Pioneer anomaly put to the test

By Aussiegirl

I just posted an article with the title Gravity's dark side, and in the article is this paragraph: In addition to fitting galaxy data very well, Moffat claims that his theory can also explain the wayward paths of the Pioneer probes. Launched in the 1970s these two spacecraft, which are now at the furthest reaches of the solar system, appear to be experiencing an anomalous acceleration towards the inner solar system. Although this may turn out to have a mundane technical explanation, it provides a classic test for alternative theories of gravity on solar-system scales. It's always exciting to discover the word "anomalous" in a scientific article, so I looked further and found another article about the Pioneer anomaly from September 2004. That's nearly two years ago, so maybe it's no longer an anomaly -- how disappointing! But according to the Wikipedia article, which you will find here, it's still a big mystery. Dark matter and revising general relativity, topics that are involved in Gravity's dark side, also turn up here. When the world gets too complicated, you can always turn to cutting-edge science and forget yourself for a while.

Pioneer anomaly put to the test (September 2004) - Physics World - PhysicsWeb

The European Space Agency is considering a unique experiment that could explain strange gravitational phenomena in the outer solar system

Since 1998 astronomers have known that the space probes Pioneer 10 and Pioneer 11 are following trajectories that cannot be explained by conventional physics. Launched in 1972 and 1973, respectively, to explore the outer planets, the Pioneer craft are now at the edge of the solar system, with Pioneer 10 being some 86 astronomical units (about 13 billion kilometres) from the Sun. But they are not quite where they should be, based on the gravitational pull of the known bodies in the solar system.

When the craft were at distances of between 20 and 70 astronomical units, researchers found that the Doppler frequency of microwave signals that were bounced off the craft drifted at a small, constant rate (see "Spacecraft anomalies put gravity to the test"). This drift meant that the craft were experiencing a constant acceleration directed towards the Sun, at a level that is 10 billion times weaker that the Earth's gravitational pull. The most obvious explanation for this anomalous deceleration is some mundane systematic effect, such as heat radiating from the craft or leakage from the propulsion thrusters. But no such mechanism has been found. [....]

The inability to explain the Pioneer anomaly with conventional forces has led to several theoretical proposals. One is that the deceleration is due to the gravitational attraction of "dark matter" - the invisible matter that astronomers think is responsible for the excess gravity that appears to affect objects on galactic scales.

Other explanations involve modifying Einstein's general theory of relativity, which many theorists think is necessary in order to merge gravity with quantum mechanics. Some of these theories suggest that gravity might attract a little harder than expected at large distances or small accelerations, so the concept of dark matter may not even be necessary.

Meanwhile, there are a number of attempts to go beyond the Standard Model of particle physics. String theory and/or supersymmetry, for example, involve higher dimensions of space that introduce new degrees of freedom and possible violations of space-time symmetries such as Lorentz symmetry. This could result in very weak forces that act on the scale of the solar system, although different theories make different predictions of the precise corrections to the spacecraft trajectories.

Some of these theoretical proposals have recently been given support by experimental results. For example, we now know that the expansion of the universe is accelerating, and some researchers have detected possible variations in the values of the fundamental constants (see "Dark energy" and "Are the laws of nature changing with time?"). However, no current proposal can explain the Pioneer anomaly. It is therefore vital to test our understanding of gravity more precisely, which is best carried out in the isolation and apparent weightlessness of space.

We have argued that it is time to settle the Pioneer issue with a new deep-space mission that will test for, and decide on, the origin of the anomaly (Class. Quantum Grav. 21 4005-4023). Any result would be of major significance. If the anomaly is a manifestation of new or unexpected physics, it would be of fundamental importance. But even if it turns out to be due to an unknown systematic mechanism, understanding the anomaly could help engineers build more stable and less noisy spacecraft that can be navigated more precisely for the benefit of deep-space experiments. [....]

Dispassionately, the most likely cause of the anomalous acceleration of the Pioneer spacecraft is on-board systematics, but the smoking gun has not yet been found. The only other possibility is the existence of new physics. This dichotomy represents a healthy win-win situation because either one of these two explanations for the Pioneer anomaly would constitute an extremely important discovery.