Exploring the Solar System

Exploring the Outer Planets

What did Pioneer 5 through Pioneer 9 accomplish?

Pioneer 5 was the first of five Pioneer probes launched into an orbit around the Sun. Generally, a solar orbit is far easier to achieve than a lunar orbit because it can be a much less precise task: Any object launched beyond Earth’s gravitational influence will naturally tend to go into solar orbit, unless carefully aimed to do otherwise. Pioneer 5 was launched on March 11, 1960, and was a sphere about twenty-five inches (sixty-four centimeters) across and weighed ninety-five pounds (forty-three kilograms). It was the first satellite to maintain communications with Earth at the then-impressive distance of twenty-three million miles (thirty-seven million kilometers). Pioneers 6 through 9 were successfully launched into solar orbit between 1965 and 1968. Each weighed about 140 pounds (64 kilograms), was covered with solar cells, and carried instruments to measure cosmic rays, magnetic fields, and the solar wind. Overall, these five Pioneer spacecraft lasted in solar orbit for decades.


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