The US space agency's Voyager 1 spacecraft has reached the outer edge of the solar system where wind from the Sun is no longer blowing outward, but sideways, NASA said.

The spacecraft was launched in 1977 and has since snapped images of Earth and other planets in the solar system and provided NASA with crucial information as it makes its long journey into outer space.

NASA researchers think Voyager 1 will leave the solar system and enter interstellar space, or the area in between the end of the Sun's influence and the next star system, in about four years.

For now, Voyager 1 is 17.4 billion kilometers (10.8 billion miles) from the Sun in "an area where the velocity of the hot ionized gas, or plasma, emanating directly outward from the sun has slowed to zero," the space agency said.

"Scientists suspect the solar wind has been turned sideways by the pressure from the interstellar wind in the region between stars."

NASA noticed that the solar wind's outward speed had slowed to zero back in June, but wanted to look at readings from Voyager 1 over the next four months to be sure.

The data was presented late Monday at the American Geophysical Union meeting in San Francisco, California.

"When I realized that we were getting solid zeroes, I was amazed," said Rob Decker, a Voyager Low-Energy Charged Particle Instrument co-investigator and senior scientist at Johns Hopkins University.

"Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing us something completely new again."

NASA described the finding as "a major milestone in Voyager 1's passage through the heliosheath, the turbulent outer shell of the sun's sphere of influence, and the spacecraft's upcoming departure from our solar system."