The spacecraft, now more than 11 billion miles (18 billion km) away, detected two distinct and related changes in its environment on August 25, 2012, scientists write in paper to be published in Geophysical Research Letters and emailed to Reuters on Wednesday.

The probe detected dramatic changes in the levels of two types of radiation, one that stays inside the solar system, the other which comes from interstellar space.

The number of particles inside the solar system's bubble in space, a region called the heliosphere, dropped to less than 1 percent of previously detected levels, while radiation from interstellar sources nearly doubled, said astronomer and lead author Bill Webber, professor emeritus at New Mexico State University in Las Cruces, New Mexico.

Scientists are not yet ready to say Voyager is in interstellar space, however