Space oddity

Nasa's Pioneer probes are in the grip of a baffling force. Could it, asks Marcus Chown, rewrite the laws of physics?

A mysterious force is tugging on Nasa's ancient Pioneer spacecraft, retarding them as they streak out towards the distant stars. It amounts to a mere 10-billionth of a "g" – gravitational force. Yet it could bring the entire edifice of physics tumbling down.

Pioneer 10 and 11 have been in space for an astonishing three decades. They were launched on 2 March 1972 and 4 December 1973, respectively, with Pioneer 10 the first spacecraft to fly by the giant planet Jupiter, and Pioneer 11 the first to visit both Jupiter and Saturn. "After those encounters, we thought the mission was over," says the Pioneer astronomer John Anderson of Nasa's Jet Propulsion Laboratory (JPL) in Pasadena. "How wrong we were."

Anderson and his colleagues continued to track the probes using the giant dishes of Nasa's Deep Space Network. "By 1980, when Pioneer 10 was 20 times farther from the Sun than the Earth – halfway between Uranus and Neptune – it was clear that the probes were not where our calculations said they should be."

Both probes were feeling a tiny force in addition to gravity. In both cases, it was of the same magnitude and directed towards the Sun, despite the fact that Pioneer 10 and 11 were travelling in roughly opposite directions. Today, the probes are way beyond the outermost planet, Pluto.

For a long time, Anderson did not publicise the "Pioneer anomaly". Everything changed, however, in the mid-1990s, with the involvement of Michael Martin Nieto, a physicist at Los Alamos National Laboratory, New Mexico. Nieto was interested in how well we know gravity within the solar system. Someone told him to talk to Anderson, which he did. "When John told me the size of the Pioneer discrepancy, I almost fell off my chair," he says.

Nieto and Anderson began working together, joined by Slava Turyshev of the JPL. The team tracked down generation-old data about the space probes from retired Nasa personnel, and painstakingly examined it, trying to find a mundane explanation of the Pioneer anomaly.

An obvious possibility was a heat leak. It would require only 70 watts of heat escaping in a direction opposite to the Sun to push the 241kg probes sunward at the observed acceleration. But was this amount of heat available? Each probe carries a plutonium heat source on the end of a long boom. A measly 70 watts was being delivered to the body of each spacecraft. "To explain the anomaly, all of that would have to be radiated in one direction, which is very unlikely," says Anderson.

The other obvious possible explanation of the Pioneer anomaly is a fuel leak. But there is no obvious evidence from spacecraft data of any such leak. More seriously, a fuel leak would have come about by accident. "It's hard to imagine such a leak happening on both probes at the same time in such a way as to produce an identical acceleration," says Anderson.

According to Anderson, the most likely explanation is still some unknown but mundane effect. Other physicists agree. "For now, in my role as Old Grumpy, I'm just waiting for the whole thing to go away," says the Nobel prizewinner Steven Weinberg of the University of Texas at Austin. Some physicists, on the other hand, are willing to stick their necks out. "I think the acceleration anomaly is real," says Bernard Haisch of the California Institute for Physics and Astrophysics in Palo Alto.

One obvious – though dramatic – possibility is that Newton was wrong and that the force of gravity does not weaken with the well-known inverse-square law in the outer solar system, but drops away more slowly with distance. There is certainly evidence of such an effect on cosmic scales. For instance, the stars orbiting the centre of spiral galaxies like ours, and galaxies orbiting within galaxy clusters appear to be in the grip of stronger-than-expected gravity. The standard explanation is to postulate the existence of a huge amount of invisible, or "dark", matter that enhances the gravity. But an alternative, Modified Newtonian Dynamics (Mond), has been proposed by the Weizmann Institute's Mordehai Milgrom. "Maybe the Pioneer anomaly is the first hint of Mond," says Nieto.

The central difficulty facing non-mundane explanations of the Pioneer anomaly is that, whatever it is, it has to affect the Pioneers but not the planets. "I think this is precisely one of the things that puts people off unusual explanations – the universality of gravity is threatened," says Bruce Bassett of Portsmouth University.

Modified gravity apart, there is always the possibility that the Pioneer anomaly is due to invisible, or "dark", matter lurking in the solar system. There is evidence that cosmic dark matter outweighs visible stars and galaxies by at least a factor of 10, so it wouldn't be too surprising if some existed near us. "The problem is that dark matter influences ordinary matter through its gravity, so you need a lot of it to explain the Pioneer motions," says Nieto. "So much that it would affect the motion of the planets in a way that we just don't see."

There is a loophole, however. It requires the local dark matter to be in the form of a tenuous cloud of "mirror matter", a hypothetical kind of matter required to restore nature's flawed left-right symmetry. Crucially, there may exist a small, non-gravitational force between mirror atoms and ordinary atoms. According to the proponents of this idea, Robert Foot and Ray Volkas of the University of Melbourne, the drag caused by only a few Earth masses of mirror matter would be sufficient to explain the Pioneer retardation. Crucially, it would not have a noticeable effect on the orbits of the outer planets, Uranus and Neptune.

Perhaps the most exciting possibility is that the Pioneer anomaly is an effect that will "pop out" of a theory such as string theory that may one day "unify" the fundamental forces of nature. A good analogy is with general relativity, Einstein's theory of gravity. Once it was formulated, all sorts of unsuspected effects popped out, like light-bending by gravity and gravitational waves. "If there were a good theoretical idea suggesting an effect of the Pioneer kind, with a connection to fundamental physics it would be very intriguing," says Frank Wilczek of Princeton University.

One thing is for sure: we are not going to make much progress without more data. Unfortunately, other probes in the outer solar system, such as the Voyagers and the Cassini probe, on route to Saturn and Titan, can't provide any useful data. Ironically, they are too sophisticated. The Pioneers are "spin-stabilised", making them a particularly simple platform to understand. Later probes were stabilised about three axes by intermittent rocket boosts. The unpredictable accelerations caused by these are at least 10 times bigger than a small effect like the Pioneer acceleration, so they completely cloak it. "Pioneers are the only game in town," says Nieto. "Actually, it's worse – Pioneer 10 is the only game in town since we lost touch with 11 in 1995."

The only way to get to the bottom of the Pioneer anomaly, say Anderson, Nieto and Turyshev, is to launch a new space probe. They are therefore proposing a mission specifically to test non-gravitational effects in the outer solar system. A probe would cost about $300m-$500m. If there was a will, says Anderson, it could be launched within five years and get to where the anomaly should be visible, in another five years. "We'd start to get answers by 2012," he says.

Nieto is hopeful that funding will be found. As Bruce Bassett says: "Anyone aware of the importance of serendipity in scientific discoveries would be unwise to discard the possibility that it is the footprint of new physics."