Two space missions launched in the 1970s continued to astound scientists this year: After touring the outer planets, Voyager 1 neared the boundary of interstellar space, while Pioneer 10 and 11 put our understanding of gravity to the test.
For years scientists scratched their heads over the “Pioneer anomaly”: Radio signals from the twin spacecraft, which are no longer in contact with Earth, showed they were decelerating more rapidly than could be explained solely by the pull of the sun or other known physical effects. Some theorists suggested that the slowdown might indicate a problem with the laws of gravity set down by Isaac Newton and Albert Einstein. Jet Propulsion Laboratory astrophysicist Slava Turyshev began an investigation and in June reported his findings. He concluded that the probes’ generators were probably radiating heat in the direction of travel; that trickle of energy acted like a small forward jet, slowing the craft. “The solar system still follows Einstein’s and Newton’s laws of gravity,” Turyshev says.
Yet it does not always adhere to scientists’ expectations. As Voyager 1 approached the heliopause—the boundary between space dominated by the sun and true interstellar space—the predicted changes in the flow of the solar wind never materialized. All the same, space physicist Robert Decker of Johns Hopkins University says several other factors suggest Voyager 1 is on the verge of crossing that mysterious zone. “We’ve never been here before,” he says. “This is really a discovery mission.”