Natural History Magazine
Something unexpected has been happening to a pair of distant space probes. Could strange new physics be the cause?
There’s no sweeping it under the rug. NASA’s twin Pioneer 10 and Pioneer 11 space probes, launched in the early 1970s and headed for stars in the depths of our galaxy, are both experiencing a mysterious, continuous force that is altering their expected trajectories. Calculations say the Pioneers should each be in a particular place, but the probes themselves have told us they’re each someplace else—as much as a quarter million miles closer to the Sun than they’re supposed to be.
That mismatch, known as the Pioneer anomaly, first became evident in the early 1980s, by which time the spacecraft were so far from the Sun that the slight outward pressure of sunlight no longer exerted significant influence over their velocity. Scientists expected that Newtonian gravity alone would thenceforth account for the pace of the Pioneers’ journey. But things seemingly haven’t turned out that way. The extra little push from solar radiation had been masking an anomaly. Once the Pioneers reached the point where the sunlight’s influence was less than the anomaly’s, both spacecraft began to register an unexplained, persistent change in velocity—a sunward force, a drag—operating at the rate of a couple hundred-millionths of an inch per second for every second of time the twins have been traveling. That may not sound like much, but it eventually claimed thousands of miles of lost ground for every year out on the road.
Contrary to stereotype, research scientists don’t sit around their offices smugly celebrating their mastery of cosmic truths. Nor are scientific discoveries normally heralded by people in lab coats proclaiming
Eureka! Instead, researchers say things like
Hmm, that’s odd. From such humble beginnings come mostly dead ends and frustration, but also an occasional new insight into the laws of the universe.
Once the Pioneer anomaly revealed itself, scientists said,
Hmm, that’s odd. So they kept looking, and the oddness didn’t go away. Serious investigation began in 1994, the first research paper about it appeared in 1998, and since then all sorts of explanations have been proffered to account for the anomaly. Contenders that have now been ruled out include software bugs, leaky valves in the midcourse-correction rockets, the solar wind interacting with the probes’ radio signals, the probes’ magnetic fields interacting with the Sun’s magnetic field, the gravity exerted by newly discovered Kuiper Belt objects, the deformability of space and time, and the accelerating expansion of the universe. The remaining explanations range from the everyday to the exotic. Among them is the suspicion that in the outer solar system, Newtonian gravity begins to fail.
The very first spacecraft in the Pioneer program—Pioneer 0 (that’s right, “zero”)—was launched, unsuccessfully, in the summer of 1958. Fourteen more were launched over the next two decades. Pioneers 3 and 4 studied the Moon; 5 through 9 monitored the Sun; 10 flew by Jupiter; 11 flew by Jupiter and Saturn; 12 and 13 visited Venus.
Pioneer 10 left Cape Canaveral on the evening of March 2, 1972—nine months before the Apollo program’s final Moon landing—and crossed the Moon’s orbit the very next morning. In July 1972 it became the first human-made object to traverse the asteroid belt, the band of rocky rubble that separates the inner solar system from the giant outer planets. In December 1973 it became the first to get a “gravity assist” from massive Jupiter, which helped kick it out of the solar system for good. Although NASA planned for Pioneer 10 to keep signaling Earth for a mere twenty-one months, the craft’s power sources kept going and going—enabling the fellow to call home for thirty years, until January 22, 2003. Its twin, Pioneer 11, had a shorter signaling life, with its final transmission arriving on September 30, 1995.
At the heart of Pioneers 10 and 11 is a toolbox-size equipment compartment, from which booms holding instruments and a miniature power plant project at various angles. More instruments and several antennas are clamped to the compartment itself. Heat-responsive louvers keep the onboard electronics at ideal operating temperatures, and there are three pairs of rocket thrusters, packed with reliable propellant, to help with alignments and midcourse corrections en route to Jupiter.
Power for the twins and their fifteen scientific instruments comes from radioactive chunks of plutonium-238, which drive four radioisotope thermoelectric generators, sensibly abbreviated RTGs. The heat from the slowly decaying plutonium, with its half-life of eighty-eight years, yielded enough electricity to run the spacecraft, photograph Jupiter and its satellites in multiple wavelengths, record sundry cosmic phenomena, and conduct experiments more or less continuously for upwards of a decade. But by April 2001 the signal from Pioneer 10 had dwindled to a barely detectable billionth of a trillionth of a watt. The probes’ main agent of communication is a nine-foot-wide, dish-shaped antenna pointed toward Earth. To preserve the antenna’s alignment, each spacecraft has star and Sun sensors that keep it spinning along the antenna’s central axis in much the way that a quarterback spins a football around its long axis to stabilize the ball’s trajectory. For the duration of the dish antenna’s prolonged life, it sent and received radio signals via the Deep Space Network, an ensemble of sensitive antennas that span the globe, making it possible for engineers to monitor the spacecraft without a moment’s interruption.
The famous finishing touch on Pioneers 10 and 11 is a gold-plated plaque affixed to the side of the craft. The plaque includes an engraved illustration of a naked adult male and female; a sketch of the spacecraft itself, shown in correct proportion to the humans; and a diagram of the Sun’s position in the Milky Way, announcing the spacecraft’s provenance to any intelligent aliens who might stumble across one of the twins. (I’ve always had my doubts about this cosmic calling card. Most people wouldn’t give their home address to a stranger in the street, even when the stranger is one of our own species. Why, then, give our home address to aliens from another planet?)
Space travel involves a lot of coasting. Typically, a spacecraft relies on rockets to get itself off the ground and on its way. Other, smaller engines may fire en route to refine the craft’s trajectory or pull the craft into orbit around a target object. In between, it simply coasts. [See “Universe” for June 2005, “Fueling Up,” and July–August 2005, “Heading Out.”] For engineers to calculate a craft’s Newtonian trajectory between any two points in the solar system, they must account for every single source of gravity along the way, including comets, asteroids, moons, and planets. As an added challenge, they must aim for where the target should be when the spacecraft is due to arrive, not for the target’s current location.
Calculations completed, off went Pioneers 10 and 11 on their multibillion-mile journeys through interplanetary space—boldly going where no hardware had gone before, and opening new vistas on the planets of our solar system. Little did anyone foresee that in their twilight years the twins would also become unwitting probes of the fundamental laws of gravitational physics.
Astrophysicists do not normally discover new laws of nature. We cannot manipulate the objects of our scrutiny. Our telescopes are passive probes that cannot tell the cosmos what to do. Yet they can tell us when something isn’t following orders. Take the planet Uranus, whose discovery is credited to the English astronomer William Herschel and dated to 1781 (others had already noted its presence in the sky but misidentified it as a star). As observational data about its orbit accumulated over the following decades, people began to notice that Uranus deviated slightly from the dictates of Newton’s laws of gravity, which by then had withstood a century’s worth of testing on the other planets and their moons. Some prominent astronomers suggested that perhaps Newton’s laws begin to break down at such great distances from the Sun.
What to do? Abandon or modify Newton’s laws and dream up new rules of gravity? Or postulate a yet-to-be-discovered planet in the outer solar system, whose gravity was absent from the calculations for Uranus’s orbit? The answer came in 1846, when astronomers discovered the planet Neptune just where a planet had to be for its gravity to perturb Uranus in just the ways measured. Newton’s laws were safe… for the time being.
Then there’s Mercury, the planet closest to the Sun. Its orbit, too, habitually disobeyed Newton’s laws of gravity. Having predicted Neptune’s position on the sky within one degree, the French astronomer Urbain-Jean-Joseph Le Verrier now postulated two possible causes for Mercury’s deviant behavior. Either it was another new planet (call it Vulcan) orbiting so close to the Sun that it would be well-nigh impossible to discover in the solar glare, or it was an entire, uncatalogued belt of asteroids orbiting between Mercury and the Sun.
Turns out Le Verrier was wrong on both counts. This time he really did need a new understanding of gravity. Within the limits of precision that our measuring tools impose, Newton’s laws behave well in the outer solar system. However, they break down in the inner solar system, where they are superseded by Einstein’s general relativity. The closer you are to the Sun, the less you can ignore the exotic effects of its powerful gravitational field.
Two planets. Two similar-looking anomalies. Two completely different explanations.
Pioneer 10 had been coasting through space for less than a decade and was around 15 AU from the Sun when John D. Anderson, a specialist in celestial mechanics and radiowave physics at NASA’s Jet Propulsion Laboratory (JPL), first noticed that the data were drifting away from the predictions made by JPL’s computer model. (One AU, or astronomical unit, represents the average distance between Earth and the Sun; it’s a “yardstick” for measuring distances within the solar system.) By the time Pioneer 10 reached 20 AU, a distance at which pressure from the Sun’s rays no longer mattered much to the trajectory of the spacecraft, the drift was unmistakable. Initially Anderson didn’t fuss over the discrepancy, thinking the problem could probably be blamed on either the software or the spacecraft itself. But he soon determined that only if he added to the equations an invented force—a constant change in velocity (an acceleration) back toward the Sun for every second of the trip—would the location predicted for Pioneer 10’s signal match the location of its actual signal.
Had Pioneer 10 encountered something unusual along its path? If so, that could explain everything. Nope. Pioneer 11 was heading out of the solar system in a whole other direction, yet it, too, required an adjustment to its predicted location. In fact, Pioneer 11’s anomaly is somewhat larger than Pioneer 10’s.
Faced with either revising the tenets of conventional physics or seeking ordinary explanations for the anomaly, Anderson and his JPL collaborator Slava Turyshev chose the latter. A wise first step. You don’t want to invent a new law of physics to explain a mere hardware malfunction.
Because the flow of heat energy in various directions can have unexpected effects, one of the things Anderson and Turyshev looked at was the spacecraft’s material self—specifically the way heat would be absorbed, conducted, and radiated from one surface to another. Their inquiry managed to account for about a tenth of the anomaly. But neither investigator is a thermal engineer. A wise second step: find one. So in early 2006 Turyshev sought out Gary Kinsella, a JPL colleague who until that moment had never met either him or a Pioneer face to face, and convinced Kinsella to take the thermal issues to the next level. Last spring, all three men came to the Hayden Planetarium in New York City to tell a sellout crowd about their still-unfinished travails. Meanwhile, other researchers worldwide have been taking up the challenge too.
Consider what it’s like to be a spacecraft living and working hundreds of millions of miles from the Sun. First of all, your sunny side warms up while the unheated hardware on your shady side can plunge to 455° below zero Fahrenheit, the background temperature of outer space. Next, you’re constructed of many different kinds of materials and have multiple appendages, all of which have different thermal properties and thus absorb, conduct, emit, and scatter heat differently, both within your various cavities and outside to space. In addition, your parts like to operate at very different temperatures: your cryogenic science instruments do fine in the frigidity of outer space, but your cameras favor room temperature, and your rocket thrusters, when fired, register 2,000° F. Not only that, every piece of your hardware sits within ten feet of all your other pieces of hardware.
The task facing Kinsella and his team of engineers was to assess and quantify the directional thermal influence of every feature on board Pioneer 10. To do that, they created a computer model representing the spacecraft surrounded by a spherical envelope. Then they subdivided that surface into 2,600 zones, enabling them to track the flow of heat from every spot in the spacecraft to and through every spot in the surrounding sphere. To strengthen their case, they also hunted through all available project documents and data files, many of which hail from the days when computers relied on punch cards for data entry and stored data on nine-track tape. Without emergency funds from the Planetary Society, by the way, those irreplaceable archives would shortly have ended up in a dumpster.
For the simulated world of the team’s computer model, the spacecraft was placed at a test distance from the Sun (25 AU) and at a specific angle to the Sun, and all the parts were presumed to be working as they were supposed to. Kinsella and his crew determined that, indeed, the uneven thermal emission from the spacecraft’s exterior surfaces does create an anomaly—and that it is indeed a continuous, sunward change in velocity.
But how much of the Pioneer anomaly can be chalked up to this thermal anomaly? Some. Perhaps even most. But not all. The team’s thermal model was based solely on the trajectory and hardware data from Pioneer 10, which displays a smaller anomaly than that of Pioneer 11. Not only that, the researchers have yet to calculate how the thermal anomaly varies with Pioneer 10’s (let alone Pioneer 11’s) distance from the Sun.
So what about the as-yet-unexplained “not all” portion? Do we sweep it under the cosmic rug in hopes that additional Kinsellan analysis will eventually resolve the entire anomaly? Or do we carefully reconsider the accuracy and inclusiveness of Newton’s laws of gravity, as a few zealous physicists have been doing for a couple of decades?
Pre-Pioneers, Newtonian gravity had never been measured—and was therefore never confirmed—with great precision over great distances. In fact, Slava Turyshev, an expert in Einstein’s general relativity, regards the Pioneers as (unintentionally) the largest-ever gravitational experiment to confirm whether Newtonian gravity is fully valid in the outer solar system. That experiment, he contends, shows it is not. As any physicist can demonstrate, beyond 15 AU the effects of Einsteinian gravity are negligible. So, at the moment, two forces seem to be at play in deep space: Newton’s laws of gravity and the mysterious Pioneer anomaly. Until the anomaly is thoroughly accounted for by misbehaving hardware, and can therefore be eliminated from consideration, Newton’s laws will remain unconfirmed. And there might be a rug somewhere in the cosmos with a new law of physics under it just waiting to be uncovered.