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This chart shows the orbital paths for Earth and the object known as
2000 SG344, with the estimated position of the two bodies during months
of the year 2030 leading to September. Astronomers said Saturday that
the object should miss Earth by at least 3 million miles.
 
  
 
 
Asteroid threat downgraded
Astronomers eliminate chance of collision in 2030
thanks to additional data, but say 'we're still watching it'
 
By Alan Boyle
MSNBC
 
 
      Nov. 4 -  One day after sounding an alert, astronomers said
additional data had eliminated any chance that a recently discovered
space object would collide with Earth in 2030. The revised forecast
shows the object passing no closer than 3 million miles. "We're still
watching it, but the 2030 event is not a concern anymore," the head of
NASA's asteroid-watching project told MSNBC.com Saturday.   

     
  
   
 
 
  
 
  

      
 

 
        THE FAINT OBJECT, dubbed 2000 SG344, was the first celestial
body to merit a rating higher than zero on the zero-to-10 Torino scale
of impact threats. That made the risk more significant than past real-
life asteroid scares but far less threatening than the scenarios
depicted in movies like "Armageddon" and "Deep Impact."
       On Friday, NASA and the International Astronomical Union said
the object had a 1-in-500 chance of hitting Earth on Sept. 21, 2030.
That may sound like a slight chance by terrestrial standards, but it
was a biggie for asteroid-watchers. In comparison, the "background
risk" that an object bigger than 0.6 mile (1 kilometer) in diameter
would hit Earth in a given year has been estimated at somewhere between
1 in 500,000 and one chance in a million.
       The estimate released Friday was based on observations made in
September using the Canada-France-Hawaii 3.6-meter telescope, plus a
review of 1999 data from the Massachusetts Institute of Technology's
LINEAR team.
       But late Friday, even more "prediscovery" data on SG344 came in
from the Catalina Sky Survey. That gave scientists such a good idea of
the object's orbit that they could see it would miss Earth in 2030.
       "The new solutions rule out the 2030 [collision]," said Don
Yeomans, manager of NASA's Near-Earth Object Program Office at the Jet
Propulsion Laboratory in Pasadena, Calif., "but it just pushes it out
40 years or so."
       He said there was still a slight risk of collision in the 2071-
73 time frame. However, additional observations of SG344 between now
and then could rule out even those possibilities, he said.  
 
 A picture taken in the wake of the Tunguska Meteorite explosion, which
occurred over Siberia in 1908, shows the devastation that even
a "small" asteroid or comet can produce.
 
      
PLAYING 'WHAT IF...'
       Even if the object were to hit Earth, it would be
no "Armageddon"-style mass-extinction event. Based on its brightness,
astronomers are guessing that the object is 30 to 70 meters (yards)
wide. The upper end of that range would put SG344 on the level of the
Tunguska Meteorite, which flattened a wide swath of Siberian forest in
1908. That blast was thought to be the equivalent of 15 million tons of
TNT - compared with a 20,000-ton yield for the Hiroshima atomic bomb.
       An impact involving SG344 could be the equivalent of 100
Hiroshima bombs, scientists said.
       "You certainly wouldn't want to be at the impact point, because
there's a good chance it would make it through the atmosphere . but at
most we would have really local consequences," said astronomer Paul
Chodas, a colleague of Yeoman's at the Near-Earth Object Program Office.
       From Earth's perspective, SG344 is slowly receding in an orbit
very close to our own and getting fainter every night. Gareth Williams
of the IAU's Minor Planet Center said that the object, currently 8.4
million miles away, is "beyond the range of all but the largest
telescopes on the planet."  
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         Within months, it will disappear from view until the 2020s.
That's why astronomers were anxious to check their data records to see
if SG344 showed up but went unnoticed. As in past cases involving
asteroid alarms, such "prediscovery observations" were key to
eliminating the perceived risk of a collision with SG344.
       The fact that the object follows an orbit so close to Earth's
raised the possibility that SG344 is actually a rocket booster that was
sent out from Earth years ago, Chodas said. The IAU pointed out that
the S-IVB rocket stages used for Apollo 8 through 12 missions, for
example, could have followed paths similar to the newly found object's
orbit.
       If SG344 is a rocket booster, the threat posed by any re-entry
into the atmosphere would be far less than if it were an asteroid,
Chodas said. He drew a parallel to NASA's Skylab space station, which
dropped through the atmosphere in 1979 and broke into fragments that
fell harmlessly into the Indian Ocean and onto Australia.
      
QUESTIONS IN THE AFTERMATH
       Benny J. Peiser, an anthropologist at Liverpool John Moores
University who specializes in the social implications of impact
threats, criticized the way the SG344 incident unfolded.
       In a posting to his widely respected Cambridge Conference e-mail
list, Peiser said Saturday that "it was extremely unwise" for
astronomers to rely on such limited data when they made their initial 1-
in-500 impact assessment, and that Friday's announcement by NASA and
the IAU was "premature and alarmist."
       Yeomans, however, said that he and other astronomers were trying
to follow guidelines dictating that announcements about asteroid risks
should be made within 72 hours after the risks are detected.
       "It's just that at 80 hours we got this data" from the Catalina
Sky Survey, he said.
         
  
    
           
     
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