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(meteorobs) 1997 XF11 press release by Brian Marsden

ONE-MILE-WIDE ASTEROID TO PASS CLOSER THAN THE MOON IN 2028

     Recent orbit computations on an asteroid discovered last December
indicate it virtually certain that it will pass within the moon's distance
of the earth a little more than 30 years from now.  The chance of an actual
collision is small, but one is not entirely out of the question.

     The asteroid, known as 1997 XF11, was discovered by Jim Scotti in 
the course of the Spacewatch program at the University of Arizona.  This
program utilizes modern electronic technology on a 36-inch telescope at
Kitt Peak that was built 77 years ago.

     After the discovery observations on December 6, observations made by
two Japanese amateur astronomers during the following two weeks showed that
the minimum distance between the orbits of 1997 XF11 and the earth was
very small.  Given also that the object was quite large as earth-approaching
asteroids go, perhaps one mile across, it was added to the list of 
"potentially hazardous objects" (PHAs) that need to be monitored, lest they
are destined to come dangerously close to the earth over the course of the
next several centuries.  There are currently 108 PHAs.

     As astronomers continued to gather data on 1997 XF11, it slowly 
began to become apparent that there would be a particularly close 
approach to the earth in October 2028.  A computation from observations 
spanning 60 days suggested that the miss distance would be 500 thousand
miles.  This distance may seem large in human terms, but it was less than
had previously been predicted in advance for any other known asteroid 
during the foreseeable future.

     Observations made on March 3 and 4 by Peter Shelus with a 30-inch
telescope at the McDonald Observatory in western Texas extended the
observed arc of 1997 XF11 to 88 days.  This time, the orbit computation
indicated a miss distance of only 30 thousand miles from the center 
of the earth; the earth's radius is about 4 thousand miles.  The time
of encounter would be around 1:30 p.m. Eastern Daylight Time on Thursday,
October 26, 2028.  That evening the object should be visible with the
naked eye.  In Europe, where it would be dark by that time, the object
should be a splendid sight as it moves from northwest to southeast
across the sky over a couple of hours.

     There is still some uncertainty to the computation.  On the one hand,
it is possible that 1997 XF11 will come scarcely closer than the moon.
On the other hand, the object could come significantly closer than
30 thousand miles.  Further observations are necessary in order to refine
the figures.  It is also possible that prediscovery observations of 1997 XF11
can be located on archival photographs.  Particularly favorable opportunities
for recording the object would have occurred in 1990, 1983, 1976, 1971 and
1957.  Ephemerides for these times are available.

     It is hoped that continuing observations will be made during the next
few months.  The object is starting to move into the dusk and to fade
week by week.  Nevertheless, it should be quite accessible for a while with
large telescopes, which in addition to helping establish whether a
collision in 2028 is possible, could usefully provide more definite
information about the object's size.  

     Further observations of 1997 XF11 should be possible with moderate-sized
telescopes equipped with electronic sensors early in the year 2000.  A
better opportunity will occur in late 2002, when the object should be
detectable with quite modest telescopes.  On that occasion the closest
approach will be on Halloween, but the miss distance will be a safe
6 million miles.

Brian G. Marsden
1998 March 11