Over the past year, Voyager 1 has made headlines every time it looks like the 36-year-old spacecraft had crossed into interstellar space. But every announcement has been marred with doubt, and Voyager 1's interstellar status has been quickly revoked.

Until last month.

Ed Stone, the principal scientist behind the Voyager mission, announced that the spacecraft is indeed flying through the unknown environment of interstellar space, making it the first in history to do so (though it hasn't yet left the Solar System behind).

This historic announcement marks more than just a technological achievement. That the Voyager spacecraft has lasted this long and continues to return valuable scientific data is an incredible triumph for the men and women behind the mission. The story of Voyager is a brilliant illustration of how a team of scientists can transform a single mission into a big science project imbued with technology to make it last well beyond its intended lifetime. And in light of this success, it's incredible we haven't seen more missions built along the Voyager model.

Voyager in a nutshell

The Voyager mission is among NASA's most well-known planetary missions. Two twin spacecrafts, Voyager 1 and Voyager 2, were launched in the fall of 1977. Each visited Jupiter then Saturn to complete their primary missions before flying off in different directions; Voyager 1 flew north from the plane on which all the planets orbit, while Voyager 2 was directed to visit both Uranus and Neptune.

After their final planetary encounters in the 1980s, both spacecraft have been rushing out towards the edge of our Solar System. And ever since, scientists have been eagerly awaiting the moment the spacecrafts would cross into interstellar space. This means leaving the heliosphere, the bubble of plasma originating from our Sun that envelopes the whole Solar System. This is what Voyager 1 has just done.

Voyager 1 didn't set out to be history's first interstellar spacecraft, and Voyager 2 didn't set out to visit all four giant planets ... but before they were dual-planet missions, NASA expected its exploration of the outer planets to be a grand affair.

Getting to this point is like the cherry on an already heavily frosted cake. Voyager 1 didn't set out to be history's first interstellar spacecraft, and Voyager 2 didn't set out to visit all four giant planets. Both launched as relatively simple dual planet flybys of Jupiter and Saturn. But before they were dual-planet missions, NASA expected its exploration of the outer planets to be a grand affair.

Origins of the Grand Tour

NASA started thinking about its future after Apollo in 1965, three years before the first manned mission of the lunar programmeme flew. There were a number of possible manned missions on the horizon ranging from exploration of our neighbouring planets to the construction of an orbital space station. But there was also a move to bring unmanned planetary exploration to the fore, and just what those missions might look like fell to the National Academy of Sciences' Space Science Board. In a meeting that summer, the board prepared a study urging NASA to shift its focus from the Moon to the planets, paying special attention to Mars and Venus, without ignoring the outer giant planets.

The study suggested NASA explore the outer planets either with a series of small reconnaissance spacecraft or with one large multi-planet survey mission. The latter mission was an attractive option. Not only was launching one spacecraft cheaper than launching a series of smaller ones, the multi-planet profile took advantage of a once-in-175-years planetary alignment that happened to be on the horizon; a favourable launch window for a multi-planet survey of Jupiter, Saturn, Uranus, Neptune, and Pluto existed between 1976 and 1980. But support for the multi-planet mission wasn't unanimous. Many scientists preferred multiple small missions that brought redundancy into planetary exploration as well as the chance to hone each mission towards answering a specific question.

Choosing between these profiled missions fell to the Outer Planets Working Group NASA established in 1969. The Working Group endorsed the multi-planet flyby mission but expanded it from one to two missions, each of which would visit three planets - a Jupiter-Saturn-Pluto mission launched in 1977, and a Jupiter-Uranus-Neptune mission launched in 1979. Two missions rather than one could visit all five planets in a shorter timeframe, simplifying the technology. Once scientists from the Space Science Board backed this decision, NASA management included this Grand Tour (GT) in its 1971 request for funding.

Perhaps the greatest champion for the multi-planet flyby mission was NASA's Jet Propulsion Laboratory. In 1967, long before NASA headquarters formally signed off on the project, JPL started promoting the the idea of the GT as a JPL mission. And the mission JPL imagined, lived up to its name. It consisted of four launches: two Jupiter-Saturn-Pluto missions in 1976 and 1977, and two Jupiter-Uranus-Neptune missions in 1979.

At the heart of all four missions was a new spacecraft to be developed by JPL called TOPS. Designed to last up to 10 years, the time each spacecraft would need to visit three planets, the heart of this new spacecraft was a self-testing and repairing computer called STAR. JPL argued that while the more durable spacecraft and sophisticated computer would increase both the cost and weight of the mission, developing these new technologies would create plenty of jobs.

Drawing from experience

As the GT idea took shape, one thing became clear: sending a single spacecraft to visit the outer planets was a hugely costly mission. Sending four was impossible. And the era of bloated budgets was fast coming to an end. When Richard Nixon assumed the presidency in January 1969, he brought even stricter budget cuts to NASA's already dwindling funding. For Nixon, space was no longer a Cold War battleground and Apollo, which he viewed as a Kennedy programme, was not worth continuing.

Nixon, instead, chose the space shuttle programme. Between the new shuttle and the existing Viking mission to land two crafts on Mars, it was clear Nixon wasn't going to approve a GT mission as well.

Unwilling to shelve the idea, NASA went back to the drawing board to consider cheaper alternatives. Luckily, the agency and JPL specifically had prior experience with planetary missions to draw from with the Mariner programme.

The Mariner series of missions was designed to launch the the first US spacecraft to other planets, specifically Mars and Venus. The programme achieved this goal: Mariner 2 became the first spacecraft to fly by Venus in 1962 and Mariner 4 managed to get a good look at Mars in 1965. The Mariner programme even saw the successful use of a planetary flyby to slingshot from one planet to the next. A Mariner-type mission to Jupiter and Saturn would be another dual flyby mission with familiar technology. It looked like exploring the outer planets would happen in a piecemeal fashion, but at least it was within NASA's budget.

NASA's 1973 budget request included funding for a pair of Mariner class spacecraft, the Mariner Jupiter-Saturn spacecraft to be launched in 1977. These missions would be two-planet alternatives to the GT. The missions were signed into existence on May 18, 1972.

Voyager 2, the only one of the pair on the right trajectory, would be able to visit Uranus and eventually Neptune. It hadn't been swift or certain, but the pieces of the former Grand Tour were finally coming back together.

From Mariner to Voyager

To reduce the overall cost, NASA decided to leave the design and construction of the Mariner Jupiter-Saturn spacecraft to JPL rather than deal an external contractor. This had the bonus effect of giving JPL scientists and engineers the opportunity to preserve their larger vision for the GT mission. Though the official word was that the Mariner Jupiter-Saturn would visit Uranus and Neptune only if the Saturn encounters were successful, the JPL team had every intention of building a pair of spacecraft that would last long enough to visit all the giant planets.

Right from the start, the team understood the mission's enormous potential, that it could be one of the truly outstanding if not the most outstanding mission in the whole planetary exploration programme. They set out to fullfill that potential.

The Mariner Jupiter-Saturn mission developed under Stone, a magnetospheric physicist from JPL who had started working on the GT idea in 1970 and was named the mission's lead scientist in 1972. As it took shape, the Mariner design was supplemented with subsystems designed to increase the mission's longevity, technology that was being used on the Viking Mars orbiters.

At NASA's order, the Atomic Energy Commission upgraded the plutonium batteries to be launched with the Mariner Jupiter-Saturn spacecraft so they might last more than ten years, solving the problem of powering the spacecraft through its eventual encounter with Neptune. An additional $7m to the programme enabled a series of scientific and technological enhancements, among which was a re-programmable computer similar to the STAR concept that had been cancelled along with the TOPS spacecraft.

The science payload, too, was developed with longevity in mind. NASA organised the mission scientists into 11 science teams corresponding to the 11 areas of investigation: imaging, radio science, infrared and ultraviolet spectroscopy, magnetometry, charged particles, cosmic rays, photopolarimetry, planetary radio astronomy, plasma, and particulate matter. As for specific objectives, the physical properties of the giant plants - surface features, periods of rotation, energy balances, and thermal regimes of the planets and moons, and investigation of electromagnetic and gravitational fields throughout the Solar System - were the main concerns.

Rolling with the punches

On March 4, 1977, about six months before launch, the twin Mariner Jupiter-Saturn spacecrafts were renamed Voyagers 1 and 2. Voyager 2 launched first on August 22 and Voyager 1 followed on September 5.

It wasn't long before systems and instruments started to fail. Before it reached Jupiter, Voyager 1's scan platform, which turns on three axis and aims the cameras, spectrometres, and photopolarimetre in the most scientifically interesting directions, became stuck. Voyager 2's scan platform similarly jammed after its encounter with Saturn.

Voyager 2 also had significant problems with its radio systems failing early in the mission, but a series of commands uploaded into the re-programmable computer ensured scientists would at least have minimal communications with their proxy when it encountered planets. And both spacecraft were affected by the high radiation levels around Jupiter; commands became difficult to send and some instruments were damaged. But the consistent threat of full failure was never realised.

When Voyager 1 left Saturn in 1980, the science return from the mission was very impressive, and Voyager 2 was deemed to be in good enough health that the mission was granted an extension. Voyager 2, the only one of the pair on the right trajectory, would be able to visit Uranus and eventually Neptune. It hadn't been swift or certain, but the pieces of the former Grand Tour were finally coming back together.

Voyager 1 is about to leave the solar system after being launched 35 years ago making it the farthest manmade object from Earth and very close to entering interstellar space [AP]

Continued success through the primary and extended missions has been due in no small part to the science team's' continued improvement to spacecraft as they fly further from Earth every minute. In upgrading the Mariner 10 camera to image Mercury, JPL engineers developed a new electronic technique that read out the image signal three times more slowly. They applied the same technique to the Voyager cameras and found that it not only facilitated data transfer from Saturn, it was a necessary procedure for imaging at Uranus.

Engineers also developed a new type of coding that promised error-free data transmission, and this was transmitted to Voyager 2 in preparation for its Uranus encounter. Once NASA's Deep Space Network of tracking stations became unable to ensure consistent communication with the increasingly distant Voyager spacecraft, JPL engineers borrowed a technique from radio astronomy and arrayed two antennas together to improve signal strength. Among the tracking sites it upgraded, NASA upgraded the facilities at the Very Large Array radio telescope in New Mexico making it at once the communications point for Voyager 2's encounter with Neptune, and a state-of-the-art facility for planetary radar astronomy.

An incredible success

This continual revision and upgrading continues to be a major part of Voyager's success, as does the team's familiarity with the mission. And more recently, the clever use of instruments to answer questions they weren't designed to answer, has allowed the science team to continue making new discoveries. Case in point, the announcement of Voyager 1's interstellar status. Plasma is the key indicator that the spacecraft is in a new region of space, but Voyager 1‘s plasma-measuring instrument failed long ago. So the team used the two antennae that measure magnetic fields instead. A change in the direction of the magnetic field, they determined, was indicative of a change in the plasma environment. This is just what Voyager 1 registered as it passed into interstellar space.

It's incredible to think that the Voyager missions that took us on a grand tour of the Solar System began life as the cheaper version of the ideal Grand Tour mission. And the mission isn't over. Both Voyager spacecraft are still talking to Earth with what instruments they have that are still working, returning information on the furthest reaches of the solar system and interstellar space.

But they can't go on forever. Starting in 2020, the science team will have to turn off one instrument per year to preserve power. In 2025, with their fuel depleted, both spacecraft will be permanently shut down. Hopefully by then, we'll have a new, long term, deep space mission in the pipeline to look forward to. Even if it's a small one that has the potential to grow into something much bigger.

Amy Shira Teitel has an academic background in the history of science and now works as a freelance science writer specialising in spaceflight history. She maintains her own blog, Vintage Space, and contributes regularly to Discovery News, Scientific American, Motherboard, DVICE.