(Sen) - One of NASA's oldest space probes, Voyager 1, appears to have reached the edge of the Solar System, 34 years after it was launched on a grand tour of the planets.
Despite its age and distance, scientists back in California are still receiving signals. Travelling at the speed of light, these take 16 hours 38 minutes to cross 17.8 billion kilometres of space (11.1 billion miles).
Recent data has shown a marked increase in the intensity of cosmic rays from outside the Solar System. This provides overwhelming evidence that Voyager 1 is leaving a protective bubble of charged particles called the heliosphere that is blown out by the Sun and surrounds its family of worlds.
That means Voyager 1 is on the brink of becoming the first probe created by humans to reach interstellar space on its journey to mingle with the stars.
The cosmic rays that are striking two High Energy telescopes aboard Voyager 1 come from supernovae that exploded long ago in our local region of the Milky Way galaxy. There was a significant increase in the level of impacts last month.
Voyager project scientist Ed Stone, of the California Institute of Technology in Pasadena, said: "The laws of physics say that some day Voyager will become the first human-made object to enter interstellar space, but we still do not know exactly when that someday will be.
"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."
Locations of the Voyager probes in the Solar System. Credit: NASA/JPL
Dr Stone added: "From January 2009 to January 2012, there had been a gradual increase of about 25 per cent in the amount of galactic cosmic rays Voyager was encountering.
"More recently, we have seen very rapid escalation in that part of the energy spectrum. Beginning on May 7, the cosmic ray hits have increased five per cent in a week and nine per cent in a month."
Space scientists are waiting for two other indicators to confirm that Voyager 1 is truly in interstellar space. One will be a dramatic fall in the level of charged particles recorded from the Sun itself inside the heliosphere bubble. The other will be a significant change in the direction of magnetic field lines surrounding the space probe.
Voyager 1's sister probe, Voyager 2, is still well within the Solar System's boundary at a distance of 14.7 billion kilometres (9.1 billion miles) but is the second most distant of spacecraft launched from Earth.