March 24: Distance is no barrier to controversy or poignancy.

Bill Webber, a senior US astronomer who has waited decades for the first human-made spacecraft to move beyond the solar system into interstellar space, believes he has won a race against time.

Webber, who began his career in the mid-1950s before the dawn of the space age, has published a research paper that says the US spacecraft Voyager 1, launched more than 35 years ago, appears to have crossed the boundary into interstellar space.

The paper in the American Geophysical Union’s journal, Geophysical Research Letters, says Voyager 1 experienced abrupt and dramatic changes in its space environment seven months ago. Those changes, Webber said, indicate that the spacecraft has exited the heliosphere, the vast bubble-like realm of space that holds the solar system and the Sun’s magnetic field.

Scientists tracking the journey of Voyager 1 as it advanced towards the edge of the solar system have been anticipating a crossing for months, but Webber’s paper has stirred a scientific controversy amid a touch of poignancy.

In a statement, US space agency Nasa asserted this week that the consensus of the Voyager 1 science team was that the spacecraft had not yet left the solar system or reached interstellar space.

Webber’s paper describes how Voyager 1 recorded sudden and significant drops in intensities of cosmic rays — charged particles — spewed by the Sun and an equally dramatic increase in the intensities of cosmic rays from interstellar space between July 28 and August 25 last year.

Webber has named his late friend and colleague Frank McDonald, a fellow cosmic ray scientist, as co-author of the paper. The two scientists, colleagues for over five decades, had been analysing data from Voyager 1 when McDonald died, only days after they had spotted the changes.

In a note at the end of the research paper, Webber said: “Frank, we have been working together for 55 years to reach the goal of actually observing the spectra of interstellar cosmic rays, possibly now achieved almost on the day of your passing.... Together we did it. Bon Voyage!”

“A big event seemed to occur on August 25,” Webber, 83, a professor emeritus at the New Mexico State University, told The Telegraph over the phone from his home. In all the months since then, the observations have remained constant. This is consistent with what we would see in interstellar space.”

Voyager 1, launched in 1977 for the orbital exploration of the outer planets of the solar system, is the most distant human-made object from the Earth. It is now about 18 billion kilometres (over 11 billion miles) from the Sun, a distance so great that it takes 17 hours for its signals to reach the Earth.

Nasa had reported in June last year that the amount of the galactic cosmic rays that Voyager 1 was encountering had gradually increased by about 25 per cent between January 2009 and January 2012.

On December 3, 2012, Nasa said that Voyager 1 had entered “a new region of space” that it called “the magnetic highway” near the edge of the solar system, a zone before the crossing into interstellar space.

In the Nasa statement issued earlier this week, Ed Stone, a Voyager 1 project scientist at the California Institute of Technology, said energetic particles can change dramatically along the magnetic highway.

“A change in the magnetic field is the last critical indicator of reaching interstellar space, and that change of direction has not yet been observed,” Stone said.

The Webber-McDonald paper says the “suddenness” of the changes in intensities of the cosmic rays and the stability of the galactic (interstellar) cosmic rays in the months since August 25 indicate that Voyager 1 has crossed a “well-defined boundary”.

“We could see that something drastic was happening — at first we thought the instruments (aboard Voyager 1) had broken down,” Webber said. “But it was just telling us what it was seeing.”

On August 25, Webber said, the spacecraft encountered a sudden increase in the levels of galactic cosmic rays, protons, helium and electrons by factors of up to two. The particles have energy characteristics of those expected in interstellar space.

Nasa scientists estimate that the spacecraft, built at Nasa’s Jet Propulsion Laboratory, has enough electrical power to continue picking up data and sending it back to the Earth until 2020, perhaps even longer.

The boundary of the solar system may be more a matter of semantics than science. Some astronomers argue that the Oort Cloud -- a shell of billions of icy bodies presumed to begin only about 750 billion kilometres away from the Sun and extending up to 15,000 billion kilometres -- is also a member of the solar family.

The Oort Cloud, proposed by Dutch astronomer Jan Oort in 1950, is believed to be made up of residual fragments of the bodies that formed the solar system some 4.6 billion years ago.

Indian space scientists not associated in any way with the Voyager 1 mission but studying aspects of the heliosphere said the boundary of the solar system has not been precisely understood but the magnetic field observations would bolster the evidence for interstellar space.

“These are the very first direct spacecraft measurements of a region of space never explored before,” said Periasamy Manoharan, a scientist at the National Centre for Radio Astronomy in Ooty. “There will be uncertainties.”

Physicists say the boundary of the heliosphere should be marked by a fading away of the Sun’s magnetic field and the emergence of interstellar magnetic fields with features distinct from those of the solar field.

“Within the heliosphere, the magnetic field has the signature of the Sun; beyond the boundary the signatures will change,” said Dibyendu Nandi, a solar physicist at the Indian Institute of Science Education and Research, Calcutta.

Four Nasa spacecraft — Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 — have been flying towards the edge of the heliosphere. Pioneer 10 was launched in 1972 but was overtaken in 1998 by Voyager 1, which is travelling much faster.