On August 20, 1977, Voyager 2 was launched, beginning humanity's most epic astronomical adventure. Two weeks later its sister spacecraft, Voyager 1, was launched, and the two explorers set out to shed light on the mysteries of our solar system by getting up close and personal with our distant neighbors for the first time ever. Forty years later, both pioneers are still operating, sending back data, and heading on their way out of our solar system to explore further than any spacecraft ever launched.

The rare alignment

The Voyager dream began in the early 1960s, when aerospace engineer Gary Flandro discovered a rare planetary alignment would occur in the late 1970s. This perfect alignment of Jupiter, Saturn, Uranus, Neptune and Pluto occurs once every 175 years, and it meant a single spacecraft could quickly visit most of the Solar System's outer planets using gravitational slingshots to leap from planet to planet.

The original project was known as the Planetary Grand Tour, and for years NASA worked on the project until it was officially canceled in 1971 due to the nearly US$1 billion proposed cost. The following year President Nixon rekindled the project, but with a caveat – the project could only budget for two probes to visit two planets, Jupiter and Saturn.

The ambitious and talented NASA engineers set to work on developing the probes, secretly designing the spacecraft with further exploration in mind. Not content with just getting to Jupiter and Saturn, the team was determined to get the probes all the way to the outer Solar System.

Send more Chuck Berry!

As the project progressed the legendary astronomer Carl Sagan worked on a series of messages to attach to the probes. Eventually the messages took the form of a golden record conceived as a cosmic "message in a bottle". The now infamous Voyager Golden Records included aboard both Voyager spacecraft are encoded with a variety of images and music from our global culture. A curious alien species will find 115 images on each record, greetings in various Earth languages and a variety of lessons in our biochemistry.

Each record also holds 27 songs, designed to be a representation of music spanning all Earth cultures. Bach is well represented with three tracks, and a variety of traditional cultural selections, such as Peruvian panpipes and Navajo chanting, are included as well. Perhaps the most contentiously argued choices came when the selection committee focused on what songs to send to represent modern music.

A Louis Armstrong jazz track and a blues song by Blind Willie Johnson were relatively uncontroversial selections, but the choice of "Johnny B. Goode" by Chuck Berry was not unanimous. After initially hesitating, Carl Sagan ultimately defended the choice against some more conservative objections, although it wasn't his first choice for a modern song. The Beatles' "Here Comes the Sun" was on the hit list for some time, and despite the band themselves apparently supporting the idea, copyright holder EMI bizarrely refused to give permission.

The name of the Voyager project only came into being months before the launch in 1977. To that point the project had been clunkily labeled the Mariner Jupiter-Saturn mission. After a public competition organized by NASA, the name Voyager was chosen, and it was all systems go. The two Voyager probes were launched in August/September 1977 and then the waiting game commenced.

A journey of a billion miles begins...

In January 1979, Voyager began its approach of Jupiter sending back photographs and data offering insights scientists could only have dreamed of just a few years prior. Active volcanoes were unexpectedly seen on Jupiter's moon Io – the first time such features ever witnessed on another body in the Solar System. Rings were also discovered orbiting Jupiter and the volatile Great Red Spot was studied in astounding detail.

Next stop was Saturn, and the two Voyagers arrived in 1980/81. After providing more extraordinary insights into Saturn, the two probes parted ways. Voyager 1 altered its flightpath for a close flyby of Titan, Saturn's largest moon, revealing it to be a smog-enclosed world. The Voyager data also suggested possible oceans of liquid methane beneath the dense atmosphere, a hypothesis that was finally confirmed by the Cassini mission 25 years later.

The success of the Voyager program led to an inevitable mission extension and the foresight of the NASA engineers in secretly future-proofing the designs allowed Voyager 2 to quickly get on its way to explore Uranus and Neptune. Voyager 1's Titan mission meant its trajectory couldn't be altered back towards the outer planets, so it began an extended mission speeding towards interstellar space.

The Voyager 2 explorations of Uranus and Neptune remain to this day the only spacecraft we have sent to explore those planets. From Uranus' bizarre axial tilt to Neptune's extraordinarily violent storms, these observations are still some of the most valuable and direct insights ever collected on these mysterious outer planets.

Where no probe has gone before

From this point forward both Voyager spacecraft commenced their final, and most epic, mission phase – exiting the Solar System. In February, 1990, at the behest of Carl Sagan, Voyager 1 turned its camera around to take a "family portrait" of our solar system from the most distant vantage point ever. One of the images became known as the Pale Blue Dot, an awe-inspiring photograph where the Earth is seen as nothing more than a tiny blue dot suspended in the vastness of space.

For the next two decades scientists tracked the Voyager craft, discovering the extent of the Sun's solar winds, known as the heliosphere. The heliosphere is the bubble-like sphere of influence of the solar winds and its outermost stretches are known as the heliopause. This is the theoretical edge of our solar system, where the sun's solar winds are brought to an end by the pressure of interstellar space.

How far out our heliosphere stretched was unknown, and it wasn't until the middle of 2012 when Voyager 1 suddenly started registering spikes in certain high energy particles that scientists began to suspect the spacecraft had finally left the Solar System. By the end of 2013 it was finally confirmed that Voyager 1 had indeed entered interstellar space as of August 2012. Voyager 2, traveling a different trajectory, and slightly slower, is expected to cross into interstellar space in late 2019 or early 2020.

Both craft are estimated to still have enough power to return minor levels of data until at least 2025, at which point they will fall silent but continue their remarkable journey out into the galaxy. In 300 years Voyager 1 will reach the mysterious Oort cloud, and in about 40,000 years it will come within one light year of the star Gliese 445, in the constellation Camelopardalis.

Voyager 2, on the other hand, will come close to the star Ross 248 in about 40,000 years, but if undisturbed it will reach the brightest star in our sky Sirius, in about 296,000 years.

It's not hyperbolic to say the Voyager program is one of the greatest human achievements to date. Whatever happens to humanity here on Earth, hundreds of thousands of years from now the intrepid spacecraft will likely still be hurtling through the galaxy. They could still be going strong years after the Sun consumes the Earth, providing evidence of our existence long after we're gone.

Safe travels Voyager – and happy 40th birthday. Here's to a billion more!

Take a pictorial trip through the spacecrafts' remarkable journeys in our Voyager mission gallery.

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