element analysis methods to understand its thermal behavior.

In March 2006, for the very last time the Earth will be in a favorable position to receive Pioneer 10's radio signal. It is possible that Pioneer 10 is still able to transmit, despite the age of its electronics, the extreme coldness of deep space, and the diminishing amount of electrical power on board. The possibility that the Deep Space Network will attempt to reacquire Pioneer 10's weak signal is currently being investigated, raising the hope, however faint, that we may yet have another data point to aid in our investigation of the Pioneers' enigmatic behavior.

The rest is here:

http://planetary.org/programs/projects/pio...ate_200601.html

[Guest_Richard Trigaux_*]

From The Planetary Society update:

Our strategy for the upcoming analysis of the newly available data ...

The rest is here:

http://planetary.org/programs/projects/pio...ate_200601.html

So this data was recovered and is now safe. Good move. Now serious work can begin.

[The Messenger]

So this data was recovered and is now safe. Good move. Now serious work can begin. 

It is an extremely difficult and tedious proposition. Each operation of a thruster must be accounted for, and every encounter with a third body - a moon, a planet, and a planetary system requires accurate mass and distance estimates.

It will be interesting to see if the Pioneer probe data pans out in the lower orbits like Cassini, Galileo, and Ulysses. Each of these probes experienced, (or rather, accelerations were measured), that exceed the 'Pioneer' effect by at least one magnitude, however, it is not possible to untangle the measured acceleration from the solar wind: It is in the same direction. (I think this measured anomally in Galileo and Ulysses is also in the opposite direction from the Pioneer probe acceleration - I don't have any numbers, other than a magnitude for Cassini.)

We will be lucky if we can hear from Pioneer in March, but if I remember correctly, the probe is no longer executing commands, so it may not be possible to enter the Doppler-repeater mode necessary for accurate distant measurements. (In this mode, the Pioneer probes echoed back an ultrastable signal sent from the Earth a harmonic frequency. If we are only listening to the Pioneer transmitter, there is not sufficent accuracy in the on-board timing to obtain accurate distance estimates.)