This artist's concept shows NASA's Voyager spacecraft against a backdrop of stars. The Voyager spacecraft were built and continue to be operated by NASA's Jet Propulsion Laboratory, in Pasadena, Calif. Caltech manages JPL for NASA. NASA/JPL-CalTech

Voyager 1 still continues to experience the bumpy effects of the "tsunami wave" it encountered in space this February.

This is the third shock wave it has encountered since entering interstellar space in 2012. The wave is still propagating outwards.

The first one was between October and November of 2012, and the second in April to May of 2013.

In fact, it was the second tsunami wave that led researchers to conclude in 2013 that Voyager 1 had left the heliosphere, the bubble created by the solar wind encompassing the sun and the planets.

The shock waves showed that a ride in interstellar space could be bumpy, and space was not the smooth medium expected, says a Nasa release.

A "tsunami wave" occurs when the sun emits a coronal mass ejection, throwing out a magnetic cloud of plasma. The pressure wave sends a shock wave when it hits charged particles or plasma in interstellar space.

"The tsunami causes the ionised 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.

Around 20 billion kilometres away from Earth and travelling at a speed of 17 km per second, Voyager 1 is the farthest craft launched from the planet.

Researchers are unsure about the reasons for the unusual longevity of this latest wave, and also on how fast the wave is moving or how broad a region it covers.

Why the density of the plasma is higher the farther Voyager goes, is also not clear.

Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977. Taking advantage of a special alignment of the outer planets that happens every 176 years, Voyager 1 took slingshots from one planet to the next, assisted by the first planet's gravity.

Both spacecraft flew by Jupiter and Saturn. Voyager 2 also flew by Uranus and Neptune.

Voyager 2, launched before Voyager 1, is expected to enter interstellar space in a few years.

Pictures of Jupiter aside, Voyager 1 also found two new moons of Saturn as well – Thebe and Metis. It sent back detailed pictures of the Galilean moons (Io, Europa, Ganymede and Callisto) as well as Amalthea.

Both spacecraft carry sound records on copper disks with sounds ranging from whale calls to the music of Chuck Berry and spoken greetings in 55 languages.

The craft is powered by a plutonium-based thermoelectric generator which will support most of its operations till 2025.