Voyager I's instruments measured new radio waves and an increase in the strength of beams of energized particles, which nearly reversed direction, on Dec. 17 as the spacecraft was about 8.72 billion miles from the sun, NASA's Jet Propulsion Laboratory in Pasadena, Calif., said in a statement.
The evidence shows the spacecraft has crossed into a new region of the solar system before the sun's influence begins to fade and interstellar space begins, said Edward Stone of NASA's Jet Propulsion Laboratory.
"We have a totally new region of space to explore," Stone said. "It's a once-in-a-lifetime opportunity."
Voyager I has passed through the so-called termination shock, where the solar wind slows, and entered the heliosheath, the last region before the heliopause, where the sun's influence begins to fade.
Solar wind is a stream of electrified particles that flows out from the sun and blows a bubble around the solar system called the heliosphere, which ends when the particles slow below the speed of sound and termination shock begins.
The heliopause is the area in which the solar wind comes in contact with the gas and dust between solar systems in the Milky Way. It should take Voyager I about 10 to 20 years to cross the heliopause.