So, it's official at last: NASA's Voyager 1 spacecraft is the first man-made object to have left the solar system for the dark and mysterious realm of interstellar space.
Scientists are excited. "Voyager 1's departure is an incredible achievement in the context of space exploration," says Warrick Couch, director of the Australian Astronomical Observatory. "It has covered about 19 billion kilometres since its launch, due largely to the sling-shot action of other planets."
Voyager reaches interstellar space
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Voyager reaches interstellar space
After decades of exploration, Voyager 1 is officially the first human-made object to venture into interstellar space. Courtesy NASA.
Melbourne University astrophysicist Alan Duffy says: "Not even NASA expected the craft to withstand the harsh environment of space for 36 years, much less bring fascinating results back of the kind of environment that exists beyond our solar system."
Compared with the Earth, the lonesome probe is 120 times further away from the sun. It moves at about 17 kilometres a second - it would take the average highway motorist 20,000 years to cover the same distance.
Amid the hullabaloo lies a vexing question: just how big is the solar system, which includes everything that orbits the sun, from planets to tiny bits of icy debris.
The controversy has raged since Galileo Galilei's first telescope revealed a world beyond the world then known. In a sense, the solar system has been expanding in size – at least in terms of human consciousness – as astronomical technologies have improved.
Size does matter
Ultimately, the solar system's size depends on where one sets the boundary. A few astronomers have suggested the outermost planet's orbit forms the limit. Since the demotion of Pluto, that's been Neptune. By that definition, the solar system would be roughly 9 billion kilometres across.
Others say it's bounded by the outer edge of the heliosphere, where the sun's sphere of influence, through the solar wind, peters out. That gives a diameter of more than 27 billion kilometres.
Still other scientists say it might extend for more than two light-years to the outer edge of the Oort Cloud, a huge spherical collection of icy debris believed to have been ejected by gravitational interactions with the giant planets.
The cloud supposedly surrounds the sun and planets, though where it ends is uncertain. If indeed it does exist, and is considered part of the solar system, that would yield a diameter of perhaps more than four light-years or roughly 40 trillion kilometres.
"There is some disagreement about the outer edge of the Oort Cloud, but most estimates suggest it is around 50,000 to 100,000 astronomical units away – in other words, one or two light years from the sun," says CSIRO astrophysicist Kurt Liffman.
"By comparison, Voyager 1 is about 120 astronomical units off. In my view, Voyager 1 has only reached the outer edge of the heliosphere."
As far as Dr Duffy is concerned, the true boundary probably lies somewhere in the region where the sun's gravity finally loses its influence. "This is many hundreds of times further out again, a dark and frozen zone filled with the leftover debris of early processes that formed the planets," he says.
With only 10 years nuclear-powered battery power left to go, Voyager 1 will most likely have shut down before reaching this region. "But it will sail on, finally approaching our neighbouring stars in 40,000 years' time," Dr Duffy explains.
The US space agency's Pioneer 10 and 11 probes, which set off in 1972 and 1973 respectively, were also the first man-made objects to start the long-haul mission of leaving the solar system.
In 1977, NASA launched its better-equipped Voyager probes, both of which flew by Jupiter and Saturn; Voyager 2 also explored Uranus and Neptune.
The Pioneers and Voyagers are all on one-way journeys out of the solar system.
They will eventually by joined by another robot brainchild of NASA's Jet Propulsion Laboratory in California.
This is the New Horizons spacecraft, which launched in 2006 and is scheduled to farewell the solar system in 2029 after paying a flying visit to one of the sentinels of the outer solar system, Pluto, and its five known mini-moons, Charon, Nix, Hydra, Kerberos and Styx.
Learn more about Voyager 1 at: http://voyager.jpl.nasa.gov/mission/index.html
Check out the solar system in greater detail at: http://solarsystem.nasa.gov/index.cfm
Discover plans for the New Horizons probe at: http://pluto.jhuapl.edu/
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