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Sky & Telscope News Bulletin - March 13, 1998

SKY & TELESCOPE'S NEWS BULLETIN
MARCH 13, 1998

NEAR-EARTH ASTEROID CAUSES COMMOTION

Although currently no known asteroids are on a collision course with the 
Earth, there are nevertheless more than 100 bodies worrisome enough for the 
Minor Planet Center to catalog them as "potentially hazardous objects." The 
purpose of this list is to identify asteroids and comets that astronomers 
should routinely check to refine their orbits.

On March 11th, Brian Marsden of the Central Bureau for Astronomical 
Telegrams announced a new contender. The asteroid, designated 1997 XF11, 
was discovered by University of Arizona asteroid hunter James Scotti on 
December 6, 1997, as part of the Spacewatch project. Using additional 
observations made over the next three months, Marsden calculated a 
preliminary orbit for the 1.4- to 2.7-km-wide rock that showed it would 
pass only 40,000 kilometers above Earth's surface on October 26, 2028. 
However, the margin of error was still relatively large -- the only near-
certainty was that 1997 XF11 would pass by us at a distance closer than the 
Moon.

The circumstances of the flyby seemed to continually change during the 
following day as other astronomers made their own analyses. Orbital 
calculations by Donald Yeomans and Paul Chodas (Jet Propulsion Laboratory) 
showed that closest approach would be some 80,000 km, with errors only half 
as great. Furthermore, the "target plane" of the asteroid did not intersect 
the Earth, and thus the probability of impact was zero. Eleanor Helin (JPL) 
reports finding a prediscovery image of the object. Incorporating positions 
from this 1990 observation moved the nominal flyby distance out to a 
comforting 950,000 km. Additional observations over the next weeks and 
years will continue to firm up these figures.

ANCIENT CRATER CHAIN ON EARTH

The Earth already has many visible scars of cosmic collisions. Now 
researchers have linked five impact features and suggest that they all 
formed at the same time as a shattered comet or asteroid struck the Earth 
-- much as the pieces of Comet Shoemaker-Levy 9 hit Jupiter in 1994. In the 
March 12th NATURE, David Rowley (University of Chicago), John Spray 
(University of New Brunswick), and Simon Kelley (The Open University) 
explain how after moving the drifting continents back to their arrangement 
214 million years ago, impact scars in France, Canada, Ukraine, and 
Minnesota lined up. The largest of the craters is 100 km across. These 
impacts are a likely influence on the mass extinction of life at the end of 
the Triassic period, where 80 percent of the species then living on the 
Earth disappeared.




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