Why is voyager 1 so fast?

Voyager 1 left Cape Canaveral on 5 September 1977 on a Titan IIIE with a Centaur upper stage, the most capable combination NASA then used for planetary missions. An additional solid-fuel kick stage attached to the probe pushed it out of Earth's gravity with enough surplus speed to reach Jupiter in just 18 months. That was already fast, but on its own it would not have been enough to leave the solar system.

The real acceleration came from the planets. When a spacecraft passes behind a planet along its orbit, it borrows a little of the planet's orbital momentum; the planet slows imperceptibly and the probe speeds up relative to the Sun. The Jupiter flyby in March 1979 added well over 10 km/s to Voyager 1's heliocentric speed, enough to exceed the Sun's escape velocity at that distance. The Saturn encounter in November 1980, planned to pass close to the moon Titan, bent the trajectory upward out of the plane of the planets and left the probe on its final hyperbolic path. Voyager 2 took a slower route past Uranus and Neptune, which is why it now moves at about 15 km/s rather than 17.

Beyond the planets, nothing much acts on the spacecraft. The Sun's gravity at 25 billion km is minuscule, and the interstellar medium is far too thin to cause drag, so Voyager 1 coasts at an almost constant 17 km/s. It crossed the heliopause into interstellar space in August 2012 and adds about 3.6 astronomical units to its distance each year. New Horizons left Earth faster than any probe, at 16.3 km/s, but without a Saturn assist it cruises at around 14 km/s today, so Voyager 1 remains the fastest human-made object leaving the solar system (data as of May 2026).