NASA's veteran Voyager 1 spacecraft has been hit by a "tsunami" from the sun as the mission pushes deeper into interstellar space.
Last year, mission scientists confirmed that the probe, which was launched on Sept. 5 1977 (a few days after sister probe Voyager 2), had officially left the sun's sphere of influence, a region known as the heliosphere. Beyond the heliosphere is interstellar space, the region that separates the stars.
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Although interstellar space is a very different environment to the heliosphere, it isn't quite empty. It contains a very thin mix of interstellar plasma that the 37-year-old Voyager spacecraft can measure and, in doing so, it can still feel the sun's presence from 12 billion miles away.
"Normally, interstellar space is like a quiet lake," said Ed Stone of the California Institute of Technology in Pasadena, Calif., the mission's project scientist since 1972. "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."
As our sun erupts with flares and coronal mass ejections (CMEs), powerful shock waves propagate throughout the heliosphere and past the planets. Eventually, the effects of these violent eruptions catch up to, and overtake, the Voyager 1 probe. In the case of this most recent event, the sun erupted about a year ago and only now is the shock wave being detected.
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The solar "tsunami" has traveled through the heliopause (the outermost boundary of the heliosphere) and washed through the interstellar plasma, causing the electrons in the plasma to "ring." The shock wave also has an impact on high-energy cosmic rays that Voyager can also detect.
"The tsunami wave rings the plasma like a bell," said Stone. "While the plasma wave instrument (on board Voyager 1) lets us measure the frequency of this ringing, the cosmic ray instrument reveals what struck the bell - the shock wave from the sun."
Since Voyager 1 entered interstellar space in 2012, this is the third such wave it recorded. Each event has helped mission scientists characterize the interstellar environment, a region where no man-made spacecraft has ever traveled through before.