Voyager 1 was launched in 1977 and has traveled farther away from the sun than any other manmade object. It’s now 19 billion kilometers from Earth, which means it has entered interstellar space. Or has it?
Interstellar space is the physical space within a galaxy not occupied by stars or their planetary systems. It contains plasma – a thin soup of charged particles. NASA announced in 2012 and again in March 2013 and again in September 2013 that it thought that Voyager 1 had passed into interstellar space. However, while the evidence each time seemed stronger, there was still some questions and calls for more proof.
And what is the proof? Ringing plasma. The sun periodically has coronal mass ejections which explode material into space and generate pressure waves, which are kind of like solar tsunamis. Three of these waves have reached Voyager 1 since 2012. The second, in March 2013, registered on its cosmic ray instrument which measures charged particles from nearby stars. It also registered on the plasma wave instrument which measures oscillations of the plasma electrons. Here’s how Ed Stone of the California Institute of Technology in Pasadena, California, the mission's project scientist since 1972, describes it:
The tsunami wave rings the plasma like a bell.
Because denser plasma oscillates faster, NASA determined that the plasma was 40 times denser than measured before, signifying it had entered interstellar space. Similar measurements were recorded in September 2013.
On July 7, 2014, NASA officials announced that in March 2014, the instruments again registered a solar tsunami that once more rang the plasma bell. Combined with data on the spacecraft’s speed, this confirms with certainty (three times is a charm at NASA) that Voyager 1 entered interstellar space in 2012.
While Voyager 1 has left the
building heliosphere, it’s still inside the solar system, whose outer edge is defined as the Oort Cloud, a cloud or shell of comets and planetesimals 50,000 AU from the sun, that it won’t reach for 14,000 to 28,000 years.
I wonder if NASA will need three solid measurements to confirm that one.