Some people had doubted that Voyager 1, launched in 1977 to explore Jupiter and Saturn, actually reached interstellar space in 2013. Those doubts are now gone.
According to NASA and the Jet Propulsion Lab (JPL), Voyager 1 has experienced a new "tsunami wave" from a solar storm. Ed Stone of the California Institute of Technology in Pasadena, CA, the mission's project scientist since 1972, explained in a statement, "Normally, interstellar space is like a quiet lake. But when our sun has a burst, it sends a shockwave outward that reaches Voyager about a year later. The wave causes the plasma surrounding the spacecraft to sing." This solar song has been detected by the spacecraft's cosmic ray and plasma wave sensors.
The solar explosions cause coronal mass ejections. This eruption of particles generate shockwaves that can be detected by Voyager even though it's almost 12 billion miles, or over 17 and a half light hours from Earth.
Here on Earth we "detect" coronal mass ejections as auroras and radio interference.
While the effects are far smaller for Voyager, they are detectable. "All is not quiet around Voyager," said Don Gurnett of the University of Iowa, the principal investigator of the plasma wave instrument on Voyager, which collected the definitive evidence that Voyager 1 had left the sun's heliosphere.
"The tsunami wave rings the plasma like a bell," added Stone in a statement. "While the plasma wave instrument lets us measure the frequency of this ringing, the cosmic ray instrument reveals what struck the bell — the shockwave from the sun."
This "ringing" of the plasma bell provided the key evidence that Voyager had entered interstellar space.
Some would still argue that Voyager 1 is still inside the solar system because it has to pass through the long period comets that come from the Oort Cloud. Since it will take Voyager 1 anywhere from 14,000 to 28,000 years to exit the Oort Cloud, and its Plutonium Radioisotope Thermoelectric Generators (RTGs) are expected to run out of power sometime after 2025, this is close as we're likely to ever get to interstellar flight.
Unless, of course, NASA's Harold White's ideas for a true faster-than-light warp drive prove to be practical. In that case, we may yet bring Voyager home.