My wife is the most gifted poet I know. I often marvel at her ability to see things with new eyes, to take experiences we have shared and look at them with such a fresh and uncluttered view that the events are transformed and new meaning extracted from them. All of which came to mind this morning in a far different context as I pondered how good science does much the same thing. A case in point in this ‘poetry of science’ is offered by a view of the edge of the Solar System made not with photons but with neutral atoms, in data gathered by the twin STEREO spacecraft. It’s a new kind of astronomy that draws on a different way of looking at the unexplored frontiers of the heliosphere.

Our Voyager spacecraft, of course, are in this region, so we’re getting new data all the time, but from an optical perspective, the outer heliosphere is invisible. This is where the solar wind — that stream of charged particles moving outward from the Sun — reaches the limits of the Sun’s influence, a place known as the heliopause. But we can get more specific still and talk about the heliosheath, a region filled with plasma on the borderline between the heliosphere and true interstellar space. Voyager 2 (having crossed the ‘termination shock,’ where the solar wind slows as it pushes against interstellar matter) is now in the heliosheath. And what the STEREO data are showing us is what happens to the energies dissipated in the area of that heliosheath.

But wait — aren’t the two STEREO spacecraft designed to study the Sun, or more specifically, solar storms as they move through space? True enough, but detectors on each have detected neutral atoms originating from the heliosheath region. What seems to be happening, according to a paper in Nature on this subject, is that ions heated up in the termination shock wound up losing their charge to cold atoms in the interstellar medium, thus beginning a flow back toward the Sun, which is what the STEREO spacecraft are detecting. Says Linghua Wang (UC Berkeley):

“We were surprised that these particle intensities didn’t depend on the magnetic field, which meant they must be neutral atoms… This is the first mapping of energetic neutral particles from beyond the heliosphere. These neutral atoms tell us about the hot ions in the heliosheath. The ions heated in the termination shock exchange charge with the cold, neutral atoms in the interstellar medium to become neutral, and then flow back in.”

Image (click to enlarge): STEREO detected energetic neutral atoms (ENAs) from the edge of the solar system, where the solar wind meets the interstellar medium. Hot ions in the heliosheath — the region between the termination shock and heliopause — are uniquely traced by ENAs and are more intense (indicated by color code) around the nose of the heliosphere, with an asymmetric double peak. The twin STEREO A and B spacecraft are shown in the sun-centered orbit they share with Earth. Last year, the Voyager 2 spacecraft passed into the heliosheath, joining Voyager 1. There, these interstellar explorers continue their journey into the farthest reaches of the heliosphere. Credit: University of California, Berkeley; L. Wang.

We’ll be watching with interest as the Interstellar Boundary Explorer (IBEX) prepares for launch later this year. The mission is designed to map lower-energy ions in the heliosheath using neutral atoms, extending our understanding of how the termination shock is put together. That’s useful information as we probe the effects of our star’s motion through the local interstellar medium, a region we will someday explore with tools specifically designed (unlike Voyager’s) for the purpose of analyzing interstellar space.

Pushing out twice the distance of Pluto, the heliosphere is the realm through which all human spacecraft have thus far moved. Innovative Interstellar Explorer or another such mission will one day establish a dedicated observatory outside it as we learn more about the conditions through which future interstellar missions may fly. The STEREO results are a useful step. The spacecraft are closer to home, but they’re showing us things we’ve never seen, and there’s a bit of poetry in that as well.

A number of papers (and an impressive cover) are devoted to the latest studies of the outer edges of the heliosphere in Nature. The paper discussed here is Richardson et al., “Domination of heliosheath pressure by shock-accelerated pickup ions from observations of neutral atoms, ” Nature 454 (3 July 2008), pp. 81-83 (abstract). Check this editor’s summary for all other related references.