Solar 'tsunami wave' helps confirm that Voyager 1 is in interstellar space

Credit: NASA/JPL-Caltech

Tuesday, July 8, 2014, 12:09 AM - The past two years have been a little confusing for NASA's Voyager 1 space probe, as its status as an interstellar traveller has been considered, debated, officially confirmed and then debated some more. However, new data looks to confirm that the intrepid spacecraft is actually in interstellar space, and the mission scientists have our Sun to thank for it.

In summer of 2012, the veteran Voyager 1 spacecraft - launched in 1977 on an outward-bound mission to tour the solar system and then fly beyond its confines - returned indications to scientists back here on Earth that it had reached a new milestone in its journey away from home: it had finally left our Sun's heliosphere behind and had entered interstellar space. This was thanks to a sensor on board the spacecraft called the cosmic ray instrument. While Voyager 1 was inside our Sun's heliosphere - the limit of the influence of the solar wind - it was detecting mostly cosmic rays from the Sun, with only a small number of cosmic rays from the galaxy. In August 2012, though, the cosmic ray instrument detected a flip in those numbers - far more from the galaxy and only a tiny number from our Sun. A third piece of data that would have cinched it is seeing the direction of the magnetic field around the spacecraft change - as the direction of our Sun's magnetic field differs from the direction of the galactic magnetic field - however the instrument that could have directly detected that had malfunctioned years before. A proxy had been worked out with a different instrument, but at the time of all these other changes going on, this proxy still didn't show any indication of any shift in the magnetic field.

However, NASA scientists lucked out when two solar eruptions - called coronal mass ejections (CMEs) - that were blasted out into space by the Sun arrived at Voyager 1's location, one in November 2012 and the other in April 2013. As the matter from these CMEs swept through that region of space, they set off vibrations in the plasma gases around the spacecraft, which were picked up by another of Voyager's detectors - the plasma wave instrument. By measuring these vibrations, the scientists were able to determine the density of the plasma, which matched what they expected to see from the gases of the interstellar medium. Thus, it confirm that Voyager 1 had, indeed, entered exited the heliosphere.

This NASA-JPL video takes the vibrations Voyager 1 detected and converts them to sound, giving us the eerie music of interstellar space:

In the time since then, though, while the mission scientists have stuck to their conclusions, doubts have been raised about them by others, based mostly on the fact that Voyager 1 still isn't seeing any indication of the expected change of the magnetic field direction. The alternative is that the spacecraft is in a different part of the edge of the heliosphere that hasn't been detected or even theorized yet, and it was possible that future detections might overturn those conclusions.

Now, though, there is even more support that Voyager is in interstellar space. This is thanks to yet another coronal mass ejection, which the scientists liken to a 'tsunami wave' in space.

"The tsunami wave rings the plasma like a bell," said Ed Stone, the Voyager mission's project scientist at the California Institute of Technology, according to a NASA-JPL statement. "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."

"All is not quiet around Voyager," said Don Gurnett of the University of Iowa, the principal investigator of Voyager 1's plasma wave instrument, said in the statement. "We're excited to analyze these new data. So far, we can say that it confirms we are in interstellar space."

Even with this confirmation that Voyager 1 has actually pierced beyond the heliosphere, and is now travelling in interstellar space, this does not mean that the spacecraft has left our solar system. While the Sun's heliosphere reaches out quite far beyond the orbit of Pluto, to about 121 astronomical units (18 billion kilometres), our solar system is considered to extend much farther. The hypothetical Oort Cloud - composed of billions of inert comets and other debris left over from the formation of the solar system, all gravitationally bound to our Sun - is thought to stretch to thousands of astronomical units away. It's taken Voyager over 36 years to make it as far as it has now. It will take an estimated 30,000 years to travel that distance.

It's worth noting, though, that even though Voyager 1 has only traveled a very small distance when you consider how big space is, it's still a monumental achievement that it has become the very first object built by human hands to reach interstellar space. We'll have to just watch in wonder from here, as it reveals even more to us about the space around our solar system.