Voyager 1 was the first in 2012, but its successor has the tech to make unprecedented observations. The Sun's heliosphere, or the region where particles and magnetic fields are altered by the Sun's influence, protects the crafts from these strikes. That crossing, around november 5, was detected by Voyager 2 in the form of a sharp drop in solar wind particles and corresponding increase in galactic cosmic rays detected by the spacecraft's instruments.
"Nobody really expected the spacecraft to last this long, to be able to continue out on their journey, to travel through the heliosheath boundary, out across the heliopause into interstellar space", Mr Nagle said. The confirmation comes from the probe's Plasma Science Experiment, according to a press release. They're both in interstellar space now, but they had very different journeys.
Even though they are out of the sun's bubble, the Voyagers are still technically in our solar system, Nasa said. Travelling at 34,000mph (54,000 km/h) Voyager 2 is now more than 11bn miles (18bn kilometres) from Earth.
The Voyager mission was launched in the 1970's, and the probes sent by NASA were only meant to explore the outer planets - but they just kept on going.
The spacecraft, which launched August 20, 1977, at a cost of $895 million, was primarily created to study the outer planets, notably Jupiter and Saturn.
For the second time in history, a human-made object has reached the space between the stars.
The solar system and beyond. At the same time, the solar wind petered out. Scientists maintain the solar system stretches to the outer edge of the so-called Oort Cloud, a sphere of icy bodies millions of miles away which will take thousands of years to traverse.
However, the Voyagers have not left the solar system "and won't be leaving anytime soon", NASA says.
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NASA says the event is likely to have occurred on November 5.
Another possible explanation is the sun's fluctuating activity, measured via outbursts such as solar flares and powerful explosions called coronal mass ejections.
"We thought it would be a much more gradual decrease", he said. It will take about 30,000 years for the spacecraft to get that far.
Q. Why is Voyager 2 crossing the heliopause important? Both are outside of the heliosphere. As NASA's Jet Propulsion Laboratory explains, engineers have to compare the readings of various spacecraft instruments that detect particles generated by the Sun as well as cosmic rays.
The approximate positions of Voyager-1 and 2. By comparison, it takes light just eight minutes to reach Earth from the Sun.
"There is a lot of science data ahead, and we anticipate we can operate for five to six or nearly 10 more years, just not with all instruments on", Dodd said.
"That's an incredible journey for this fantastic little ... spacecraft". It takes almost 17 hours for a radio signal, traveling at the speed of light from Earth, to reach Voyager 2 - as a comparison, it takes a little more than eight minutes for light to travel from the sun to Earth. But NASA's New Horizons spacecraft, which flew by Pluto and is about to conduct the most distant-ever rendezvous in the solar system, could continue operating into interstellar space, giving us a third functioning interstellar probe in coming decades.