A Nasa file picture of the Voyager spacecraft. (AFP)

New Delhi, Sept. 13: An American spacecraft launched 36 years ago, ferrying images and sounds from Earth, including a greeting in Bengali and a classical Indian raga, has become the first human machinery to leave the Sun’s zone of influence and enter interstellar space.

Scientists tracking the epic journey said instrument readings from the Voyager-1 spacecraft indicate that it had slipped out of the heliosphere, the bubble-like zone of solar influence, and moved into interstellar space last year on or around August 25.

In a paper published today in the US journal, Science, the researchers described changes in the electron plasma density in Voyager-1’s environment that they say provide “strong evidence” for its crossing a boundary into interstellar space.

Their study also vindicates US astronomer Bill Webber who had first proposed in March this year that Voyager-1 had crossed the boundary on August 25, 2012. But, as had been reported in The Telegraph on March 25, the US National Aeronautics and Space Administration had then questioned Webber’s claim.

“We now have definitive measurements — they tell us Voyager-1 is indeed in interstellar space,” Donald Gurnett, a professor of astronomy at the University of Iowa and first author of the paper in Science told The Telegraph over telephone.

Gurnett and his colleagues studied instrumental readings from Voyager-1 between April 9 and May 22 this year and observed an electron density of 0.08 per cubic centimetre, which falls within the values expected in interstellar space.

“We believe this is humankind’s historic leap into interstellar space,” Ed Stone, a senior Voyager project scientist at the California Institute of Technology, Pasadena, said in a media release.

The ageing Voyager-1, launched by Nasa in 1977 to study Jupiter and Saturn, is 18.6 billion kilometres from the Sun, a distance so great that its signals take 17 hours to reach Earth. Nasa scientists estimate that it has electrical power to continue relaying data to Earth until at least 2020.

Voyager-1 and its successor, Voyager-2, carry a 12-inch gold-plated copper disk containing sounds and images that portray the diversity of life and culture on Earth. Among the recordings are clips of western classical music, folk music from Asia, Africa, Europe, and the Americas, an Indian raga, Jaat Kahan Ho, sung by 20th century legend Kesarbai Kerkar, and spoken greetings in 55 languages, including Bengali and seven other Indian tongues.

In March, Webber with his late colleague Frank McDonald had co-authored a paper in the journal Geophysical Research Letters, that described changes in cosmic rays in the environment of Voyager-1 that they said marked its entry into interstellar space.

Their paper had said the “suddenness” of the changes in the intensities of cosmic rays and the stability of interstellar cosmic rays suggest that since August 25, Voyager-1 had crossed the boundary.

Gurnett today said that while the changes in cosmic rays did mark the crossing of the boundary, the new electron plasma density measurements serve as critical additional data in support of the spacecraft’s entry into interstellar space.

“This is a huge milestone,” said Dibyendu Nandi, a solar physicist at the Indian Institute of Science Education and Research, Calcutta. “An object that has crossed the heliopause is beyond the influence of the Sun.”

Astronomers caution that it would be inappropriate to suggest that Voyager-1 has left the solar system. Many view the Oort Cloud — a shell of billions of icy bodies that starts about 750 billion kilometres from the Sun and extends up to 15,000 billion kilometres — as a member of the solar family.

Scientists estimate it will take Voyager-1 about 40,000 years to approach another star system.

The spacecraft’s entry into interstellar space has thrown a fresh challenge to astronomers who had long expected that the magnetic field in the heliosphere with signatures of the Sun would abruptly change in interstellar space.

Gurnett and his colleagues said the absence of a change in the magnetic field raises the possibility that the interstellar magnetic field “may be linked to the solar magnetic field via some mechanism” that still remains to be deciphered.

Another spacecraft Pioneer-10 launched in 1972 had been the most distant human-made object in space until 1998, when Voyager-1 overtook its distance. Voyager-2, launched only 16 days before Voyager-1, is presently about 15 billion kilometres from the Sun.