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When the two Voyager spacecraft were launched in 1977, then-US president Jimmy Carter left a message on the disc carried by each probe into space: “We are attempting to survive our time so we may live into yours.” Probably against the odds, we have achieved the first part of his sentence; whether the latter happens, we will have to wait and see. Probably a very long time.

Forty years on, the Voyagers have fulfilled their astronomical promise, yet it is those golden discs (long-playing records) containing music, human greetings, art and photographs from Earth that caught the popular imagination.

More crucially, the discs indicated where anybody or anything that found one could locate our planet – using the positions of pulsars – meaning aliens could still, potentially, turn up on our doorstep.

But as new movie The Farthest reminds us, Voyager has been more than an attempt to contact fellow life forms. The two-spacecraft mission was first proposed in 1972, when astronomers realised that the alignment of Jupiter, Saturn, Uranus and Neptune in 1977 would make it possible to visit all four on a single flight.

Such an alignment occurs only once every 176 years (although the talking heads in the movie seemed a little unsure) and allowed the slingshot effect of the planets’ gravity to be used to speed the probes on their way. President Richard Nixon was on the ball, and the probes launched in 1977.

No equal

Frankly, the project’s grandeur has no equal. Landing on the moon took the plaudits, but Voyager was the big deal. All we knew before it launched was that “it was big” out there, as likeable mission designer Charley Kohlhase says in the film.

After it, we knew that Jupiter had lightning and learned more about two of its moons: Io was volcanic and  Europa icy. Saturn’s moon Titan turned out to have an atmosphere, and Uranus a crazy magnetic field and a weirdly shaped moon, Miranda. And we know that Neptune has a Great Dark Spot.

In fact, Voyager sent back so much new information as it passed by the planets that one researcher describes it as like “trying to drink from a fire hose”.

But 40 years on – and to the irritation of some mission staff – the record meant for aliens still stands out. Conceived by Frank Drake, a theoretical physicist described by a colleague as “a cross between Einstein and Edison”, the disc was metal so it would last and, helpfully, was provided with a stylus for playing it.

Cosmologist Carl Sagan was the architect of the record’s contents, although his then-wife Linda Salzman put in much legwork making the recordings. He described it as the “ultimate message in a bottle”, and despite tabloid fears of alien invasion, most team members hoped it would be found.

As the self-effacing Drake says, with only slight foreboding, “this may be the only evidence that we ever existed”.

Chuck Berry in space

If it is discovered, aliens will find greetings in numerous languages, photographs and 27 pieces of music – the only piece of popular music is Chuck Berry’s Johnny B. Goode (The Beatles apparently failed to make the cut because of “licensing issues”). Mission control’s in-joke has run and run: centuries later, NASA finally receives a message from aliens that says “Send more Chuck Berry…”

What aliens will make of the disc is anybody’s guess. Even if they figure out what the stylus is for, how will they know whether to play it at 33 or 45 rpm? And where might those aliens come from?

The most poignant part of the movie is not on the travails of troublesome launches, the loss and re-establishment of communications, or the fears that Jupiter’s magnetic field would fry the probe’s innards.

It is when the protagonists talk of the moment Voyager 1 finally left the solar system, crossing the heliopause around 25 August 2012, the first human-made object to enter interstellar space. Tears are shed.

This was the only chance for generations that humans had to make a grand tour of the outer planets – and we took it. That, in the context of the politics in which we lived, and still live, is quite extraordinary.

directed by Emer Reynolds