Voyager 1, launched in 1977 to explore the outer planets, has passed into a new region on its way out of the solar system, scientists have said.
The spacecraft, now more than 11 billion miles (18 billion km) away, detected two distinct and related changes in its environment on 25th August 2012, scientists write in a paper to be published in Geophysical Research Letters.
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.
Scientists are not yet ready to say Voyager is in interstellar space, however.
In a statement on Wednesday, Voyager project scientist Edward Stone said more evidence is needed to indicate Voyager has left the solar system. "It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space," Stone said.
Voyager 1 and a sister spacecraft, Voyager 2, were launched 16 days apart in 1977 to fly past Jupiter, Saturn, Uranus and Neptune.
Voyager 2 is following a different path out of the solar system and is not believed to have reached the magnetic highway toward interstellar space yet.