Dino death: a stellar weapon ...

Two astronomers have refined previous estimates of how close certain types of stars must come to the Oort Cloud, a proposed reservoir of comets at the fringes of the solar system in order to trigger a comet shower that might wreak havoc on Earth. The comets most susceptible to the tug of a passing star are those with highly elongated orbits--that is, bodies whose farthest point from the sun is about 30,000 times Earth's distance from the sun and whose closest approach might equal Neptune's distance from the sun. If such comets are jostled out of position, they could venture near Earth as a comet shower about 2 million years later.

Although researchers have evidence of only one large body slamming into Earth around the time that the dinosaurs began dying out, some scientists believe the creatures may have taken several million years to become extinct and that a series of comets may have contributed to the death toll.

Over the past year or so, researchers at NASNs Jet Propulsion Laboratory (JPL) in Pasadena, Calif., have zeroed in on the triple star system Algol and the single star Gliese 710 as nearby bodies that might have disturbed the Oort Cloud in the recent past or may do so in the near future.

Recently, however, Lawrence A. Molnar and Robert L. Mutel of the University of Iowa in Iowa City examined the same stars but took into account some subtle effects--the motion of the sun as it orbits the Milky Way compared to the motion of nearby stars orbiting the galaxy, and the differences in the galaxy's gravitational pull on the sun and on neighboring stars. Using the Very Long Baseline Array, a suite of 10 radio telescopes spread out between Hawaii and the Virgin Islands, the researchers also measured the velocity of Algol with great accuracy, enabling them to trace more precisely its path back in time.

The new analysis indicates that Algol, now 63 light-years from Earth, came within 13 light-years of the solar system 4.3 million years ago, not as close as the JPL team had initially estimated. Molnar and Mutel also find that Gliese 710 will come within 3.1 light-years of the solar system 1 million years from now. The new figures reveal that neither star comes close enough to shake up the Oort Cloud and generate a comet shower.

Algol didn't miss by much, however. Given its mass, six times that of the sun, and its relatively low velocity, about 6 kilometers per second (km/sec), this star system could have induced a shower from as far away as 4 light-years, Molnar and Mutel report. In comparison, Gliese 710, which has half the mass of the sun and a relative velocity of about 14 km/sec, would have to pass within a light-year of the sun--adjacent to or just inside the outer edge of the Oort Cloud. The re searchers plan to search the Hipparcos catalogue, a recently released listing of very accurate distances and velocities for over 100,000 stars, for better candidates.