NEW DELHI (TIP): Voyager 1, the spacecraft that is now over 19 billion kilometers from the Earth, has reported a sudden burst of tinkling bells from around it. Travelling in a region where no manmade spacecraft has ever been before, the instruments aboard Voyager 1 registered this ‘singing’ in the otherwise silent space around it and informed mission control back at Earth. NASA scientists have figured out that this is what happened: the Sun goes through periods of increased activity, where it explosively ejects material from its surface, flinging it outward.

These events, called coronal mass ejections, generate shock, or pressure, waves. Three such waves have reached Voyager 1 since it entered interstellar space in 2012. “Normally, interstellar space is like a quiet lake,” said Ed Stone of the California Institute of Technology in Pasadena, California, 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.”

Data from this newest tsunami wave generated by our sun confirm that Voyager is in interstellar space – a region between the stars filled with a thin soup of charged particles, also known as plasma. 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. Cosmic rays are energetic charged particles that come from nearby stars in the Milky Way galaxy.

The sun’s shock waves push these particles around like buoys in a tsunami. Data from the cosmic ray instrument tell researchers that a shock wave from the sun has hit. Meanwhile, another instrument on Voyager registers the shock waves, too. The plasma wave instrument can detect oscillations of the plasma electrons. “The tsunami wave rings the plasma like a bell,” said Stone. “While the plasma wave instrument lets us measure the frequency of this ringing, the cosmic ray instrument reveals what struck the bell – the shock wave from the sun.” Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977.

Both spacecraft flew by Jupiter and Saturn. Voyager 2 also flew by Uranus and Neptune. Voyager 2, launched before Voyager 1, is the longest continuously operated spacecraft and is expected to enter interstellar space in a few years.