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RE: The Pioneer anomaly
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Pioneer Anomaly and the Kuiper Belt mass distribution
Authors:
O. Bertolami, P. Vieira

Pioneer 10 and 11 were the first probes sent to study the outer planets of the Solar System and Pioneer 10 was the first spacecraft to leave the Solar System. Besides their already epic journeys, Pioneer 10 and 11 spacecraft were subjected to an unaccounted effect interpreted as a constant acceleration towards the Sun, the so-called Pioneer anomaly.
This anomaly can be interpreted as a constant acceleration with a magnitude of a = (8.74 ± 1.33) × 10^-10 ms^-2 and is directed towards the Sun.

One of the possibilities put forward for explaining the Pioneer anomaly is the gravitational acceleration of the Kuiper Belt.
In this work the researchers examine this hypothesis for various models for the Kuiper Belt mass distribution.
They find that the gravitational effect due to Kuiper Belt cannot account the Pioneer anomaly.
Their conclusions suggest that only through a mission, the Pioneer anomaly can be confirmed and further investigated.
This mission would study the mass distribution of the Kuiper Belt.


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RE: Pioneer 10 and 11
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Pioneer 10 and 11 were two of NASA’s most successful space missions of the 1970s. Launched in 1972, Pioneer 10 was the first spacecraft to travel through the Asteroid belt and the first to obtain close-up images of Jupiter.
Pioneer 11 was launched in 1974 and went on to make the first direct observations of Saturn in 1979. But there was something else that made them special: each carried a golden plaque with a message from mankind to alien civilisations.


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The plaques, for instance, depict our solar system and the path followed by the spacecrafts. The trajectory shows their way past Jupiter and out of the solar system. However unrealistic the aim of contacting aliens may be, there is no doubt of the symbolism. The Pioneers were the product of a generation of astronomers eager to communicate with the cosmos. It now seems that the cosmos has responded in a surprising way.
In 1980, John Anderson, one of the leaders of the Pioneer team at the Jet Propulsion Laboratory (JPL) in California, noticed something unexpected. Astronomers should have been able to exactly predict the trajectory of their spacecrafts by using the standard Newtonian-Einsteinian laws of gravity. But Anderson’s calculations were all wrong. The spacecrafts were not where they were supposed to be. It was as if they were being slowed down by an unknown, mysterious force.
Anderson was later joined in his efforts by Michael Martin Nieto, from the Los Alamos National Laboratory and Slava Turyshev, a colleague at JPL. Turyshev was one of the first Soviet scientists to work in the United States after the Cold War. When he first heard of the Pioneer anomaly, he volunteered to work on the project for free.
Convinced that there was an error in the way astronomers were calculating the trajectories, Turyshev studied more than 60 possible effects that could cause an abnormal reaction in the spacecraft. Every conceivable cause was taken into account, from gas leaks to the effect that ocean waves, hitting the shores in the Mojave Desert, had on the radio instruments receiving Pioneer’s transmissions. After more than a decade of careful analysis, they realised that these effects were not even close to explaining the Pioneer anomaly.
The mystery then got even more interesting when similar effects were reported on two other spacecrafts, Ulysses and Galileo. How can we explain the same anomaly occurring simultaneously in four different spacecrafts? The scientific community was now paying attention.
Today, an increasing number of people believe that the Pioneer anomaly could actually be a discovery, a very big one. The call for new laws of gravity is getting stronger.
In fact, a solution may have already been proposed a long time ago. It dates back to 1983, when Mordehai Milgrom, a physicist at the Weizmann Institute in Israel, first considered a modification to the standard laws of gravity.
Back then, one of biggest riddles in physics was related to the motion of galaxies. Galaxies were observed to be rotating in a way that physicists could not understand. Milgrom, using a different law of gravity, was able to successfully predict such behaviour.
Nonetheless, his attempt was considered heresy against one of the most well-established physical theories. The Universal Law of Gravity was first formulated by Isaac Newton and had managed to describe how the planets moved with one single equation. But in the 17th century, Newton knew nothing about galaxies. Stacy McGaugh, a physics professor at the University of Maryland, suggests that Newton would probably be the first to propose a different law if he knew the facts about the galaxies as well.
Milgrom’s theory on galaxies did not predict the Pioneer anomaly, but an increasing number of people feel that it may well do so. There are already several proposals for more complete versions of modified gravity in which the anomaly is accounted for. There is still a long way to go, though, both in developing the theory and getting attention.
I’m still shocked by how so few of us have heard about this theory, let alone know anything serious about it.” - Stacy McGaugh.
In the meantime, at the Jet Propulsion Laboratory, Turyshev and his colleagues have initiated the development of a dedicated mission to study the Pioneer effect and understand the source of the anomaly. Whether it is a gas leak or modified gravity, Turyshev feels obligated to go to the bottom of this. “What if this is something that nature is telling us?” he asks, “We must be careful and patient to listen and understand what we are being told”.
Sadly, the main characters of this story will not be around for its conclusion. NASA has received its last, very weak, signal from Pioneer 10 in 2003, and Pioneer 11 sent its last signal in 1995. The spacecrafts are probably plodding their way into deep space, in a way no-one yet understands.

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-- Edited by Blobrana at 18:05, 2005-05-20

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Distant asteroids could help reveal the nature of the mysterious force that has nudged NASA's 33-year-old Pioneer 10 spacecraft about 400,000 kilometres off course.
The so-called Pioneer Effect could be accounted for by a force pulling the probe towards the sun with a strength of just one ten-billionth of the gravity at Earth's surface. But no one has managed to explain the nature of this force, and many suspect that it is just a systematic error in the data or a fault of the spacecraft design.
Gary Page of George Mason University in Fairfax, Virginia, and his colleagues have identified 15 asteroids that might also be subjected to the mysterious force. The asteroids' orbits all stretch far into the outer solar system. This is crucial because the Pioneer anomaly only shows up beyond about twice the distance from the sun to Saturn.
Of the 15 candidates, the best is 1995SN55. This 370-kilometre-wide space rock has spent the past 54 years in the anomaly zone, so it should have experienced the largest perturbation. And tantalisingly, it is not where predictions say it should be.
"It could be lost because of the Pioneer effect. Asteroids are just big and dumb and go where gravity tells them."
Michael Martin Nieto of the Los Alamos National Laboratory in New Mexico thinks the asteroid study is a great idea, but cautions that it is unlikely to reveal the whole truth.
"It will only test whether gravity is the cause. The anomaly could also be caused by a subtle effect in inertia or even time." So a spacecraft mission may be needed.
Page and his colleagues are applying for telescope time to track the distant asteroids precisely...


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The Pioneer 10 and 11 spacecraft exhibited an `error` in their trajectories that `drag` them 8.74e-8 cm/sec/sec back towards the sun, after they had ventured passed a distance of 20 AU. This anomalous acceleration has come to be referred to as the Pioneer Effect.
If this represents a real phenomenon, then observations of minor planets may provide the key investigate the gravitational field in the outer solar system, and measure the Pioneer Effect.

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-- Edited by Blobrana at 17:31, 2005-04-29

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