NASA's Voyager 1, launched in 1977, is nearing the outer boundary of the solar system and may already be "dancing on the edge" of outer space, the experts behind the pioneering craft said.

In a lecture marking the 35th anniversary Wednesday of the space craft's launch, 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 fueled 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 at the lecture on Tuesday at 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.

The twin Voyager craft -- Voyager 2 was actually launched first, on August 20, 1977, followed by Voyager 1 on September 5 -- were designed primarily to study the biggest planets in our solar system, Jupiter and Saturn.

Taking advantage of a planetary alignment, they fulfilled that mission before pushing on to Uranus and Neptune, beaming back stunning images of the first two in 1979 and 1980, and the latter pair in 1986 and 1989.

But with those jobs complete and both craft still functioning perfectly, project managers decided to keep mining information as the devices fly further into the void and towards the very edge of our solar system.

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 boundary of the Sun's influence and the next star system -- within two years.

NASA has described Voyager 1 -- now 11 billion miles (18 billion kilometers) 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 gives it just a 100,000th of the computer memory of an eight-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.