Nasa's Voyager 1, launched in 1977, is nearing the edge of the solar system and may already be "dancing on the edge" of outer space, the scientists behind the project said on Tuesday.
In a lecture marking the approaching 35th anniversary of the Voyager project, Ed Stone said it could be "days, months or years" before it finally breaks into interstellar space.
Earlier this year a surge in a key indicator fuelled hopes that the craft was nearing the so-called heliopause, which marks the boundary between our solar system and outer space.
Scientists were intrigued in May by an increase in cosmic rays hitting the spacecraft, which for decades has snapped images of the Earth and other planets in the solar system as it has made its long journey into outer space.
But measurements since then have fluctuated up and down, indicating that, while the craft is near to the edge, it may still not get there for some time.
"The question is, how much further is it to the heliopause?" Stone asked rhetorically at the lecture at the headquarters of the Jet Propulsion Laboratory (JPL) in Pasadena, California.
"We don't know. whether we're dancing along the edge of a new region which is connected to the outside," added Stone, a Voyager project scientist from the California Institute of Technology in Pasadena.
Noting that Voyager 1 moves a billion miles every three years, he said: "It's hard to imagine that it's going to be too much longer, but I can't tell you if it's days, months or years. I really can't tell you."
But he underlined how important a milestone it will be.
"Crossing into interstellar space – that will be a historic moment when the first object launched from Earth finally leaves the bubble," he said.
Before May's surge in cosmic rays researchers had said they expected Voyager 1 would leave the solar system and enter interstellar space – between the end of the Sun's influence and the next star system – within two years.
Nasa has described Voyager 1 – now 11 billion miles away from the Sun – and its companion Voyager 2 as "the two most distant active representatives of humanity and its desire to explore."
The scientists controlling Voyager 1 – whose 1970s technology means it has only a 100,000th of the computer memory of an 8 gigabyte iPod Nano – decided to turn off its cameras after it passed Neptune in 1989, to preserve power.
Assuming the craft continues to function normally, they will have to start turning off other on-board instruments from 2020, and it is expected to run out of power completely in 2025, said Stone.
Edited By Cen Fox Post Team
