From the 2011 Fall Meeting of the American Geophysical Union (AGU): Voyager 1 at the edge of the solar system

For two days I will be reporting from San Francisco, home every year to the Fall Meeting of the American Geophysical Union. This is by far the largest meeting that I attend on a regular basis. This year, more than 20,000 are registered to attend sessions on topics ranging from the deep ocean to the interiors of Earth and other planets, to climate change, to planetary atmospheres, to seismology, and mitigation of the effects of natural disasters. The full program is here. I will be Tweeting quite a bit and am trying to include abstract numbers with Tweets to make them easy to look up (both for you readers and for me!) In general, the theme for today will be the Saturn system, with a smattering of other outer solar system stuff; tomorrow will be Dawn, Dawn, Dawn.

This morning I attended a press briefing on Voyager 1, which is still making discoveries even as it has nearly exited the solar system entirely. This is a cool story, and a beautiful example of the scientific method in action. Science rarely works so neatly. To set the story: Voyager 1 is the most distant still-functioning spacecraft, so it is performing pioneering exploration beyond the solar system. For its entire life, it has been traveling within a region of space dominated by our Sun. The solar wind blows outward in all directions. At some point, though, the pressure of the solar wind is balanced by the pressure of the interstellar medium. This point (actually not a point but a comet-shaped region of space) is called the heliopause, and it makes a convenient place to mark the boundary between the solar system and the rest of the universe.

Voyager 1 is very close to the heliopause. Last year at this time, the Voyager team reported that the outward-directed speed of the solar wind had dropped nearly to zero. With this observation and a mental model of the way the boundary between the solar wind and the interstellar medium should work, they formed a hypothesis: we are near the heliopause, and the direction of the energetic particles that Voyager 1 can measure should be shifting from the outward and east-west directed flow to a north-south one, the direction of the interstellar medium. So the simple experiment that the scientists needed to do to test their hypothesis was to measure the north-south flow of energetic particles. They predicted that they should be seeing increased north-south flow, matching the interstellar medium.

The problem is, Voyager 1 lost its plasma instrument a long time ago (in 1980) and can only measure two of the three components of the solar wind direction: out-and-in (out from and in toward the Sun) and east-west. To measure north-south, they would have to rotate the whole spacecraft 70 degrees and hold in a gyrocontrolled orientation -- a maneuver the spacecraft hadn't done for 21 years. ("An amazing feat," one scientist remarked.) But they did it four times this year, to check to see if they were right about their hypothesis.

Guess what? They were wrong. The wind wasn't shifting to a north-south direction. There was basically no flow north-south, either. And the out-and-in component got so slow this year that it sometimes even blew back at the spacecraft, a completely unexpected finding. The wind hadn't shifted direction; it had stagnated.

They think now that they're seeing an effect of the prolonged solar minimum, when there were less charged particles emanating from the Sun. When there's more pressure from solar wind, the heliosphere grows; when there's less, the heliosphere shrinks, and you get this wide stagnant boundary region between the heliosphere and interstellar space. At least, that's their new hypothesis! They said it should take Voyager 1 weeks to months to pass through this region.

Asked to reflect on this particular discovery in the context of Voyager 1's long mission, Robert Decker remarked that with this mission, "there's something new and surprising every 6 to 12 months," something that they have a hard time explaining. Those unexpected discoveries are what make space exploration so fun!