The Voyager 1 spacecraft is still riding a massive 'tsunami wave' that first began in February, Nasa has revealed.
It is the longest-lasting shock wave that researchers have seen in interstellar space.
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'Most people would have thought the interstellar medium would have been smooth and quiet. But these shock waves seem to be more common than we thought,' said Don Gurnett, professor of physics at the University of Iowa.
He presented the new data at the American Geophysical Union meeting in San Francisco.
A 'tsunami wave' occurs when the sun emits a coronal mass ejection, throwing out a magnetic cloud of plasma from its surface. This generates a wave of pressure.
When the wave runs into the interstellar plasma - the charged particles found in the space between the stars - a shock wave results that perturbs the plasma and causes in to 'sing'.
'The tsunami causes the ionized gas that is out there to resonate -- 'sing' or vibrate like a bell,' said Ed Stone, project scientist for the Voyager mission based at California Institute of Technology in Pasadena.