The Voyagers rode a planetary lineup that comes round once every 175 years
In the 1960s an engineer noticed that the outer planets would soon line up so that one spacecraft could swing from each to the next. NASA cut the grand version to pay for the Space Shuttle, but two leaner probes launched in 1977 anyway. Nearly fifty years on, both are still sending data from interstellar space.
Gary Flandro at the Jet Propulsion Laboratory spotted the rare geometry in 1964: a probe could use each planet's gravity as a slingshot to reach the next. The resulting Grand Tour, aimed at every outer planet and Pluto, was cancelled in December 1971. A pared-down plan followed, two identical craft derived from the Mariner series and first called Mariner 11 and 12. A naming contest in March 1977, only months before launch, produced Voyager.
The mission plan had a built-in hedge. Voyager 1 took a faster route designed for a close look at Saturn's hazy moon Titan, a pass that flung it out of the planets' plane for good. Voyager 2, actually launched first, kept an option: had the Titan flyby failed, it would have been diverted there, giving up the rest. Voyager 1 succeeded, so Voyager 2 carried on and is still the only craft to visit Uranus and Neptune.
Each probe weighed 815 kilograms at launch, 105 of them science instruments, and draws power from three radioisotope generators. Onboard, six custom computers share roughly 32,000 words of memory, and an eight-track digital tape recorder stores plasma data for playback twice a year. Bolted to the side of each is the Golden Record. The designers expected about a decade of contact.
Instead, the Voyagers traced the edge of the Sun's influence. Voyager 1 passed the termination shock, where the solar wind drops below the speed of sound, in December 2004; Voyager 2 reached it about 1.6 billion kilometres closer to the Sun, showing that the bubble is lopsided. Voyager 1 crossed into interstellar space on 25 August 2012 and Voyager 2 on 5 November 2018. By 2024 Voyager 1 was 24.5 billion kilometres out, moving at 17 kilometres per second, the most distant human-made object, with instruments being switched off one by one to save power.
Source: Voyager program
