15 November 2019
Voices from interstellar space.
Back in 1977, NASA launched the twin probes Voyager 1 and Voyager 2. Voyager 1 left the solar system in August 2012, and now, nearly 18 billion kilometres from the Earth, Voyager 2 has finally left as well. It's outside the "heliosphere", or the bubble of particles ejected from our Sun, and so is now in interstellar space.
How can we tell that Voyager 2 has crossed the heliosphere? The probe has a few sensors that are still working. These can measure the temperature, pressure and density of plasma particles blown out of the Sun. Scientists at NASA describe the plasma inside the heliosphere as "solar wind" and the particles outside as the "galactic wind". The characteristics of these winds are different.
Voyager 2's measurements all fell significantly in around November 2018, and levels have remained low. This indicates that it wasn't just a random zone of low plasma density, but Voyager really had crossed the boundary.
Incidentally, to see what a heliosphere actually looks like, the Hubble Space Telescope took a photo of one around a young star a while back. I've attached the photo. You can see it looks like a bow shock in space. There's definitely a border there, and both Voyager craft are now outside that.
So where next? Voyager 2 is now travelling towards the Oort Cloud. This zone is outside the heliosphere, but here the Sun's gravity is still stronger than other stars. There are dirty snowballs here too, and they're still pulled towards the sun, if only weakly. The Oort Cloud is the domain of sleeping comets.
It's going to be a while before Voyager 2 gets there, though. The Oort Cloud is big and starts well beyond the heliosphere. It'll probably take around 300 years to even arrive.
The probe is on course for Sirius, but will take nearly 300,000 years to get there. By then, its nuclear batteries will have run out, and the probe will be a cold, dead relic.
Will humans remember the Voyager missions?