(BBC 15 June 2012) The Nasa mission, which launched from Earth in 1977, could leave our Solar System at any time.
It is now detecting a sharp rise in the number of high-energy particles hitting it from distant exploded stars.
The observation was predicted, and is another indication that Voyager will soon reach its historic goal.
"The laws of physics say that someday Voyager will become the first human-made object to enter interstellar space, but we still do not know exactly when that someday will be," Ed Stone, the Voyager project scientist at the California Institute of Technology in Pasadena, said in a Nasa statement.
"The latest data indicate that we are clearly in a new region where things are changing more quickly. It is very exciting. We are approaching the Solar System's frontier."
Voyager 1 is travelling at about 17 km per second (38,000 mph), and is almost 18 billion km (11 billion miles) from Earth.
The vast separation means a signal from the probe takes more than 16 and a half hours to arrive at Nasa's receiving network..."