What was considered a giant leap for man's thirst to explore space may have been a scientific miscalculation of gigantic proportions.
In 2012, Nasa's Voyager 1 spacecraft was believed to have achieved what was once unimaginable - becoming the first manmade object to breach interstellar space and move beyond mankind's solar system - more than 11 billion miles distant and 36 years after it was launched.
But, in the nearly two years since that historic announcement, it has become increasingly uncertain about whether Voyager 1 really crossed the threshold.
Some scientists say that the spacecraft is still within the heliosphere - the region of space dominated by the Sun and its wind of energetic particles - and has not yet reached the space between the stars.
To put an end to all doubts, two Voyager team scientists have developed a test that they say could prove once and for all if Voyager 1 has crossed the boundary.
The new test is outlined in a study accepted for publication in the journal of the American Geophysical Union.
The scientists predict that, in the next two years, Voyager 1 will cross the current sheet - the sprawling surface within the heliosphere where the polarity of the sun's magnetic field changes from plus to minus. The spacecraft will detect a reversal in the magnetic field, proving that it is still within the heliosphere.
But, if the magnetic field reversal doesn't happen in the next year or two as expected, that is confirmation that Voyager 1 has already passed into interstellar space.
"The proof is in the pudding," said George Gloeckler, a professor in atmospheric, oceanic and space sciences at the University of Michigan and lead author of the new study.
Gloeckler has worked on the Voyager mission since 1972 and has been a vocal opponent of the view that Voyager 1 has entered interstellar space.
He said that, although the spacecraft has observed many of the signs indicating it may have reached interstellar space, like cosmic rays, Voyager 1 did not see a change in magnetic field that many were expecting.
"This controversy will continue until it is resolved by measurements," Gloeckler said.
The Voyager 1 and 2 spacecraft were launched in 1977 to study Jupiter and Saturn. The mission has since been extended to explore the outermost limits of the Sun's influence and beyond. Voyager 2, which also flew by Uranus and Neptune, is on its way to interstellar space.
Gloeckler and co-author, Len Fisk, also a professor in atmospheric, oceanic and space sciences at the University of Michigan, are basing their new test on a model they developed.
The model assumes that the solar wind is slowing down and, as a result, that the solar wind can be compressed. Based on this assumption, the study says Voyager 1 is moving faster than the outward flow of the solar wind and will encounter current sheets where the polarity of the magnetic field will reverse, proving that the spacecraft has not yet left the heliosphere.
The scientists predict this reversal will most likely happen during 2015, based on observations made by Voyager 1.
"If that happens, I think if anyone still believes Voyager 1 is in the interstellar medium, they will really have something to explain," Gloeckler said. "It is a signature that can't be missed."
Nasa had called it a day in history as important as "Ferdinand Magellan's first circumnavigation of Earth, the first planetary flyby in 1962, when Nasa's Mariner 2 went by Venus and Neil Armstrong's first step on the moon".
It said in 2012 "Voyager 1 is the most distant object made by human hands. We've never entered the interstellar medium before, or actually sampled interstellar space, before now. It is another first in the history of exploration".
The Voyager team believes Voyager 1 entered interstellar space on Aug 25, 2012. The team came by this conclusion by combining data about the changes in the charged particles observed, the magnetic field data and the new plasma data from April to May 2013.
Voyager 1 was 11.3 billion miles (18.3 billion kilometres, or 122 astronomical units) away from the sun at that time. It was 11.3 billion miles (18.2 billion kilometres, or 121 astronomical units) from Earth.
Voyager 1 is about 12 billion miles (19 billion kilometres) away from the sun and Earth (as of Sept 9, 2013).
Ed Stone, Voyager project scientist said "The Voyager team needed time to analyse those observations and make sense of them. But we can now answer the question we've all been asking -- Are we there yet? Yes, we are".
New and unexpected data indicate Voyager 1 has been traveling for about one year through plasma, or ionized gas, present in the space between stars.
Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977.
Both spacecraft flew by Jupiter and Saturn. Voyager 2 also flew by Uranus and Neptune. Voyager 2, launched before Voyager 1, is the longest continuously operated spacecraft.
It is about 9.5 billion miles (15 billion kilometres) away from our sun.
"Voyager has boldly gone where no probe has gone before, marking one of the most significant technological achievements in the annals of the history of science, and adding a new chapter in human scientific dreams and endeavours," said John Grunsfeld from Nasa.
