The Year in Science: Space 1997

The Pioneer 10 space probe—the farthest-traveling man-made object in the universe—came to the end of its mission this past year, after a quarter of a century of scientific exploration. When NASA launched it in 1972, the probe was meant to survive only a 21-month mission to Jupiter. Crossing the asteroid belt without incident was its first achievement; then it sent us the first close-up photographs and scientific measurements of Jupiter, confirming that the giant planet is mostly gas and liquid. Curving past Jupiter, it became the first spacecraft to use a planet’s gravitation to fling itself along, a maneuver that later became standard procedure. Finally it showed that our solar system is larger than previously thought: when the mission ended, with the probe 6.2 billion miles from Earth, it was still detecting solar-wind particles, indicating that it had not yet crossed the heliopause—the boundary between the solar system and interstellar ...