Finding a place for the sun in a cloud.

At present, the solar system appears embedded in a tenuous cloud of interstellar gas. As the sun and planets move through the enveloping material, the cloud feeds hydrogen, helium, and other types of atoms into the solar system.

These interloping atoms penetrate to the inner planets, where the atoms interact with the solar wind and, when ionized, with Earth's magnetic field. This interstellar material may even influence Earth's climate.

Using data from a variety of satellites, astronomers have started to construct a detailed picture of the gaseous environment in which the solar system resides (SN: 5/22/93, p.326). Now, a new analysis of satellite data suggests that the sun first entered this interstellar cloud just a few thousand years ago.

"Over the last several million years, the solar system has traversed a region of space devoid of [interstellar matter] and appears to have only recently encountered the surrounding interstellar cloud," says Priscilla C. Frisch of the University of Chicago. Moreover, "the sun appears to be skimming the cloud surface."

Frisch describes her analysis and reports her conclusions in the Sept. 2 SCIENCE.

To determine the cloud's geometry in the solar system's vicinity, Frisch focused on measurements of cosmic rays and the spectra of light emitted by stars. She used data from seven spacecraft -- including the Hubble Space Telescope, the Extreme Ultraviolet Explorer, and Voyagers 1 and 2 -- orbiting throughout the solar system.

The data support the notion that the initial encounter between the solar system and the surrounding cloud occurred between 2,000 and 8,000 years ago. The measurements also indicate that the interstellar cloud is not uniform but consists of at least two clumps.

As additional data on the shape and extent of these cloud components become available, the uncertainty in the encounter date could be reduced. "It may be possible to determine with precision the encounter epoch and therefore to establish whether relatively low-density interstellar clouds are capable of affecting the terrestrial climate," Frisch says.

Frisch's analysis also indicates that the surrounding interstellar cloud has not been violently heated within the past million years. She argues that this gas represents the quiescent, residual material from one or more supernova explosions, which created a "superbubble" centered on a group of nearby stars known as the Scorpius-Centaurus Association (SN: 4/17/93, p.244).

Considered separately, the motions of the sun and the interstellar cloud are nearly perpendicular. Hence, the sun appears to be skimming the cloud near its edge. This relative motion also indicates that the solar system creates a bow shock -- a strong, leading disturbance where the speeding solar system impacts the expanding cloud.