NASA launched sister spacecrafts Pioneer 10 and Pioneer 11 in 1972 and '73, respectively. Their joint mission was to gather information about the asteroid belt, as well as Jupiter, Saturn, and their moons. As the probes hurtled past these objects, they measured many properties of their atmospheres and surfaces; they also took beautiful and now famous photographs of Jupiter's red spot and Saturn's rings.
Then, after the Pioneers completed their "flyby" missions, they kept going. Carrying identical plaques depicting a man and a woman, the atomic transition of hydrogen, and the location of the Sun and Earth within the galaxy--a message to aliens--they are now exiting the solar system in opposite directions: 10 heads toward the constellation Taurus, and 11 aims for Aquila.
In 1995, 11 sent its last blip of data back to Earth. 10 stayed in contact longer, sending sporadic bursts of signal until 2003. After their lines went dead and all was said and done, the Pioneers had indeed lived up to their family name. They provided decades of data for scientists to analyze, and from them we have learned a great deal about distant regions of the solar system.
But as NASA scientists sifted through the last signals sent by the spacecrafts from deep space, a problem emerged. It seemed minor at first - just a small discrepancy between the incoming data and the engineers' predictions, which (I assume) must happen occasionally at NASA. But no amount of rechecking the data or reevaluating predictions could force the two to sync up. The divergence wouldn't go away, because it reflected the divergence of the Pioneer spacecrafts themselves.
On their way out of the solar system, as they struggled through the gravitational field of the Sun and planets, the Pioneers were slowing down. Of course NASA expected this: they had made staggeringly precise calculations of the gravitational pull on the spacecrafts at every point in space, and they knew the exact rate at which the probes ought to decelerate during their uphill climb against gravity. The problem was, 10 and 11 were decelerating too much. Each year, they were 5,000 kilometers farther behind where they should have been on their respective paths. 5,000 kilometers is very little in the context of space travel, to be sure, but it isn't trivial. Some additional, undeniable force is pulling the probes inward toward the Sun, a force about 10 billion times weaker than gravity, and we have no idea what it is.
The scientific community caught wind of the so-called Pioneer anomaly in the late 90s, and in the years since, they have studied, theorized, and passionately argued about it via two international conferences and hundreds, or possibly thousands, of academic papers.
Why do they care so much? Well, the thinking is this: the anomalous effect could stem from errors in the machinery of the Pioneers, such as gas leaks, asymmetrical solar heating, or an unaccounted-for force exerted backwards by the probes' radio transmissions. Theoretically, these types of issues could throw things off course, and if NASA plans to execute deep space missions in future, they need to figure out what's going on.
But as engineers go over every detail of the spacecraft designs, analyze every possible source of error, and find no problem, the erroneous machinery explanation seems less and less likely. Alternatively and much more seriously, physics itself, not the spacecrafts, might be broken. The Pioneer anomaly could be our first clue of a mysterious, extremely weak force that exists in the Universe.
Many scientists who've dedicated their careers to contemplating the Pioneer anomaly think NASA ought to plan a mission specifically aimed to test it. If a spacecraft with an entirely different design and instrumentation shows the same lagging effect in deep space as the Pioneers, then the "new physics" explanation of the anomaly moves forward. If a new spacecraft shows no anomalous motion, then the broken Pioneers idea gets a boost.
Until such a mission, scientific fascination mixed with psychological confusion will continue to surround the Pioneer anomaly. Revolutions in physics often stem from results that subtly deviate from predictions, like when the precession of the perihelion of Mercury led to the overthrow of Newtonian mechanics by Einstein's theory of general relativity. The seed of revolution may be germinating once again. But the maddening thing is, maybe it's not!