Long after the human race has vanished or evolved into something else, long after the sun has swollen into a red giant and incinerated Earth 5 billion years from now, at least one human artifact will continue drifting to the far reaches of the galaxy, safely preserved for eternity by the near-perfect vacuum of interstellar space.
Pioneer 10, launched in 1972 on what was expected to be a 21-month voyage to Jupiter, is now some 8 billion miles from home. On January 23, tracking stations picked up the last feeble transmission from the probe's plutonium-powered radio transmitter, which can no longer muster a signal strong enough to reach Earth.
As project scientists listened to that final fading whisper, they were left to ponder a mystery: The spacecraft seems to be defying the laws of gravity. Pioneer 10 has been slowing down, as if the gravitational pull on it from the sun is growing progressively stronger the farther away it gets.Pioneer 10 is not the only spacecraft acting strangely. Pioneer 11, launched in 1973, also slowed down as it pulled away from the sun, right until NASA lost contact with it in 1995. And there's some evidence of similar bizarre effects on two other probes: Ulysses, which has been orbiting the sun for 13 years, and Galileo, which plunged into Jupiter's atmosphere last month.
Is it possible that a single, identical malfunction struck all these vehicles? Or is some unknown force in the universe slowing them down? For Michael Martin Nieto, a theoretical physicist at Los Alamos National Laboratory in New Mexico, the mystery involves much more than a few hunks of spacefaring hardware; it reveals that there might be something wrong with our understanding of gravity, the most pervasive force in the universe.
"We don't know anything," he says. "Everything about gravity is mysterious."
