Data showed Voyager 1 spacecraft, launched in 1977, had entered interstellar space, the U.S. National Aeronautics and Space Administration (NASA) said on Monday.

Interstellar space is a region between the stars filled with a thin soup of charged particles, also knows as plasma. NASA said last September that Voyager 1 might have left the sun's heliosphere and entered interstellar space on Aug. 25, 2012.

"Normally, interstellar space is like a quiet lake," said Ed Stone, the Voyager mission's project scientist. "But when our sun has a burst, it sends a shock wave outward that reaches Voyager about a year later. The wave causes the plasma surrounding the spacecraft to sing."

NASA said three such waves had reached Voyager 1 since it entered interstellar space in 2012. The first was too small to be noticed when it occurred and was only discovered later, but the second was clearly registered by the spacecraft's cosmic ray instrument in March of 2013.

Thanks to the second wave, the mission team acquired evidence that Voyager had been flying for more than a year through plasma that was 40 times denser than measured before -- a telltale indicator of interstellar space.

A third wave was registered in March of this year, and data showed that the density of the plasma was similar to what was measured previously, confirming the spacecraft was in interstellar space.

According to NASA, the mission has not left the solar system -- it has yet to reach a final halo of comets surrounding our sun -- but it broke through the wind-blown bubble, or heliosphere, encasing our sun. Voyager is the farthest human-made probe from Earth, and the first to enter the vast sea between stars. Endi

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