New observations from NASA's Voyager 2 spacecraft, which launched from Earth 30 years ago, show that the solar system is asymmetrical and not spherical, scientists said Monday.

The discovery comes after the U.S. space agency'sprobe crossed a threshold in the solar system called the "termination shock" on Aug. 30, 2007, almost three years after its twin, Voyager 1, reached the same milestone.

The termination shock is 13.5 billion kilometres from the sun, or 90 times more distant from the sun than the Earth is. It marks the beginning of a region of space called the heliosphere where the charged particles from the sun — called the solar wind — collide with a mix of particles that act as a buffer zone between our solar system and high-energy particles emanating from other stars in the galaxy.

Voyager 2 was also almost a billionkilometres closer to the sun when it crossed the shock than Voyager 1 was, suggesting that the heliosphere is not spherical but instead asymmetrical in shape. Scientists said the shape of our heliosphere was likely because the interstellar magnetic field is pitched at an angle to the plane of the Milky Way.

"The magnetic field is disturbing an otherwise spherical surface," said Voyager mission scientist Edward Stone of the California Institute of Technology at a meeting of the American Geophysical Union in San Francisco.

Earlier in December, scientists at the University of California Riverside created a computer model that suggested Voyager 2 would cross the threshold sometime in late 2007 or early 2008.

UC-Riverside physics professor Gary Zank said Voyager 2's passage through the threshold was significant because, unlike Voyager 1, it still had a working instrument to make direct measurements of speed and temperature.

The probe, he said, would also provide a window into the origins and characteristics of cosmic rays. Only a tiny fraction of cosmic rays reach the Earth from outside the solar system because the particles collide with, and are redirected by, the solar wind, he said.

Voyager 2 launched in 1977 and flew by Jupiter, Saturn, Uranus and Neptune between 1979 and 1989. Like Voyager 1, it carries with it a gold record containing pictures, sounds and information from Earth.

The nuclear power sources on the two spacecraft should last until 2020. The two probes are expected to reach the heliopause, which marks the end of the solar system, in about a decade.