3 July 2008
A cool bit of news from beyond Mars this week: Voyager 2 has relayed new info on the shape of the solar system!
New data from the spacecraft, published yesterday in Nature, indicates that Voyager 2 passed through the termination shock in the heliosphere back in August/September of last year. Without the jargon, that means that Voyager 2 reached the location where the solar wind goes from super-sonic to sub-sonic, which creates a shock wave (like the ones that form when jets go super-sonic).
The interesting part of this news is that Voyager 2, traveling through the southern hemisphere of the heliosphere, measured the termination shock at about 87 AU (13 billion kilometers or 8 billion miles) from the Sun, which is about 10% less than Voyager 1 measured in December 2004 while traveling through the northern hemisphere. This means that the solar system is lopsided – at least as defined by the extent of the solar wind.
So why is the heliosphere lopsided? The authors suggest that this might be due to the fact that the magnetic field of the interstellar medium is oriented 60 degrees away from the flow of the interstellar medium, which is in the plane of the galaxy. Because the magnetic field is twisted, it puts more pressure on the southern boundary of the heliopause than on the northern boundary, causing the whole heliosphere to bulge to the north.
While many news articles are saying that this means that the Voyagers have left the solar system, it’s important to keep in mind that this is a fairly arbitrary way to define the edge of the solar system. After all, the Voyagers haven’t even gotten close to the heliopause, where the interstellar medium starts, or to the Oort cloud, the collection of icy bodies that orbit the sun in orbits that extend out to something like 50,000 AU, or one light year. (The Oort cloud is hypothesized to be the source of long-range comets that come screaming into the solar system occasionally.)