Since today marks the 35th anniversary of the launch of the Voyager 1 spacecraft, it seems appropriate to commemorate the mission—and to note how it continues to provide data about the far edges of the Solar System.
Where exactly are those edges of the Solar System? According to theory, the boundary of the Solar System is marked by a region known as the heliopause, where the solar wind—particles streaming from the Sun—meets the plasma of interstellar space. In this region, beginning about 90 times the distance from Earth to the Sun, models predicted that the solar wind's particles would be deflected by the interstellar material, much as water is pushed aside by the bow of a ship.
However, new measurements provided by the venerable Voyager 1 probe have failed to find the expected flow, deepening the mystery of the boundary between our Solar System and interstellar space. This adds to an earlier surprise, when Voyager's instruments measured zero outward velocity in the solar wind, a measurement that has now held constant for over two years. In a Nature paper, Robert B. Decker, Stamatios M. Krimigis, Edmond C. Roelof, and Matthew E. Hill concluded that Voyager 1 is not actually close to the heliopause, despite expectations. The researchers further suggested that the models for interactions between the solar wind and interstellar plasma may require reevaluation.
The solar wind is a plasma: a mixture of electrons and (mostly) protons streaming out from the Sun. This flow varies a lot over time, based on the solar cycle, but its particles consistently move outward, or radially.
At some point, the wind will necessarily run into the material in the region between stars. This contains more plasma, along with non-ionized atoms and molecules, which are collectively known as the interstellar medium (ISM).
By any reasonable theory, when the solar wind meets the ISM, there should be a transition. The usual model describes the solar wind carving out a region in the ISM known as the heliosphere, and the place where the solar wind is deflected by the pressure from the ISM is the aforementioned heliopause.
In April 2010, Voyager 1 reached a point about 113 AU from the Sun and saw the solar wind velocity began slowing down dramatically. (1 AU, or astronomical unit, is the average distance from Earth to the Sun, which is about 150 million kilometers.) This discovery led to speculation that the craft was reaching the heliopause.
If that was true, then the solar wind particles should be deflected in the transverse or meridional direction. Voyager's instruments happen to be good at measuring outward, or radial, flow velocity, so the spacecraft had to be reoriented five times over a 10-month period so that it could determine the meridional velocity of solar wind particles.
Two years of data revealed that the meridional velocity was 3±11 kilometers/second, or between -8 and +14 km/s. (Negative velocity in this case represents flow in the opposite direction expected by theory.) In other words, while the most likely velocity value is 3 km/s—within the instrumental limits, the flow is zero. That's certainly nowhere close to the predicted value of approximately 25 km/s.
Unlike the earlier "Pioneer anomaly," these data don't appear systematic: the five separate measurements were all consistent with zero meridional flow, but varied widely in most likely value.
These results are troublesome for several reasons. The thickness of the region where Voyager 1 has measured low radial velocity is now at least 7.5 AU (and counting). In that region, there has been very little fluctuation in the solar wind, even though we know the solar wind should vary strongly in time, thanks to the solar cycle. Additionally, combining the three components of the solar wind velocity shows the shape of the flow to be very different from what we expected.
The authors concluded that the heliopause either differs radically from theory, or Voyager is still not close to it. Both of these scenarios would require another look at the model for the solar wind-ISM interaction, since the current calculations don't explain the strange reduction in solar wind velocity or the direction of flow. It's possible the edge of the Solar System is farther out still—and might look very different from what we expected.