29.

The Voyager program consisted of only two unmanned probes, Voyager 1 and Voyager 2, both of which were funded by NASA and were launched in 1977 to study the gas giant outer planets of the solar system. Voyager 2 is about 14 billion km away from the sun and is the longest continuously operating NASA spacecraft . Voyager 1 is about 17 billion km away from the sun and is the most distant active spacecraft. Periodic contact has been maintained with both probes since the crafts’ radioactive power supplies are still functioning . Carl Sagan, Murmurs of Earth: The Voyager Interstellar Record ( New York : Random House , 1978 ).

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