"The Voyager team needed time to analyse those observations and make sense of them. But we can now answer the question we've all been asking -- 'Are we there yet?' Yes, we are," Stone said.
The findings are published in the US journal Science.
Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977. Both spacecraft flew by Jupiter and Saturn. Voyager 2 also flew by Uranus and Neptune.
Voyager 2, launched before Voyager 1, is the longest continuously operated spacecraft. It is about 15 billion km away from our Sun. Scientists are not certain when Voyager 2 is expected to cross into interstellar space, but they believe it's not very far behind.
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