For nearly 50 years, NASA's Voyager 1 mission has competed for the title of deep space's little engine that could. Launched in 1977 along with its twin, Voyager 2, the spacecraft is now soaring more than 15 billion miles from Earth.

On their journeys through the solar system, the Voyager spacecraft beamed startling images back to Earth—of Jupiter and Saturn, then Uranus and Neptune and their moons. Voyager 1's most famous shot may be what famed astronomer Carl Sagan called the "pale blue dot," a lonely image of Earth taken from 6 billion miles away in 1990.

But Voyager 1's trek could now be drawing to a close. Since December, the spacecraft--which weighs less than most cars--has been sending nonsensical messages back to Earth, and engineers are struggling to fix the problem. Voyager 2 remains operational.

Fran Bagenal is a planetary scientist at the Laboratory for Atmospheric and Space Physics (LASP) at CU Boulder. She started working on the Voyager mission during a summer student job in the late 1970s and has followed the two spacecraft closely since.

To celebrate Voyager 1, Bagenal reflects on the mission's legacy—and which planet she wants to visit again.

Astronaut Thomas P. Stafford, who commanded a dress rehearsal flight for the 1969 moon landing and the first U.S.