Full-scale mockup of the Voyager spacecraft on public display at JPL, with yoomans for scale. The spacecraft's Radioisotope Thermoelectric Generators are visible on the left of center, with the Voyager Golden Record and the optical spectroscopy instruments visible to the right. Thanks to David Calhoun for the image!

Old Spacecraft Do Learn New Tricks: Discovering a New Region of the Heliosheath

This month’s issue of the journal Science contains a series of papers dedicated to the physics results produced using data collected from the Voyager 1 spacecraft over the last year or so. It’s really quite lovely that we’re getting useful, novel science data out of Voyager 1 and turning it into published science right up to this day, far beyond the intended end of the Voyagers’ original mission of planetary exploration. Voyager 1 is way out there, collecting this unique, priceless empirical data in an environment that is completely outside any other observational or experimental accessibility. (Voyager 2 lags a little behind, slightly closer to Earth.)

The twin Voyager spacecraft left Earth in late 1977, nearly 36 years ago. Today, Voyager 1 is 18.67 billion kilometers away from Earth (124.83 AU1), leaving the solar system at an impressive 1.64 million kilometers per day (about 19 km/​s).

Last year Voyager 1 entered an unexpected region of space where it observed a sharp decrease of charged particles from the Sun and an abrupt increase in particles from interstellar space, whilst still embedded in the solar magnetic field. This is the focus of one of the major papers published recently — Magnetic Field Observations as Voyager 1 Entered the Heliosheath Depletion Region.

Magnetic field measurements taken by by Voyager 1 show that the spacecraft crossed the boundary of an unexpected region, in terms of its magnetic environment, five times in 2012. The magnetic field strength increased across this boundary, but the direction of the magnetic field did not change significantly across any of these five boundary crossings.

Basically, these observations indicate that Voyager 1 has not passed through the heliosheath (the region where the solar wind is slowed, compressed and made turbulent by its interaction with the interstellar medium as the heliopause is approached — essentially the transition point where the influence of the solar wind stops, where it is balanced by the solar wind of the other stars) and entered the interstellar medium, but it has entered a new region — essentially a new region of the heliosheath, different from the heliosheath before it, but clearly not yet the interstellar medium.

This indicates that the heliosheath is not just a simple, homogeneous layer but something with more complexity in its structure, with different regions with different properties, containing enormous bubble-​​like structures on the order of 1 AU across. In this case, the authors have termed this newly observed region the heliosheath depletion region. This is something not present in previous understandings or models of heliospheric physics, something that is a brand new experimental discovery.

These exciting and unique modern observations require previous models of the physics of the sun’s magnetic environment and its interaction with the interstellar medium to be revisited — exciting stuff, with much more exciting data expected from this program over the coming years as Voyager 1 really enters the interstellar medium — probably.

[Featured image: Full-​​scale mockup of the Voyager spacecraft on public display at JPL, with humans for scale. The spacecraft’s Radioisotope Thermoelectric Generators are visible on the left of center, with the Voyager Golden Record and the optical spectroscopy instruments visible to the right. Thanks to David Calhoun for the image!]

  1. 1 AU (Astronomical Unit) is roughly 150 million kilometres, or the average distance from the Earth to the Sun. []