Revisionists' view of solar system environs.

Astronomers are zeroing in on the nature of the gaseous environment in which the sun and nearby stars make their home. In addition to precisely measuring such properties as the distribution of gas in and around the solar system, a trio of new studies provide supporting evidence that the solar neighborhood consists of diffuse clouds embedded in a hot, virtually gas-free region. However, the origin of this void may differ from what astronomers had proposed.

Examining reflected sunlight near Mars, a French-U.S. research team has confirmed that hydrogen atoms slow down as they enter the solar system. Using the Hubble Space Telescope's Goddard High Resolution Spectrograph, the team found that in the solar system these atoms move at a speed of about 20 kilometers per second. Previous Hubble measurements, reported by a separate research group headed by Jeffrey L. Linsky of the Joint Institute for Laboratory Astrophysics in Boulder, Colo., are consistent with hydrogen atoms moving at a faster speed outside the solar system, about 25 km/sec. The slowdown confirms an earlier finding, using a ground-based telescope, reported by Rosine Lallement of the French research agency CNRS, in Verrieres-le-Buisson, and P. Bertin, a CNRS colleague.

In contrast to hydrogen, helium atoms don't change their speed as they cross into the solar system, notes John T. Clarke of the University of Michigan in Ann Arbor, a coauthor of the new study. Clarke and his colleagues suggest that the hydrogen slowdown occurs near the edge of the solar system, where the solar wind - a swiftly moving stream of charged particles -- slams into the slower moving interstellar wind. At this boundary, protons grab electrons from the hydrogen atoms. In the process, the hydrogen atoms decelerate. According to the researchers, helium atoms keep up their speed since the solar wind protons can't steal their electrons as efficiently.

Using estimates of the density of solar wind protons, Clarke and his colleagues calculate that the edge of the solar system lies about 100 astronomical units from the sun, or 100 times the distance from the Earth to the sun. If their calculation proves correct, the Voyager 1 spacecraft should pass the solar system's edge by 2010, the researchers note. Clarke, Lallement, and Jean-Loup Bertaux of CNRS report their work in the May 21 SCIENCE.

In another Hubble study, Linsky and his colleagues measured the local ratio of deuterium (a heavy isotope of hydrogen) to hydrogen in two directions: toward the nearby star Capella and another neighbor of the sun, Procyon. Results of a preliminary analysis indicate that the interstellar gas along the Procyon line of sight may have a smaller deuterium-to-hydrogen ratio than that in the direction of Capella. Future Hubble observations along other lines of sight should reveal if the local ratio truly varies. Linsky notes that because hydrogen and deuterium have not been produced since the Big Bang, the present-day ratio can be used to determine the amount of ordinary matter in the universe a few hundred seconds after the birth of the universe.

Examining a bigger piece of interstellar space, Barry Y. Welsh of NASA headquarters and his colleagues have used ultraviolet data from two orbiting observatories, ROSAT and the Extreme Ultraviolet Explorer, to map the contours of a huge void in space that extends about 600 light-years across, well beyond the solar system. Researchers had previously speculated that this giant void represents a low-density bubble blasted out by a supernova explosion (SN: 4/17/93, p.244).

But the new data, presented last week by Welsh at an astrophysics meeting in Noordwijk, the Netherlands, indicate the void has a highly misshapen contour and bears little resemblance to a single bubble. At the meeting, Fred Bruhweiler of Catholic University in Washington, D.C., proposed that the void marks the intersection of several bubbles, each sculpted by a separate supernova explosion or strong stellar wind.