The Misshapen Solar System

Having traveled far beyond the planets in their 28.5-year journey, the two Voyager spacecraft are providing new information on the heliosphere, the teardrop-shaped bubble that separates the solar system from interstellar space.

At this week's Joint Assembly Meeting in Baltimore of the American Geophysical Union (AGU) and several other geophysics-related societies, Ed Stone of the California Institute of Technology in Pasadena, Calif., reported that the heliosphere is deformed, according to Voyager observations, with the teardrop's rounded edge bulging at the top, corresponding to the northern hemisphere of the solar system, and squashed at the bottom, corresponding to the southern hemisphere. (See pictures and movies on the NASA Web site.)

As Rob Decker of Johns Hopkins University Applied Physics Laboratory explained, the asymmetry is due to a magnetic field from interstellar space pushing on the southern hemisphere. The field is about 1/100,000 the strength of Earth's field but its effects can be felt for billions of miles, since it is acting over a large area on the very dilute gas at the solar system's edge.

The interstellar field even squashes an important spherical zone inside the heliosphere, called the termination shock. Analogous to the circle that forms when water splatters on a sink, the termination shock represents the boundary at which the rapidly traveling solar wind (the stream of charged gas from the sun) slows down abruptly and piles up.

Voyager 2's measurements indicate that the southern part of the termination sphere might be a billion miles closer to the sun than the northern part. Moreover, forces from the solar wind cause the termination shock to breathe in and out roughly every dozen years.

Voyager 1 has already ventured beyond the termination shock, to the heliosheath, the region where solar wind and interstellar gas mix. So in a way, the end of the solar system is not clearly defined. Stone guesses it could be another 10 years, or 3 to 4 billion miles, before the two spacecraft pass through the heliopause (the very outermost boundary of the heliosphere) and enter purely interstellar space. The spacecraft have about another 15 years of power left in them.

Session SH02 at the meeting
See the Joint Meeting home page
See pictures and movies on the NASA Web site