THE veteran spacecraft hurtles towards the stars. Out at the darkest edge of the Solar System, far beyond Pluto, there should be nothing but the feeble gravity of the receding Sun to slow it down. Yet a mysterious extra force seems to be tugging on the spacecraft. And 24 billion kilometres away in the other direction, the probe’s twin is feeling an identical force.
Pioneers 10 and 11 may be old and battered, but in their twilight years they are giving physicists a few sleepless nights. The probes were launched in 1972 and 1973, and Pioneer 10 became the first spacecraft to fly past Jupiter. Pioneer 11 followed it past Jupiter and then became the first to visit Saturn. “After those encounters, we thought that essentially the mission was over,” says astronomer John Anderson of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena. “How wrong we were.”
NASA kept track of the probes to see if their trajectories might reveal the influence of a possible tenth planet or the predicted Kuiper asteroid belt. Microwave signals sent to each space probe were bounced back automatically by a transponder, and the Doppler shift in the wavelength of the returning signal revealed the probe’s velocity. And with frequent velocity measurements, their positions could be worked out. When the NASA engineers compared their actual positions with the predicted trajectories, things didn’t add up.
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“In the early days, we couldn’t be sure of the discrepancy because there was a bigger non-gravitational effect in the inner Solar System – the outward pressure of sunlight,” says Anderson. “But by 1980, when Pioneer 10 was halfway between Uranus and Neptune, it was clear the probes were not where our calculations said they should be.”
Pioneer 10 is experiencing a mysterious deceleration towards the Sun. Its magnitude is tiny: less than a nanometre per second per second, a mere ten-billionth of the gravity at Earth’s surface – but every year that’s passed has simply confirmed the persistence of the effect. And it adds up. Today, Pioneer 10 is 80 times as far from the Sun as Earth is, and it’s 400,000 kilometres behind schedule, roughly the distance between the Earth and the Moon. And although NASA lost touch with Pioneer 11 in 1995, up to that point it was experiencing exactly the same deceleration as its sister probe, also directed roughly towards the Sun.
For a long time, Anderson didn’t publicise the Pioneer anomaly. “We were convinced it had to be some mundane effect on board – a leak of fuel, perhaps, or heat,” he says. But in 1994, physicist Michael Martin Nieto from Los Alamos National Laboratory in New Mexico raised another possibility. Nieto noticed that on most cosmic scales we don’t know the effects of gravity with any real precision, and he began to wonder how precisely we understand gravity even within the Solar System. Don Yeomans of JPL suggested Nieto should talk to Anderson. He did, and got a big shock. “When John told me the size of the Pioneer discrepancy, I almost fell off my chair,” Nieto says. “To me it was huge.”
To Nieto, the mysterious deceleration sounded like it could be explained by some kind of modification of the laws of gravity. When predicting how the probes ought to move, NASA engineers assume that the only force acting on them is gravity from the Sun and planets. The basic equation they use goes right back to Newton: the strength of gravity falls off with the inverse square of distance. So if gravity behaves just a little bit differently, it could explain the Pioneer anomaly.
Nieto and Anderson began working together along with Slava Turyshev of JPL. Their first task was to rule out any mundane explanation such as a technical fault. Having first ruled out a software error, the team tracked down generation-old data about the space probes’ design from retired NASA personnel and painstakingly examined it. One possibility was a heat leak. Just 70 watts of radiation escaping in the direction away from the Sun would be enough to slow the 241-kilogram probes’ outward velocities at the observed rate. But do the probes have enough power left to be doing this?
Each one carries four radioisotope thermal generators, radioactive plutonium sources that originally generated about 2500 watts of heat. The generators could easily slow the probes by the amount observed if they were located at the front of each craft. But they aren’t. Engineers were worried that they would damage the probes’ instruments, so they stuck them out to the side of the spacecraft on the end of long booms.
So could the power delivered by the generators still end up as heat radiated forwards? Thermoelectric devices convert the original heat to electricity, which is carried to the main body of each spacecraft. But with the decay of the plutonium, and wear and tear on the converters, that only amounts to a measly 70 watts. “To explain the anomaly, all of that would have to be radiated in one direction,” says Anderson. And that’s all but impossible.
The other early idea, a fuel leak, was even easier to dismiss, as the probe’s internal sensors would have picked it up. And in any case it’s hard to imagine identical accidents producing identical leaks on both probes at the same time.
What, then, does that leave? According to Anderson, the most likely explanation is still some unsuspected error to do with the probes or the tracking system. “For the life of me, though, I can’t think of what it could be,” he says. But Nieto, though he still thinks that an error is most likely, hopes there is more to it. “I admit I want it to be something profoundly important, some entirely new physics.”
Could it really be that the force of gravity breaks the inverse square law in the outer Solar System? Intriguingly, there is evidence that that might happen on even larger, galactic scales. For instance, stars orbiting the centres of spiral galaxies like our own, and galaxies orbiting within galaxy clusters, appear to be moving too fast, as if in the grip of stronger than expected gravity. The standard answer is to postulate huge amounts of invisible “dark matter” exerting its own gravitational influence. But Mordehai Milgrom of the Weizmann Institute in Israel has proposed an alternative explanation, known as modified Newtonian dynamics. In MOND, the inverse square law only applies where the gravity is relatively strong. Where it is very weak, gravity fades more slowly with distance.
The physics community is sceptical about MOND because any evidence for it is largely empirical. “There is no fundamental theory beneath it,” says Nieto. Yet it could explain the Pioneer results. The only problem is, why doesn’t it affect the orbits of the planets too – especially the outer planets, Uranus, Neptune and Pluto? That’s the central difficulty facing any explanation of the Pioneer mystery, says Nieto. But here he makes an observation. Unlike the planets, which are gravitationally bound within the Solar System by the Sun, the Pioneer probes are on escape courses. “Could it be that there’s something that affects unbound bodies but not gravitationally bound bodies like the planets?”
Look in the mirror
It’s always possible that the anomaly is evidence for dark matter after all. If there is dark matter scattered throughout our Galaxy, it could be slowing the Pioneers down. Most of the hypothesised kinds of dark matter couldn’t do this strongly enough to explain the anomaly, because they only affect ordinary matter through the feeble force of gravity. But there is one theoretical possibility: “mirror matter”.
Mirror matter would be like ordinary matter, except at the quantum level it is opposite to matter in the way a right hand is opposite to a left hand. Since it would interact only very weakly with ordinary matter, it could exist all around us without our knowing. However, there may be a small, non-gravitational force between mirror atoms and ordinary atoms (Â鶹´«Ă˝, 17 June 2000, p 36), and according to Robert Foot and Ray Volkas of the University of Melbourne, the drag caused by only a few Earth-masses of mirror matter spread throughout the Solar System would be enough to explain the Pioneer measurements. And crucially, it would not have a noticeable effect on the orbits of the planets. Unlike the featherweight Pioneers, the massive planets could plough through a mirror-matter headwind almost completely unperturbed.
MOND and mirror matter are just the start of it. Perhaps not surprisingly, proponents of all sorts of speculative theories have latched onto the Pioneer anomaly, suggesting ways that their own theories can explain it.
For example, Bernard Haisch of the California Institute for Physics and Astrophysics in Palo Alto has a radical alternative. Rather than unexpectedly strong gravity, the slowing could be due to a reduction in inertial mass. “If the inertia of the Pioneer spacecraft is slightly less that it should be, then the same gravitational force would give an anomalous Sun-directed acceleration,” he says. In Haisch’s theory, inertia is not an inherent property of an object, but a kind of drag produced by short-lived “virtual” particles predicted by quantum mechanics (Â鶹´«Ă˝, 3 February 2001, p 22). But it’s not clear why these virtual particles should behave differently as you get further from the Sun.
Some other ideas are more tentative still, not so much attempts to explain the anomaly as possible links with other oddities in modern physics. Could the Pioneer puzzle be linked in some way with the acceleration in the expansion of the Universe? A kind of universal energy called quintessence is one theory to explain the accelerated expansion, and Bruce Bassett of the University of Portsmouth wonders whether it could also affect the Pioneers somehow (though it would have to work in the opposite sense from its effects on the Universe, decelerating one while accelerating another). Bassett also suggests that the Pioneer conundrum might just be linked to another big puzzle in physics: the apparent variation of something called the fine-structure constant. This number describes how strong the electromagnetic force is, and thereby how strongly light interacts with matter. By observing light from distant quasars, a team led by John Webb at the University of New South Wales has found evidence that the fine-structure constant was slightly different billions of years ago (Â鶹´«Ă˝, 11 May, p 28). “It’s possible that whatever caused variation of the fine-structure constant in the distant past might have a present-day manifestation,” says Bassett.
The fact that serious physicists must resort to vague speculations may be telling. Perhaps the Pioneer mystery is something we’re not equipped to explain; perhaps we’ll only understand it via some as yet unformulated theory that unifies the fundamental forces of nature. A good analogy is general relativity, Einstein’s theory of gravity. Once it was formulated, all sorts of unexpected things popped out, such as gravitational waves and an explanation for shifts in Mercury’s orbit.
In the meantime, the only way to whittle down the possibilities is to get better measurements of the effect. Other space probes in the outer Solar System, such as the Voyagers and the Cassini probe, en route to Saturn and Titan, are no use because their propulsion systems are too sophisticated. The orientations of the Pioneers were stabilised by setting them spinning, whereas the later probes use intermittent rocket boosts to keep them correctly aligned. The unpredictable accelerations caused by these spurts are at least 10 times as big as the Pioneer acceleration, and make it impossible to measure the effect.
To get to the bottom of the problem, Anderson, Nieto and Turyshev are proposing a new mission specifically to test non-standard gravitational effects in the outer Solar System. Ideally, says Nieto, the probe should look for acceleration in four specific directions. If the acceleration is along the probe’s spin axis, it must be something relatively mundane to do with the spacecraft; if it is along the probe’s trajectory, that would imply some kind of drag, perhaps due to mirror matter. If the acceleration is towards the Sun, it might imply a modification of gravity. And finally, if it is towards the Earth, it could indicate that time is running increasingly more slowly on the Pioneers – what you might call an acceleration of time.
Flipping the axis of the spinning spacecraft by 180° would create an opposite acceleration if it were due to something on board. It would also be interesting to see if the anomalous acceleration could be detected by an on-board accelerometer, Weinberg says. If not, it would mean the gravitational field within the Solar System was misbehaving. And if the acceleration were detected, it would have to be some non-gravitational force.
Such a probe would cost between $300 million and $500 million. It could be launched within five years, says Anderson, and reach the zone where the anomaly should be detectable in perhaps another five years. But Nieto admits that neither NASA nor the European Space Agency is likely to fund a probe solely to examine this puzzle. So one possibility is that it could be incorporated into the proposed Pluto/Kuiper Belt mission to the outer reaches of the Solar System. The drawback is the other instruments on board. “That would make it more difficult to discern the effect,” says Nieto. “However, we might be able to live with that if we were able to give input into the physical design of the spacecraft from the beginning.”
So will it happen? Nieto is hopeful. “I’m sure the mission will get studied,” he says. Part of the reason for his confidence is that physicists are increasingly interested in the Pioneer anomaly. Already this year, the journal Physics Review D has published a 50-page paper by Anderson and his colleagues reviewing all the possible explanations. It’s an unusually long paper for the journal. “The anomalous acceleration of Pioneer is indeed extremely interesting,” says Bassett. “Anyone aware of the importance of serendipity in scientific discoveries would be unwise to discard the possibility that it really is the footprint of new physics.”
