That last explanation would have been big news indeed. Much of what we know about the universe — for example, the existence of dark matter, which seems to swaddle and shape the galaxies, and of dark energy, which seems to be speeding up the expansion of the universe — comes from presuming that Einstein’s General Theory of Relativity, which describes gravity as the warping of space-time geometry — is correct over cosmic distances.

General relativity has passed every test on Earth. Without correcting for it, GPS systems would not work. But some theorists have suggested that if gravity behaved differently over large distances from what Einstein thought, it would relieve astronomers of the embarrassing need to posit that 96 percent of the universe consists of various kinds of unknown dark stuff. A similar, but larger, kind of deviation from Einsteinian theory could explain the Pioneer anomaly, as it is called.

Pioneers 10 and 11 were launched in 1972 and 1973, respectively, and are now both about 10 billion miles out. They were last heard from in 2003, when the radio signal from Pioneer 10 got too weak to be detected. They were the first spacecraft to go past Jupiter and Saturn (though their biggest impact on pop culture until then might have been a controversy over the nude human figures on a plaque designed for the benefit of any distant aliens who might find them).

In 1998, however, when John D. Anderson of the Jet Propulsion Laboratory and his colleagues discovered that the spacecraft were running a little late on their timetable to eternity, it seemed as if general relativity might be up for grabs — allowing the news media to ask their favorite science question: Was Einstein wrong? There was talk of a special deep space probe whose only mission would be to track its own movements.

The effect was slight — slowing the spacecraft by about 300 miles a year — but the crack interplanetary navigators at J.P.L., who can slip a probe through Saturn’s rings or buzz the moons of Jupiter, take great pride in their knowledge of the forces and foibles of the solar system.