Science

Last edited Sun Nov 24, 2013, 11:00 AM - Edit history (1)

In the latest twist in the story, the craft seems to be traversing an unexpected ‘dead zone’. This week, Robert Decker, a space scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, and his colleagues report1 in Nature that at Voyager 1’s current location, some 121.6 astronomical units (18.2 billion kilometres) from the Sun, the average velocity of solar particles has dropped to nearly zero. (Voyager 2, which is about 3 billion kilometres closer to the Sun and moving in a different direction, has yet to detect the same reduction in velocity.)

Decker’s team first reported2 the change last year, when it had measurements of the particles’ velocity only in the radial direction, outwards from the Sun. At the time, the team thought that the change was a sign that the craft was nearing the heliopause, where solar particles are expected to collide with powerful winds generated by supernovae that exploded some 5 million to 10 million years ago. The collision would force the solar particles to stop moving outwards and push them sideways, like a stream of water hitting a solid surface.

The plasma situation changed radically beginning on April 9, 2013, when Voyager's Plasma Wave Sensor picked up oscillations in the electron density. The rate of oscillation is correlated with the plasma density; prior to April, there had been no measurable oscillations. However, between April and May, the researchers found a gradual increase in the plasma density until it reached about 80,000 electrons per cubic meter—a number very close to what is measured in the ISM. (The theoretical density within the heliopause is around 2,000 electrons per cubic meter.)

So, has Voyager 1 left the Solar System? The only anomalous measurement remaining is the magnetic field direction, which doesn't correspond to the standard theories of the heliopause. With the new plasma density numbers, the case for Voyager's departure is stronger than before, leaving the magnetic problem in the hands of the theoreticians.