[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Asteroid Will Miss Earth By Comfortable Distance In 2028

I think Brian Marsden did the right thing, but the news media seems to blow
things out of proportion.

This situation with Asteroid 1997 XF11 is very similar to Comet Swift-Tuttle
from about 5 years ago.  Comet Swift-Tuttle was recovered in late 1992, and
prelimary analysis by Brian Marsden indicated a chance (albeit a very
small chance) that the comet could collide with the Earth in the year 2126.
The news media got wind of this and trumpeted an Earth impact in their
headlines, often leaving out the details that there was a 
large error margin and that that there was a very low probablity of impact.  
Additional observations of the comet were 
obtained, including matching the comet up with a previous appearance in 1737.  
Based on the additional data, Marsden recomputed the comet's orbit and
found the comet would miss the Earth by a comfortable 15 million
miles in 2126, and retracted the collision possiblity.

Note the similarity with 1997 XF11?

More details on the story is available here:

http://www.as.wvu.edu/~jel/skywatch/swfttle.html

Then there was Comet Shoemaker-Levy 9.  The comet was discovered near
Jupiter in 1993.  There was a large error margin with the initial orbit, which
was refined over time as more observations were made.  Marsden first
determined that the comet was in a temporary orbit around Jupiter.  Later on, 
he determined that half of the fragments could possibly collide with Jupiter.
Don Yeomans and Paul Chodas then chipped in, and they concluded that the
entire comet train would impact Jupiter with a 64 percent probablity.
With additional observations, the impact probablity became 100%, and the
rest was history.

The fact is, when a new object is discovered, there will be a large leeway
in its orbit determination, which will get refined over time.  There will be
more instances of Comet Swift-Tuttle and asteroid 1997 XF11 in the future - 
it is only inevitable.  The thing to make clear is up front is the initial 
orbit is "work in progress" and there are inherent large errors initially 
which will decrease with time as new data is obtained.

Ron Baalke
Follow-Ups: