The Voyager spacecraft

The Voyager's flight makes history

After 36 years hurtling through space at around 18km per second, Voyager 1 has crossed an incredible boundary. It is the first manmade object to reach interstellar space. We look at the past of a spacecraft that will outlive every conceivable human future.

What was the single greatest achievement in the history of 20th century space exploration? Not Sputnik, which was just a simple metallic sphere stuffed into the nose cone of an existing Soviet missile. It wasn't the flight of Yuri Gagarin, the first man in space, whose trajectory was enabled in a similar fashion. The first manned lunar landing in 1969 touched our imaginations, but the trip took only a few days there and back.

Arguably, the second most incredible thing we've ever done – as engineers and curious, conscious beings – is to explore the outer planets of the solar system via robot emissaries, so that once unimaginable worlds, and their hosts of small moons, would become familiar to us. But perhaps the most incredible space project, initiated nearly four decades ago, was to build machines that have every chance of journeying to other stars, and surviving intact long after the human race, and all its Earthly constructions, have crumbled into dust.

In the late 1960s, it was hard enough simply to reach the Moon, but Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, California, was gearing up for an unprecedented journey across the solar system. Dr Ed Stone of the California Institute of Technology (CalTech, which manages JPL) was involved at the outset. "Every 176 years, the giant outer planets, Jupiter, Saturn, Uranus and Neptune are lined up on the same side of the Sun," he explains. "We knew this was due to occur over the late 1970s time frame, so we decided to launch two robotic probes on a journey of exploration to those planets."

JPL devised a series of missions that would exploit planetary gravitational fields, so that each world would divert and accelerate a passing probe towards the next target. The plan was nicknamed the Grand Tour. Twin probes, Voyager 1 and Voyager 2, were launched a month apart, in August and September 1977. Two years later they swung past Jupiter and returned stunning images of a world in turmoil, with complex banded cloud systems, and a permanent red-tinged hurricane, the 'Great Red Spot', that could swallow several Earths.

Jovian gravity accelerated the Voyagers towards Saturn, where they encountered a ring system more complicated and finely structured than anyone had expected. Some of the rings twist around each other like braided ropes, under the influence of stunningly complex gravitational interactions between Saturn and its moons. Voyager 1 was diverted so that it could study some of those smaller worlds, while Voyager 2 continued outwards to Uranus in 1986, and Neptune three years after that.

After the main planetary encounters were completed, both Voyagers continued to transmit data about the interplanetary environment. As Voyager 1 slipped the bonds of Saturn's gravity and drifted deeper into space, its last transmitted image showed the Earth as a tiny spark in the darkness, shepherded by a single bright but distant star, the Sun.

Crossing the line

After 36 years hurtling through space at around 18km per second, Voyager 1 has reached an incredible milestone, recently announced to the world's press by Dr Stone and his colleagues. At nearly 19 billion km from Earth, the spacecraft is now "the most distant human-made object," and Stone is confident that it has escaped the solar system completely. "We hoped 40 years ago that this would happen, but none of us knew how long it might take to get there. We are in interstellar space for the first time."

How can Dr Stone be so sure this is the key moment? How does he know where the solar system ends and interstellar space begins? Perhaps surprisingly, the answer to this question is determined at the smallest scales of matter and energy.

Space is not empty. The Sun's furious energies hurl the outermost layers of its atmosphere across the solar system. This constant energetic stream, known as the solar wind, is made up of plasma, atoms that have been shredded into subatomic particles (positively charged atomic nuclei and detached negatively charged electrons). When solar storms hurl particularly intense bursts of plasma in Earth's direction, satellites can be knocked out, and terrestrial power supplies disrupted.

But there comes a point, at the outermost edges of the solar system, where the pressure of this wind is depleted, and the opposing pressure of interstellar gas, and charged-particle streams from other powerful galactic sources of radiation, answers back, forcing the solar wind to retreat. This is the 'heliopause', where the Sun's influence is finally defeated by the wider forces of the cosmos. "The solar wind creates a shockwave," explains Dr Stone, "pushing against the interstellar environment, and being pushed back in return. Voyager 1 is now passing through that shockwave and into the realms beyond."

In about 40,000 years' time, Voyager 1 will approach the star Gliese 445 in the constellation Camelopardalis. Voyager 2 trails behind at 15.4 billion km from the Sun. It may be another three years before it joins its twin on the other side of the heliopause on its way to a distant rendezvous with Ross 248, a star in the constellation of Andromeda.

Stone is amazed that the Voyagers are still functioning. "When we built them, the space age itself was young, so we had no way of knowing that any spacecraft could last that long. But they just keep going." Even so, the Voyagers will never relay to us any discoveries among the stars. They are, at last, running out of power. Their radioisotope thermoelectric generators convert heat released by the decay of plutonium-238 into electricity. These will cease functioning some time around the year 2025.

Interstellar fleet

Voyager 1's recent transition from interplanetary to interstellar spacecraft has made the news this year, but there are three more probes destined for the stars. One of them still has work to do in this system, but the others completed their mission decades ago. Nasa's New Horizons probe was launched in January 2006 to study the 'dwarf planet' Pluto. It should arrive in July 2015. Then it will investigate the Kuiper Belt, a hitherto unexplored region inhabited by stray fragments of rock and ice left over from the solar system's formation more than four billion years ago. After that, the little probe will head towards the constellation Sagittarius.

Two more starbound probes were launched half a decade before the Voyagers. Pioneer 10 was the first spacecraft to fly by Jupiter in December 1973, coming to within 130,000km of the gas giant's upper atmosphere. It is now on a two million year-long journey to the red star Aldebaran in the constellation Taurus. A sister ship, Pioneer 11, rehearsed the gravitational 'slingshot' techniques used in the Voyager project. It is now 16 billion km from the Sun. Although the Voyager probes were launched more recently, their rockets and interplanetary trajectories placed them on a faster track for interstellar space.

From Earth with love

Powerless they may become, but the Voyagers' mechanical structures could survive for millions or even billions of years. It is unlikely that any alien intelligences will intercept them, but not impossible. If they do, special messages from 'us' to 'them' are waiting. Each Voyager carries a gold-plated metal disc, engraved with a spiral of dips and dents, using the best phonograph technology available in the 1970s, prior to the age of DVDs and laser discs.

The outer protective case includes a diagram of how the stylus, included in the kit, can be used to 'play' the record. Images of life on Earth, and a variety of natural sounds, such as surf, wind and thunder, are encoded, along with the songs of birds and whales. Humanity is represented in 55'languages, and by greetings from President Jimmy Carter and the UN Secretary General of the time, Kurt Waldheim.

Rectangular plaques on each Pioneer show where our species lives, and its biological form. At top left of the diagram is a schematic of two hydrogen atoms bound up in a molecule of dihydrogen, or 'molecular hydrogen'. This is such a basic material in the universe, it should be known to any technological alien civilisation. Its properties can be used as a yardstick for both time and physical length throughout the universe. That gets rid of the problem of metric, imperial or other units of measurement.

As a further size check, the binary equivalent of the decimal number 8 is shown between tote marks indicating the height of the two human figures, standing next to the Pioneer spacecraft itself, which is also shown on the plaque. The radial pattern to the left represents the position of our Sun relative to 14 pulsars (stars that emit extremely regular energy pulses) and to the centre of the galaxy. The binary digits on the other lines denote time. There is also a diagram showing Pioneer 10's journey, accelerating past the largest planet in our solar system with its antenna pointing back to its origin on the third planet.

Would you understand the plaque, as drawn by people of your own species? Perhaps not at first, but intelligent aliens might dedicate their brightest minds to the task of unravelling its mysteries, so maybe they will have better luck. For now, the plaque is better understood (by us, at any rate) as a message to ourselves rather than to aliens. It signifies that we at least tried to head towards the stars, even while clashing ideologies and social problems afflicted us on Earth. It is a technological cave painting, created as much because we felt like it as for any other reason: a flattering portrait of ourselves as we would like to be seen.

Attitudes have changed since 1972, and we no longer share all the assumptions in the drawn symbols. Why, a concerned 21st century person may ask, is the man waving, and not the woman? Nasa decided that if both humans held their hands up in greeting, the aliens would think that all of us walked around all day with one arm in the air. It made sense to show a variety of postures. But there's one assumption that can only be pleasing to E&T readers. Nasa assumed there's a basic language that intelligent aliens must share with us: mathematics.

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