A Breeze from the Star Signs

Credit: American Scientist. More The Sun's gravity deflects the interstellar helium breeze and causes it to pile up downstream from the sun. This concentration is helpful to spacecraft observing the wispy-thin flow.

It's a hot cloud, the gas temperature is 6000 C, about the same as the temperature of the sun's surface. It's also very wispy, only 0.264 atoms per cubic centimeter. The sun's magnetic field has little trouble deflecting this diaphanous material before it crosses the orbit of Pluto. Only a trickle (0.015 atoms per cubic centimeter) penetrates the inner solar system.

One day the solar system might run into something more massive. There are clouds in the galaxy thousands of times denser than the Local Interstellar Cloud. University of Chicago astronomer Priscilla Frisch has studied what might happen if we plowed into one of those. Writing in the magazine American Scientist she reports, "a cloud with 1,000 atoms per cubic centimeter could compress the sun's magnetic field to within a few AU of the sun. (1 AU or "one astronomical unit" is the distance between the sun and Earth). Planets such as Saturn, Uranus, Neptune and Pluto would be fully exposed to interstellar atoms and molecules. Interstellar gas would overwhelm the solar wind at 1 AU," transforming the space-environment of our planet.

More An artist's concept of the Local Interstellar Cloud.

The first signs of such a transformation could be the helium breeze thickening or shifting directions, heralding something new to come.

ACE has already detected changes. "We see strange gusts, ebbs and flows," says Gloeckler. "We doubt these variations are interstellar." Instead, the sun is probably responsible. The helium breeze must blow through the much denser solar wind, which can push the breeze around. Sunspots also affect the breeze. Ultraviolet radiation shining from sunspots ionizes the breeze and changes the way it appears to instruments like SWICS.

"What we're doing now," explains Gloeckler, "is learning how solar activity affects the breeze. When we can reliably account for the sun, in detail, then we can use these measurements to diagnose interstellar space."

What's out there? What's coming? The answer lies in a breeze from the stars of the 13th house.