Which Voyager probe was launched first?

Voyager 2 was launched first, on August 20, 1977, while Voyager 1 followed on September 5, 1977.

The numbering can be misleading. Voyager 1 was given the faster trajectory and therefore reached Jupiter and Saturn before Voyager 2, despite launching later. Voyager 2 followed a longer route designed to preserve the possibility of continuing beyond Saturn.

That route became one of the mission's greatest successes. Voyager 2 flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. It remains the only spacecraft to have visited Uranus and Neptune.

A common mix-up is confusing launch order with encounter order. Voyager 1 was launched second but overtook Voyager 2 and became the first to reach Jupiter and Saturn. Both probes were built by NASA's Jet Propulsion Laboratory and launched aboard Titan IIIE-Centaur rockets.