Kevin's Watch

Pioneer anomaly put to the test

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.
Anomalous trajectory
Anomalous trajectory

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.

Attempts to test the anomaly using other spacecraft such as Galileo and the Voyager probes have proved unsuccessful, and the deep-space missions that are currently being developed - for example the Laser Interferometer Space Antenna (LISA) and the Jupiter Icy Moons Orbiter (JIMO) - will not be designed to test the properties of the Pioneer anomaly. Given this situation, we concluded that the anomaly could no longer be ignored.

At its Cosmic Vision workshop in Paris this month, the European Space Agency (ESA) will consider plans for a number of experiments and missions that will test gravity in new ways, one of which is designed to test the Pioneer anomaly directly. If the anomaly is an indication of new physics, finding its origin might change our understanding of the laws of nature at a very basic level and turn our cosmic backyard into the new terra incognita.
Theoretical proposals

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.
Pioneering mission

Thanks to new technologies such as precise accelerometers, improved launch techniques and optical navigation methods, we have come up with a proposal for the most precisely tracked spacecraft ever to go into deep space. The craft is also designed to eliminate essentially all on-board effects that might mask the result, such as forces due to radiated heat. And its hyperbolic orbit, like that of the Pioneer probes, will allow it to distinguish between the different types of effect that might be causing the anomaly.

Such a mission could also be an excellent opportunity to develop and test new technologies for spacecraft design, in-space propulsion, on-board power and many other developments that may ultimately find their way into many other space and terrestrial applications.

In what turned out to be a gratifying and most encouraging surprise, a number of our European colleagues had also became interested in developing technologies that would enable the precise testing of the Pioneer anomaly. So now, almost seven years after we and our co-workers Philip Laing, Eunice Lau and Tony Liu published the initial analysis of the anomalous deceleration, interest has grown to the point that ESA is considering a mission that would test the Pioneer anomaly to the level of a thousand times better than the announced value of this mysterious force.

Researchers at NASA's Jet Propulsion Laboratory, the University of Bremen and the Los Alamos National Laboratory are also preparing to reanalyse earlier, less precise, Pioneer data from the time when the craft were closer to the Sun. This should provide valuable information about the anomaly in earlier stages of the trajectory, and could also reveal other interesting properties of the effect - particularly during planetary fly-bys.

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.

Wayfriend wrote:Why haven't the ruled out simple particle acretion? Picking up "space dust" would imperceptibly slow down the probes (transfer of inertia), and if it was being acreted at a constant rate this would be hard to distinguish from "accelleration towards the sun".

Malik23 wrote:Wayfriend, it may not list in in this article--I read several--but space dust was considered. I think the density of the dust was too low to account for this slowdown.

Emotional Leper wrote:My bet is with a planet or planetoid. Pluto is as reflective as a lump of coal that is something like four dozen AU away. That's a damn dim thing to look for. The only reason we can still FIND the Pioneer today is because it's broadcasting.

Damelon wrote:

Emotional Leper wrote:My bet is with a planet or planetoid. Pluto is as reflective as a lump of coal that is something like four dozen AU away. That's a damn dim thing to look for. The only reason we can still FIND the Pioneer today is because it's broadcasting.

My guess is that your right. Neptune was found because Uranus' orbit didn't quite work out. Probably some other unaccounted for body is having an effect on Pioneer.

Damelon wrote:

Emotional Leper wrote:My bet is with a planet or planetoid. Pluto is as reflective as a lump of coal that is something like four dozen AU away. That's a damn dim thing to look for. The only reason we can still FIND the Pioneer today is because it's broadcasting.

My guess is that your right. Neptune was found because Uranus' orbit didn't quite work out. Probably some other unaccounted for body is having an effect on Pioneer.

dlbpharmd wrote: But that doesn't quite add up either - because along with slowing Pioneer down, wouldn't the course be affected also?

Data from the Galileo and Ulysses spacecraft indicate a similar effect, although for various reasons (such as their relative proximity to the Sun) firm conclusions cannot be drawn from these sources. These spacecraft are all partially or fully spin-stabilised; the effect is harder to measure accurately with three-axis stabilised craft such as the Voyagers.

The New Horizons spacecraft to Pluto is spin-stabilised for much of its cruise, and there is a possibility that it can be used to investigate the anomaly. New Horizons may have the same problem that precluded good data from the Cassini mission - its RTG is mounted close to the spacecraft body, so thermal radiation from it, bouncing off the spacecraft, may produce a systematic thrust of a not-easily predicted magnitude, several times as large as the Pioneer effect. Nevertheless efforts are underway to study the non-gravimetric accelerations on the spacecraft, in the hopes of having them well modeled for the long cruise to Pluto after the Jupiter fly-by that occurred in February 2007.

Wayfriend wrote:I'm skeptical of a dark matter theory since dark matter would be everywhere and so would be unlikely to pull in a certain direction.

Loremaster wrote:I will have to double check the following statement, but I think that it's not uniform.