Early 2012. The Voyager 1 space-probe is millions of kilometres beyond the orbit of the dwarf planet Pluto. In fact, it’s in a region of space filled with scattered rocks and constantly perturbed by charged particles streaming in from outer space. Has it left the Solar System, then? Nobody is sure.
Late 2012. Scientists still aren’t sure if Voyager 1 has crossed over into the interstellar medium. The ISM is the region of the universe between stars, where the probe would definitely have been outside the Solar System. The probe’s batteries had been low for a while. An important instrument on-board that could’ve ‘sniffed’ at the charged particles and known where the probe was is dead. Only something like luck could save the day.
June 2013. Three papers published in Science discuss changes in the magnetic fields around the probe. Some measurements indicate Voyager 1 is in the ISM. Others say it’s just entered a new region of space, a ‘transition zone’ between the Solar System’s outermost fringes and the first tastes of the universe beyond.
August 2013. Luck finally struck. A storm on the surface of the Sun had ejected a massive burst of its own charged particles, way back in March 2012. They coursed in waves throughout the Solar System. When the waves met the charged particles Voyager 1 was swimming in, there was a resonating, a twang in the electromagnetic field. Some other instruments could pick that up well. It was confirmation that Voyager 1 was out and away.
September 2013. The announcement was made to much celebration.
But in December 2014, there was a surprise.
When the charged particles from the Sun, called a coronal mass ejection, meet the sea of charged particles in the ISM, it’s like a big wave hitting a placid shore. There is a tsunami, a disturbance spreading outward like ripples in water. Scientists don’t know how potent these tsunamis can be, but they assumed not too much because of the distances involved as well as the timescales.
They were wrong. On December 15, NASA reported that Voyager 1 was still recording the effects of a tsunami that had been unleashed 10 months ago, in February. As Don Gurnett, professor of physics at the University of Iowa, noted, “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.”
Just like a small ball floating on the surface of a pond bobs up and down when ripples pass under it, Voyager 1’s instruments pick up a bobbing of the electromagnetic field around it. These oscillations can be translated to and relayed as a sound with rising and falling pitches. Listen to it here.
One of the telltale signs that Voyager 1 is in interstellar space is that the sea of particles – or plasma – it’s cruising through gets thicker, as if more viscous. Based on observations, the plasma density has been increasing the farther out Voyager 1 goes. “Is that because the interstellar medium is denser as Voyager moves away from the heliosphere, or is it from the shock wave itself? We don’t know yet,” said Ed Stone, project scientist for the Voyager mission at Caltech.
If you’ve listened to the audio file, you’ll see how eerie it feels. The Sun’s coronal mass ejection behaves like a lighthouse in this sense. As its light – in the form of the charged particles – sweeps through space, the little boat called Voyager 1 finds its way in a rough and uncharted sea, one bob at a time. Here’s to the Sun keeping it going.