Washington: NASA's Voyager 1 spacecraft is nearing the edge of the solar system and is set to enter the interstellar space, nearly 36 years after its launch.

Data from Voyager 1, now more than 18 billion kilometres from the Sun, suggest the spacecraft is closer to becoming the first human-made object to reach interstellar space.

Research using Voyager 1 data and published in the journal Science provides new detail on the last region the spacecraft will cross before it leaves the heliosphere, or the bubble around our Sun, NASA said in a statement.

Three papers describe how Voyager 1's entry into a region called the magnetic highway resulted in simultaneous observations of the highest rate so far of charged particles from outside heliosphere and the disappearance of charged particles from inside the heliosphere.

Scientists have seen two of the three signs of interstellar arrival they expected to see: charged particles disappearing as they zoom out along the solar magnetic field, and cosmic rays from far outside zooming in.

However, scientists have not yet seen the third sign, an abrupt change in the direction of the magnetic field, which would indicate the presence of the interstellar magnetic field. "If you looked at the cosmic ray and energetic particle data in isolation, you might think Voyager had reached interstellar space, but the team feels Voyager 1 has not yet gotten there because we are still within the domain of the Sun's magnetic field," said Ed Stone, Voyager project scientist at the California Institute of Technology, Pasadena.

Scientists do not know exactly how far Voyager 1 has to go to reach interstellar space. They estimate it could take several more months, or even years, to get there. The heliosphere extends at least 13 billion kilometers beyond all the planets in our solar system. It is dominated by the Sun's magnetic field and an ionized wind expanding outward from the Sun.

The study focused on observations made from May to September 2012 by Voyager 1's cosmic ray, low-energy charged particle and magnetometer instruments, with some additional charged particle data obtained through April this year.

Voyager 1 was about 18 billion kilometres from the Sun on August 25 when it reached the magnetic highway, also known as the depletion region, and a connection to interstellar space. This region allows charged particles to travel into and out of the heliosphere along a smooth magnetic field line, instead of bouncing around in all directions as if trapped on local roads.

For the first time in this region, scientists could detect low-energy cosmic rays that originate from dying stars. "We saw a dramatic and rapid disappearance of the solar-originating particles. They decreased in intensity by more than 1,000 times, as if there was a huge vacuum pump at the entrance ramp onto the magnetic highway," said Stamatios Krimigis from the Johns Hopkins University.

(Agencies)

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