"It's a milestone and the beginning of a new journey," said mission chief scientist Ed Stone at NASA's Jet Propulsion Laboratory in Pasadena.
Launched 36 years ago on a tour of the outer planets, Voyager 1 has become the first spacecraft ever to leave the solar system. It is approximately 11.5 billion miles from the sun.
"Reaching this historic milestone is even more exciting because it marks the beginning of Voyager's exploration of the space between the stars," said Voyager project scientist Edward C. Stone in a statement.
NASA believes Voyager 1 actually made its exit more than a year ago, but it was not until recently that the space agency had the evidence to convince it that the spacecraft had finally plowed through the hot plasma bubble surrounding the planets and escaped the sun's influence. A chance solar eruption caused the space around Voyager 1 to echo like a bell last spring and provided the scientists with the data they needed, convincing them the boundary had been crossed in August of last year.
"This event makes me think of Carl Sagan, who helped people appreciate our cosmic explorations. He might have said that this is only a first step. We have left our own star behind, but there are billions and billions of other stars just in our own galaxy. Science is an endless frontier, and we are on our way," said Andrew Ingersoll, member of the Voyager project's atmospheric-science team, in a statement.
Voyager 1's odyssey began in 1977 when the spacecraft and its twin, Voyager 2, were launched on a tour of the gas giant planets of the solar system. Voyager 2 hopscotched to Uranus and Neptune, while Voyager 1 used Saturn as a gravitational slingshot to power itself past Pluto. It may take another three years before Voyager 2 joins its twin on the other side.
Voyager 1 still has hundreds of years to go before bidding adieu to the last icy bodies that make up our neighborhood, NASA said. It will now study exotic particles and other phenomena in a never-before-explored part of the universe and radio the data back to Earth.
The spacecraft also carries a gold-plated disc containing multicultural greetings, songs and photos -- just in case it bumps into intelligent species.
Some scientists said they remain unconvinced, partly by the absence of a change in magnetic field direction.
"I'm actually not going to believe it for another year or two until it's been solidly outside for a while," said Harvard astrophysicist Jonathan McDowell.
Eventually, the Voyagers will run out of nuclear fuel and will have to power down their instruments, perhaps by 2025.
The Associated Press contributed to this report.