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NASA

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 art 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, re cord 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.