Artist's impression of the two Voyager spacecraft as they approach interstellar space.

Credit: NASA/JPL

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PARIS: Millions of textbooks depicting our Solar System as spherical have got it all wrong, according to studies of data sent back from deep space by NASA's probe, Voyager 2.

In fact, the Sun's zone of influence – called the heliosphere – turns out to be seriously asymmetrical, scientists say.

The heliosphere comprises space dominated by the solar winds, or particles, blasted out by the Sun. It goes way beyond the orbit of Pluto, which circles the Sun at a distance of nearly six billion kilometres.

Crossing thresholds

Launched in 1977 on a historic trek of the outer planets, Voyager 2 has now crossed the turbulent boundary, known as the "termination shock," where the heliosphere yields to interstellar space. Its twin probe Voyager 1, crossed the same threshold four years earlier at a different spot some 1.5 billion kilometres farther from the Sun.

This difference proves that the heliosphere is not even close to perfectly round, but is almost oblong, or egg-shaped, according to the studies, published today in the British journal Nature.

The "bottom" of the egg is flattened by a permanent clash of particles, as the outbound solar wind smashes into atomic debris hurtling in from interstellar space, the scientists theorise.

Voyager 2 also crossed the "termination shock" several times within the space of a single day, showing that the boundary is in perpetual flux, like the ebb-and-flow of a tide.

University of Arizona astronomer Randy Jokipii paid tribute to the two Voyagers, which have been operating faithfully since their launch in 1977.
Crossing the heliosphere "opens a new age of exploration," he said. "The stream of in situ and remote data from the outer reaches of the heliosphere has revolutionized our view of how the Sun interacts with the Galaxy."

Far out

For decades to come, the two spacecraft – speeding outward at more than 17 kilometres per second – will be the only source of local observations of the far limits of our Solar System.

The probes were originally sent to fly by and observe Jupiter and Saturn, which they did with thrilling results, including the discovery of active volcanoes on Jupiter's moon Io, and unknown intricacies in Saturn's rings. After that their mission was reconfigured to explore space beyond the Solar System's planets.

They became the first man-made objects to enter these cold, dark reaches, powered by long-life nuclear batteries in the absence of solar energy.

The spacecraft are so distant that commands from Earth, travelling at light speed, take more than a dozen hours to reach them. Each Voyager logs approximately 1.6 million kilometres per day.

Should they ever encounter extraterrestrial intelligence, the probes each carry a time capsule, a "golden record" of sounds and images about life on Earth in the mid-1970s.