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 ...
