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On September 5, 1977, NASA launched the Voyager I spacecraft. Nearly 40 years later, the phenomenally successful Voyager I and its sister spacecraft, Voyager II (which actually launched earlier), continue to return data and receive commands.
 
The primary mission of the Voyager program was to explore the planets Jupiter and Saturn, as well as their moon and ring systems. Voyager II went on to explore our solar system’s other gas giants, Uranus and Neptune. As of 2014, it is still the only spacecraft to have flown by those planets.
 
Voyager I has traveled more than 19.1 billion kilometers (11.9 billion miles) from Earth—the farthest of any spacecraft. Voyager I has actually left our solar system and is now traveling through the interstellar medium at about 520 million kilometers (325 million miles) per year. The spacecraft is expected to transmit data until about 2025.
asteroid
Noun

irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter.

comet
Noun

celestial object made up of ice, gas, and dust that orbits the sun and leaves a tail of debris.

command
Noun

an order or direction.

data
Plural Noun

(singular: datum) information collected during a scientific study.

gas giant
Noun

one of the four enormous outermost planets in the solar system (Jupiter, Saturn, Neptune, Uranus), composed mostly of gases instead of rock. Also called a Jovian planet.

interstellar medium
Noun

material in the space between star systems, mostly hydrogen plasma and cosmic dust.

launch
Verb

to start or instigate.

mission
Noun

important goal or purpose.

Noun

natural satellite of a planet.

NASA
Noun

(National Aeronautics and Space Administration) the U.S. space agency, whose mission statement is "To reach for new heights and reveal the unknown so that what we do and learn will benefit all humankind."

outer space
Noun

space beyond Earth's atmosphere.

phenomenal
Adjective

very impressive.

Noun

large, spherical celestial body that regularly rotates around a star.

primary
Adjective

first or most important.

receive
Verb

to get or accept.

ring system
Noun

thin circles of dust, gas, and rocks that orbit a planet.

solar system
Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

spacecraft
Noun

vehicle designed for travel outside Earth's atmosphere.

transmit
Verb

to pass along information or communicate.

travel
Noun

movement from one place to another.

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