The famous finishing touch on Pioneers 10 and 11 is a gold-plated plaque affixed to tilt side of tile 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 space craft's provenance to any intelligent aliens who might stumble across one of the twins. (I've always had nay 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 tile ground and oil its way. Other, smaller engines may fire on route to refine the craft's trajectory or pull the craft into orbit around a target object. In between, it simply coasts. 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 tilt 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 tilt 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 art 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 ill tile sky but misidentified it as a star). As observational data about its orbit accummulated over the following de cades, 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, uncataloged 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 radio-wave 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, requited 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 Turyshey chose the latter. A wise first step. You don't want to invent a new law of physics to explain a mere hardware malfunction.