SLAVA TURYSHEV is a man on a mission. Two missions in fact. The researcher from NASA’s Jet Propulsion Laboratory in Pasadena, California, is about to re-fly the two most controversial spacecraft in history. The Pioneer 10 and 11 probes were launched in 1972 and 1973 and are now drifting in deep space beyond the outermost planets of the solar system. NASA lost contact with Pioneer 10 in 2003, but though the probes are now long gone, they are anything but forgotten. That’s because Pioneer 10, when last heard from, seemed to be off course by around 400,000 kilometres and nobody has ever been able to figure out why. Did some malfunction nudge the craft off its expected trajectory, or are there deeper forces at work?
Pioneer 10’s sister ship is also being pushed off course, and an analysis of the Galileo spacecraft to Jupiter and the Ulysses solar probe hints that they are affected by a similar anomaly. This has led some physicists to suggest that the Pioneer probes may have exposed a fundamental flaw in our understanding of how objects move through space under the influence of gravity. Turyshev’s mission might be the best chance we have to see if our best theory of gravity, Einstein’s general theory of relativity, needs an upgrade.
Turyshev will not, however, be launching two replica craft into space. His missions will be virtual ones, in which he will replay the data from the real flights to see if he can detect where and how things started going wrong.
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The project nearly failed at the first hurdle. Though the Pioneers have been flying for more than 30 years, the anomaly in their trajectories is based on observations from only a small portion of their journeys: 11.5 years for Pioneer 10 and just 3.75 years for Pioneer 11. Turyshev needed the data for the rest of the flights, but when he went in search of it he found that the magnetic tapes and floppy discs had gone astray. After much searching, all he could find were 400 reels of magnetic tape, stored in boxes under a staircase at JPL. The tapes contained the full 30 years’ worth of measurements about the spacecrafts’ trajectories, but there was a snag. NASA was about to scrap the outdated computers that could read the tapes. Alarmed by NASA’s decision, a campaign group in Pasadena called The Planetary Society lobbied its members to raise money to save the computers. And that was not the end of Turyshev’s problems.Years of heat and humidity had started to take their toll on the tapes, and he feared they might have become unreadable.
“This is the first time in history that a space mission has been reconstructed”
Luckily, he knew someone who could help. Software engineer Kyong Lee at JPL was able to coax the tapes into revealing their precious data, which he re-recorded on DVD. Last year Turyshev had another stroke of luck when he visited NASA’s Ames Research Center in Moffett Field, California. “I had gone to Ames to give a lecture, specifically to interest people in investigating the Pioneer anomaly,” he says. He knew that sitting somewhere in the research complex were the project documents, including records from 114 onboard sensors that recorded the spacecrafts’ manoeuvres and spin rate, as well as “housekeeping” data. Throughout the Pioneers’ three-decade voyages, the craft beamed back this telemetry data to Earth so that engineers on the ground could monitor each spacecraft’s vital signs. All this data had been painstakingly stored on floppy discs by mission engineer Larry Kellogg, who figured that it could one day be used to construct a computer program to teach children how a real spacecraft operated.
With access to both the tracking measurements and telemetry data, Turyshev was confident that he had everything he would need to build a virtual model of the spacecraft, and investigate whether a malfunction was causing the anomaly. His optimism was short-lived, though. Straight after the talk, managers at Ames told him they had earmarked the data discs for destruction because they were taking up too much space. The first dumpster had already been delivered and was sitting outside waiting to be filled. In desperation, Turyshev offered to return at the weekend with a truck of his own to collect the material. He had no idea where he was going to store it, but he knew he could not allow this priceless information to be destroyed. In the end, the Ames managers relented, and found a place to store the 60 filing cabinets’ worth of data.
The next problem was how to read the obsolete discs. Kellogg introduced Turyshev to Viktor Toth, a C++ programmer and space enthusiast living in Ottawa, Canada. Toth volunteered to write a program that would extract the 40 gigabytes of data from the old floppies and record it onto DVDs. That has now been done, and the data is about to be released to groups throughout the world so that the virtual re-flight of Pioneer can begin.
Several teams will pore through the telemetry data, watching for every firing of the thrusters and every malfunction, no matter how slight. Each event will then be checked against the tracking data, which tells them how fast and in what direction the spacecraft is moving. In this way they will be able to reconstruct, moment by moment, the condition and movement of the spacecraft (see “Anatomy of an anomaly”). The job will take at least a year to complete. There are nearly 95,000 measurements to correlate for Pioneer 10 alone, covering 29 years of the mission during which the spacecraft travelled more than 12 billion kilometres (see Diagram). Nothing similar has ever been attempted before, Turyshev says. “This is the first time in history that a space mission will be reconstructed.”
Constant force
The investigators are faced with a painstaking task, as the malfunction they are looking for is likely to be a tiny one. “The leak of a single molecule of gas from the spacecraft will give a momentary acceleration similar in size to the Pioneer anomaly,” says Dario Izzo, a mission analyst from the European Space Agency’s Advanced Concepts Team in Noordwijk, the Netherlands. Thankfully, the force acting on the craft is a persistent one, so if a malfunction of the spacecraft’s systems is the cause it must be long-lived too.
With this in mind, the teams plan to concentrate their attention on the prime suspect for the anomaly: the radioisotope thermoelectric generator (RTG) on each Pioneer probe. The RTGs, which convert heat from the radioactive decay of plutonium-238 into electricity to power the spacecraft, are held on a boom away from the spacecraft’s body. Even from here a little waste heat from the generator might reach the probe. This could raise the temperature on one side of it, producing a feeble thrust as the energy radiates away into space. Turyshev has high hopes that the telemetry data will allow the researchers to study the impact of the RTG for the first time. “The plutonium’s half-life is 87.7 years, and I believe that we will able to see this decay in the telemetry and account for its effects,” he says. Once researchers have done that, and other checks, whatever remains is the Pioneer anomaly. “We may end up with a smaller Pioneer anomaly than at present – or better with nothing at all,” Turyshev says.
But what if the teams convince themselves that there is no onboard explanation for the anomaly? Their next step then will be to measure the precise direction in which the force is acting. The best we can say at the moment is that it seems to point back towards the sun, but that is only because the limited measurements and analysis available till now make the Earth’s direction virtually indistinguishable from the sun’s. Signals from the earlier part of the mission should resolve this, because the difference between the sun’s and the Earth’s directions should be more noticeable in measurements taken when the spacecraft was closer to Earth. If the anomalous force points towards the Earth, it will mean that it is an artefact of the way the tracking stations on Earth collected the signals, and the mystery of the Pioneer anomaly will be solved.
Turyshev’s effort comes at a time of growing interest in the anomaly. Two years ago, when ESA asked researchers to submit ideas for missions to launch between 2015 and 2025, several of the proposals were for experiments to investigate the Pioneer anomaly. This led Izzo and his team at ESA to analyse various strategies to study it. “If I had the money I would launch a mission just to study the gravitational field of the outer solar system,” he says. The central component of the mission would be a small sphere of precisely known mass with a highly reflective surface. The sphere would move as gravity dictates, along a trajectory known as a geodesic. A conventional spacecraft would follow, firing a laser beam at the sphere’s reflective surface to track its movements. As well as investigating the Pioneer anomaly, such a mission would be the most precise test yet of general relativity.
Planetary puzzle
Izzo concedes, though, that this is probably not going to happen, at least not as the dedicated mission he would like. “I have to be realistic,” he says, and that means piggybacking the mission onto something that will tour the outer solar system. He and his collaborator Andreas Rathke at EADS Astrium in Germany propose doing this on ESA’s next mission to Jupiter or one of the other outer planets.
Not everyone thinks the Pioneer anomaly deserves the attention it is getting. Lorenzo Iorio of the Italian National Institute for Nuclear Physics in Bari points out that if the anomaly is due to gravity, then it should act on the planets as well as spacecraft. Yet the outer planets are moving entirely as expected, Iorio says, so the anomaly must arise from a fault with the Pioneer craft. Another doubter is Daniel Whitmire, an astronomer at the University of Louisiana, Lafayette. In a study in 2002 of the behaviour of comets from the farthest realms of the solar system, he failed to observe any strange gravitational effects. He is also quick to dismiss a suggestion by Gary Page of George Mason University in Fairfax, Virginia, that the unexplained disappearance of an asteroid called 1995SN55 occurred because it was pulled off course by the same forces that act on the Pioneer craft. None of this persuades Izzo to give up the hunt. “There are many complicating factors and uncertainties in planetary orbits that will mask the Pioneer anomaly,” he says.
The achievements of Pioneers 10 and 11 as the first missions to visit Jupiter and Saturn are already being overshadowed by the mystery of their unexpected trajectory through deep space, but with the stakes so high, even those who found the Pioneer anomaly are determined to remain sceptical. “I’m trying to remain open-minded,” Turyshev says. “That is why I want to reanalyse the data from the full Pioneer 10 and 11 missions to see if we can explain the anomaly with things that were happening on the spacecraft.” If after that the anomaly persists, it might be time to start thinking the unthinkable: that two distant spacecraft have provided the first clear evidence for a breakdown in Einstein’s theory of gravity.
Retro rockets
Pioneers 10 and 11 were ideally suited to detecting the anomaly because of their relatively simply design. Launched on 2 March 1972 and 5 April 1973 respectively to explore the outer solar system, they relied on angular momentum to keep them stable in the same way as a child’s spinning top stays upright when it is rotating. This spinning motion means that mission scientists hardly ever needed to fire the thrusters. So the Pioneers spent months at a time moving solely under the influence of the solar system’s gravitational field.
By the time NASA launched the pair of Voyager spacecraft in the early 1980s, a technique called three-axis stabilisation had been developed. This compares a snapshot of the position of the stars with accurate maps to watch for movements of the spacecraft. Small thrusters are then used to nudge the spacecraft back into position every week or so. By maintaining a fixed orientation in space, it is easier to point cameras and other instruments at their targets.
However, constantly using the thrusters overwhelms any Pioneer-like anomaly in the Voyager spacecraft. Any mission to investigate the anomaly will have to return to using the old-fashioned spin stabilisation method.
Anatomy of an anomaly
The “Pioneer anomaly” first showed up in the late 1970s when John Anderson at NASA’s Jet Propulsion Laboratory and others began analysing the trajectory of Pioneer 10 in the hope of finding a previously undiscovered planet in the outer solar system. They reasoned that the gravity of such a planet would pull their little spacecraft off course by a minute but noticeable amount. And to their delight, they found a tiny signal, equivalent to a force 10 billion times weaker than Earth’s gravity.
Yet there was a problem. Instead of pointing into the dark reaches of the outer solar system where they expected any undiscovered planet to lurk, the force pointed back towards the inner solar system. It was as if the sun’s gravity were tugging the spacecraft a little bit harder than expected.
Most astronomers think the drift is the result of a minor malfunction on the spacecraft or an illusion created by a fault in one the tracking stations on Earth. Even those who discovered the anomaly are working under this assumption. But while its source remains unexplained, no one can be certain.
